Gravity, Morphology, and Bedrock Depth of the Rathdrum Prairie

Transcription

Gravity, Morphology, and Bedrock
Depth of the Rathdrum Prairie, Idaho
Guy W. Adema
Roy M. Breckenridge
Kenneth F. Sprenke
Idaho Geological Survey
University of Idaho
Moscow, Idaho 83844-3014
Technical Report 07-2
ISBN 1-55756-514-6
Contents
Abstract............................................................................................................................ 1
Introduction . ................................................................................................................... 1
Geologic Setting ............................................................................................................. 2
Basement Rocks ............................................................................................... 2
Tertiary Geology .............................................................................................. 3
Quaternary Geology ......................................................................................... 4
Previous Geophysical Investigations .............................................................................. 7
Seismic Surveys ............................................................................................... 7
Gravity Surveys . .............................................................................................. 9
Gravity Data Collection .................................................................................................. 9
Purves Data ...................................................................................................... 9
Cady and Meyer Data .................................................................................... 21
New Observations .......................................................................................... 21
Gravity Data Reduction ................................................................................................ 23
Instrument Calibration Correction ................................................................. 23
Tidal and Instrument Drift Corrections .......................................................... 23
Terrain Corrections . ....................................................................................... 24
Latitude and Elevation Corrections . .............................................................. 25
Bouguer Corrections ...................................................................................... 25
Correlation of Data Sets ................................................................................. 25
Removal of Regional Trend ........................................................................... 26
Gravity Data Modeling ................................................................................................. 27
Gravity Model Interpretation ........................................................................................ 30
Conclusion .................................................................................................................... 34
Acknowledgments ........................................................................................................ 34
References . ................................................................................................................... 34
Figures
Figure 1. Location of the Rathdrum Prairie .................................................................... 2
Figure 2 Maximum extent of the Cordilleran ice sheet .................................................. 4
Figure 3. Maximum terminal extent of the Lake Pend Oreille lobe ............................... 5
Figure 4. Locations of geophysical work performed on the Rathdrum Prairie .............. 7
Figure 5. Seismic reflection profile . ............................................................................... 8
Figure 6. Locations of eight gravity measurements . .................................................... 26
Figure 7. Bouguer anomaly map of the Rathdrum Prairie . .......................................... 27
Figure 8. Bouguer gravity map showing regional trend ............................................... 28
Figure 9. Residual Bouguer gravity map of the Rathdrum Prairie ............................... 29
Figure 10a. The Idaho Road (Washington) profile ....................................................... 30
Figure 10b. The Corbin Road profile ............................................................................ 31
Figure 10c. The Idaho Road (Idaho) profile ................................................................. 31
Figure 10d. The Idaho Highway 41 profile . ................................................................. 32
Figure 10e. The Hayden Avenue profile ....................................................................... 32
Tables
Table 1. Principal Gravity Station Data ................................................................... 10-20
Table 2. Principal Facts for Primary Benchmark . ................................................... 22-23
Table 3. Digital Elevation Models ................................................................................ 25
Table 4. Modeled Aquifer Characteristics .................................................................... 33
iii
Gravity, Morphology, and Bedrock
Depth of the Rathdrum Prairie, Idaho
Guy W. Adema
Roy M. Breckenridge1
Kenneth F. Sprenke
ABSTRACT
The Rathdrum Prairie overlies part of a regional
ground-water source, known as the Rathdrum PrairieSpokane Valley aquifer, that covers 1,055 square km in
Spokane County, Washington, and Kootenai and Bonner
counties, Idaho. The aquifer is considered a sole-source
water supply for the greater Coeur d’Alene and Spokane
metropolitan areas. The 615-square-km part in Idaho is
called the Rathdrum Prairie aquifer. It extends from Lake
Pend Oreille south and west to the Idaho-Washington
state line. The aquifer occupies a glacially scoured
trough filled with highly permeable, coarse-grained,
catastrophically deposited glacial outwash.
Five geologic cross-sections of the valley have been
created using 630 gravity measurements, 146 of which
were collected specifically for our study to complement
existing data. The data were modeled in 2¾ dimensions,
and the regional trend caused by crustal thickening
to the east has been removed. The models present a
generally smooth valley floor with an incised channel
in the ancestral, subsurface valley in the western half
of the prairie. The bedrock-sediment interface appears
to be slightly sloped to the east with the deepest point
between Idaho Road (Idaho) and Hayden Avenue where
sediments may extend over 350 m below the surface.
Westward, near the state line, the sediments appear to
thin to a thickness of 216 m.
INTRODUCTION
The Rathdrum Prairie-Spokane Valley aquifer (Figure 1)
is a valley-fill aquifer that extends from the southern end
of Lake Pend Oreille, south to Coeur d’Alene Lake, and
west to Spokane. The valley is bound by Mount Spokane
to the north, the Mica Peak uplands to the south, and
1
Idaho Geological Survey, University of Idaho, Moscow, ID 83844
2
the Coeur d’Alene Mountains to the east. The aquifer is
identified as the sole-source water supply for the greater
Coeur d’Alene and Spokane metropolitan areas. Our
study will focus on the 615-square-km part in Idaho, the
Rathdrum Prairie aquifer.
In 1978, the aquifer was designated “sole source,”
which qualified it for protection under the 1974
Federal Safe Drinking Water Act. Since then, the
area’s rapid growth in both population and irrigation
demand continues to strain the aquifer’s capability.
Consequently, to answer civic concerns about the aquifer
requires reconstructing the broader geologic history of
the region. This must begin with a clear understanding
of the subsurface geology of the Rathdrum Prairie;
however, developing an accurate predictive model of
its structure has been difficult because of its complex
history. Nonetheless, knowing what the limits are for
this critical aquifer depends on scientific analysis that
measures its actual extent, recharge potential, and future
expectations. Determining the depth and geometry of
the bedrock surface is the first step. Our study brings
together new geophysical information and an improved
interpretation technique to more fully analyze the basin
geometry and its influence on hydrologic assumptions.
The Rathdrum Prairie aquifer is primarily composed
of valley-fill deposits of glaciofluvial origin. Extensive
gravels, deposited by catastrophic floods from Glacial
Lake Missoula, fill the ancestral valley of the Rathdrum
River (Breckenridge, 1989). The Missoula Floods, as
these events are known today, were caused by the periodic
impoundments and sudden releases of water from Glacial
Lake Missoula, which was reestablished many times
during the Pleistocene when the Pend Oreille lobe of the
Cordilleran ice sheet repeatedly blocked the Clark Fork
River (Bretz and others, 1956; Bretz, 1959). The latest
Department of Geological Sciences, University of Idaho, Moscow, ID 83844
Gravity, Morphology, and Bedrock Depth of the Rathdrum Prairie, Idaho
Adema, Breckenridge, and Sprenke
Figure 1. Location of the Rathdrum Prairie, Idaho (from Wyman, 1994).
periods of lake-emptying cycles occurred from 12,000 to
17,000 years ago (Waitt and Atwater, 1989). Proglacial
and flood processes deposited clasts with lithologies
derived from local terranes that include the Precambrian
Belt Series, the Cretaceous and Tertiary plutons and
associated rocks, the lower Paleozoic sediments, and the
Miocene basalt (Breckenridge, 1997). Some evidence
suggests the aquifer overlies the Latah Formation, which
is characterized by thick units of shale and clay with
some sands and gravels (Newcomb and others, 1953;
Pardee and Bryan, 1926). The Latah beds are thought
to lie unconformably over pre-Tertiary sediments,
metasediments, and granitic rocks. Intercalated with the
Latah Formation, and near the margins of the aquifer,
lies Miocene basalt of the Columbia River Basalt Group
(Breckenridge and Othberg, 1998a and 1998b).
These new data target coverage to specific profiles and
validate the previous work. The combined data set was
reduced and modeled with techniques not applied in the
earlier studies.
The thickening of the continental crust beneath the
northern Rocky Mountains (Winston and others, 1989;
Harrison and others, 1972) east of the Rathdrum Prairie
imposes a regional trend on gravity data. The regional
gradient is steep and obscures near-surface contributions
to the gravity field. The effects of this gradient were
not incorporated in the most extensive previous gravity
survey of the prairie (Purves, 1969). The models
developed in our investigation account for the regional
trend and, as a result, more accurately define the basin
geometry.
GEOLOGIC SETTING
Our study employs gravity techniques to determine
the subsurface geology. Previous gravity surveys (Purves,
1969; Hammond, 1974; Cady and Meyer, 1976a) and
seismic refraction and reflection studies (Newcomb and
others, 1973; Gerstel and Palmer, 1994) have focused on
relatively small sections of the aquifer and do not produce
an overall subsurface view of the Rathdrum Prairie. Our
study measured the gravity at 146 new locations on the
prairie to complement the same number of existing ones.
BASEMENT ROCKS
The country rocks surrounding, and presumably
underlying, the Rathdrum Prairie are mostly
Precambrian, but intrusions as young as Eocene are also
present. These pre-Miocene igneous and metamorphic
units are described in Lewis and others (2002). The
units include low-grade metasedimentary rocks of the
Belt Supergroup and high-grade (amphibolite facies)
metamorphic rocks whose protolith is either the Belt
Supergroup or the basement rocks that predate the
Belt metasedimentary rocks. The high-grade rocks are
exposed in the Priest River metamorphic core complex
west of the Rathdrum Prairie. Deformed granitic rock
(orthogneiss) of probable Cretaceous age is included
in the core complex. Plutonic rocks of Cretaceous age
are also present as intrusions within the low-grade Belt
Supergroup. Relatively undeformed Eocene igneous
rocks are exposed as plutons northwest of the Rathdrum
Prairie. A few Eocene rhyolite and dacite dikes are also
present.
Clark Fork Valley, and perhaps into Montana (Conners,
1976).
Younger Miocene basalt flows eventually overrode
the entire Rathdrum Prairie region. The separate
events and long interludes created alternating layers of
basalt and Latah Formation interbeds, as observed by
Hammond (1974) in well sections. The Latah sediments
were deposited in lakes formed where the basalt flows
impounded westward-flowing drainage systems. Kiver
and Stradling (1989) suggest that basalt and Latah
sediments filled the valley to a present-day elevation
of 730 m. Breckenridge and Othberg (1998a, 1998b)
delineate where these younger basalt flows filled the
embayments of the region. The increasing elevation of
basalt flows in the Miocene caused lake levels to the east
to rise accordingly. Eventually, levels were high enough
to force the drainage pattern to alter from a westerly
to a northerly direction along the margin of the basalt
(Connors, 1976; Savage, 1965). The new drainage flowed
north along the Purcell Trench through what is now Lake
Pend Oreille and then westward to the Columbia River.
The Latah sediments of the Rathdrum-Spokane valley
may have been completely covered by one or more
basalt flows during the final extrusions of the Columbia
River basalt.
The pre-Miocene igneous and metamorphic units
generally act as an aquiclude and constitute the
impervious basement to the Rathdrum Prairie aquifer.
Because of their complexity and local variation, the
pre-Miocene rocks are treated as one unit for gravity
modeling purposes. Differentiating the various host
rocks and intrusions and assigning unique physical
properties to each require more comprehensive data than
are available.
TERTIARY GEOLOGY
The rocks of Tertiary age include the Latah Formation
and the Columbia River Basalt Group. These rocks were
deposited on a mature erosional surface that developed
during the Late Cretaceous and Early Tertiary. The
geography included ridge crests and mountain tops with
gently rounded forms (Molenaar, 1988) and rugged
canyons with depths exceeding 600 m (Connors, 1976).
The west-flowing, Early Tertiary, ancestral Rathdrum
River occupied a valley under the present-day Spokane
Valley and Rathdrum Prairie (Savage, 1967).
The downcutting from the late Miocene to the
early Pleistocene removed as much as 180 m of Latah
sediments (Anderson, 1927). Developing drainages
probably eroded much of the exposed Latah beds and
some of the marginal basalt. The Coeur d’Alene drainage
developed along the basalt marginal valley now occupied
by Coeur d’Alene Lake. Those sediments, protected
by overlying basalt flows, were largely unaffected by
this erosional cycle, but a substantial amount of basalt
may have been eroded during this time. The thickness
of remaining Latah sediments is difficult to estimate
owing to the cover of Pleistocene drift and the scarcity
of evidence from boreholes. So far, researchers have not
determined the extent of the Latah sediments. Anderson
(1940) discovered a 300-m-thick bed of Latah Formation
below an exposed basalt flow in a well west of Hayden
Lake. Similarly, seismic velocities, intermediate to those
of bedrock and gravels, were interpreted by Newcomb
(1953) to represent Latah Formation near the state line.
He estimated the thickness of these sediments to be
300 m.
During the Miocene, flows of the Columbia River
Basalt Group spread northeastward from the Columbia
Plateau and filled these deep canyons. The basalt
dammed drainageways, including the Rathdrum River,
and these dams allowed lakes to form in which the
sand, silt, and clay beds of the Latah Formation were
deposited. The Latah sediments consist predominantly
of white, yellow, orange, and brown lacustrine silt and
clay, along with some fluvially deposited sand and gravel
units (McKiness, 1988). The older basalt flows did not
extend to the eastern and northern Rathdrum Prairie, and
a relatively thick section of Latah Formation is believed
to have formed at these locations, as evidenced in three
water-well logs of Hammond (1974). Pardee and Bryan
(1926) suggest that over 500 m of Latah sediments
accumulated in the Spokane area. Such deposits probably
extended throughout the Lake Pend Oreille basin, up the
The Missoula Floods eroded much of the Columbia
River Basalt Group and Latah Formation, significantly
altering the geologic features that had developed
during the Miocene. Many Tertiary geologic and
geomorphic features were either destroyed or obscured.
QUATERNARY GEOLOGY
exposure, and removed large amounts of rock material
from the valleys extending north of the main Rathdrum
Prairie (Conners, 1976; Savage, 1964).
The glacial and interglacial periods began between
2 Ma and 3 Ma in this region. The Cordilleran glacier
complex was the dominant geomorphic force during the
Pleistocene in North America. The epoch began with
high alpine glaciers in the Cascades, Coast Range, and
northern Rocky Mountains. As these glaciers coalesced
in major valleys, the thickening ice eventually formed
a massive sheet at least 2,200 m thick, over 3,400 km
long, and over 800 km wide (Conners, 1976). A series
of massive glacial lobes occupied the major north-south
trending valleys in northern Idaho and produced the
primary landscaping event of the period. Erosion by the
advancing ice lobes deepened valleys, smoothed bedrock
Determining the southernmost extents to the various
lobes of the Cordilleran ice sheet is difficult owing to
the lack of exposed glacial and proglacial features.
Richmond (1986) has compiled the stratigraphy and
chronology of well-documented glacial deposits. The
current consensus on the southernmost margin of
the ice sheet is shown on Figure 2, as summarized in
Richmond (1986) and modified by Breckenridge (1989).
The locations shown are revisions of previous estimates
taken from numerous studies of Quaternary deposits
throughout the northwest.
Figure 2. Maximum extent of the Cordilleran ice sheet (Breckenridge, 1989). BRL = Bull River lobe; FL = Flathead lobe;
PTL = Purcell Trench lobe; PRL = Priest River lobe; TRL = Thompson River lobe. Crosshatched area = Glacial Lake Missoula.
Arrows show routes of major flood outbursts.
Of interest to our study is the Lake Pend Oreille lobe,
sometimes referred to as the Rathdrum lobe. This tongue
of the Cordilleran ice sheet extended southward between
the Selkirk Range and Cabinet Mountains. Flint (1937)
and Bretz and others (1956) originally mapped the Lake
Pend Oreille lobe well onto the Columbia Plateau. Weis
and Richmond (1965) extended it through the Rathdrum
Prairie-Spokane Valley to the present location of
Spokane. A more refined estimate of the southern extent
of the lobe is shown in Figure 3, taken from Breckenridge
(1989) who mapped it as extending only to the present
southern shore of Lake Pend Oreille.
Oscillations of the Lake Pend Oreille lobe significantly
affected the formation of the Rathdrum Prairie-Spokane
Valley aquifer. Most sediments in the valley are thought
to have been deposited catastrophically. With each
major advance of the lobe in late Wisconsin time, an
ice dam formed at Clark Fork and impounded the Clark
Fork River to create another glacial Lake Missoula.
The ice dams eventually became unstable, producing
periodic failures and recurrent, sudden flooding of the
downstream reaches. With each failure, as much as 2.1
x 107 m3sec-1 of water was released (Baker, 1973). These
events are now referred to as the Missoula Floods. The
Figure 3. Maximum terminal extent of the Lake Pend Oreille lobe of the Cordilleran ice sheet (from Breckenridge, 1989).
water released from Lake Missoula flowed forcefully
across the Rathdrum Prairie, scoured the region now
known as the Channeled Scabland, and continued down
the Columbia River to the Willamette Valley and the
Pacific Ocean.
1998c). The most recent flood events are thought to have
occurred between 17,200 and 11,000 years ago (Waitt,
1985). The number of flooding episodes is unknown,
but at least fifty have been associated with depositional
rythmites (Waitt, 1980). Other researchers have found
evidence for numerous episodes in other localities
(Sieja, 1959; Chambers, 1971, 1984; Breckenridge and
Othberg, 1998c).
Evidence for the Missoula Floods was introduced by
J Harlan Bretz in the 1920s (Bretz: 1923, 1925, 1928a,
1928b, 1928c, 1930a, 1930b, 1932). Bretz identified
erosional and depositional features in the Channeled
Scabland. For him, the only explanation for these unique
landforms was a relatively brief, but enormous flood,
which he called the Spokane Flood. Bretz investigated
such examples as erratic boulders, deeply notched cliffs,
streamlined loess hills, gravel deposits, and the Portland
delta.
Several theories compete to explain the catastrophic
ice dam failures and have been summarized by
Breckenridge (1989). Those theories include jökulhlaup
releases (Waitt, 1985), flotation of the ice (Thorarinsson,
1939), the enlargement of subglacial tunnels by water
(Liestol, 1956), the deformation of ice by water pressure
(Glenn, 1954), and the enlargement of tunnels by icebergs
(Aitkenhead, 1960). Whatever the mechanism of icedam failure, the geomorphic expression of the Missoula
Floods has been well documented and serves as the most
recent significant event affecting the composition of the
Rathdrum Prairie-Spokane Valley aquifer.
Scientists were slow to accept his thesis. For years,
most denounced the idea as outrageous and physically
impossible. Prominent among the old guard attacking
this perceived heresy was R.F. Flint (1935, 1936, 1938)
who, in rebuffing Bretz, decreed the scablands merely the
result of normal proglacial discharge. The controversy
finally prodded J.T. Pardee to show Bretz his own
work on a large Pleistocene lake in western Montana.
Pardee (1910) had been studying this evidence of a flood
source, which he named Lake Missoula, before Bretz
presented the idea of massive floods. Finally, Pardee
(1942) published his work on glacial Lake Missoula.
The paper strongly complemented Bretz’s great flood
hypothesis and represents a much-cited example of how
the collaboration of evidence advances ideas in science.
Both men agreed that the Lake Pend Oreille lobe of the
Wisconsin ice sheet had impounded a huge lake and that
the lake had drained catastrophically. Later, this idea
would be enlarged to include several episodes of ice
dams and floods.
The widely accepted geologic model of the Rathdrum
Prairie has the ancestral Rathdrum River valley first
being filled with Miocene basalt and Latah sediments
and then with Missoula Flood deposits. The Missoula
Floods eroded much of the basalt and Latah sediments
and also obscured their exposures with extensive flood
deposits. These flood deposits are principally composed
of fine to coarse gravels of glaciofluvial origin derived
from glacial outwash of the Purcell Trench lobe. The
provenance of the gravels includes Precambrian Belt
Supergroup rocks, Tertiary and Cretaceous plutons, lower
Paleozoic sediments, and Miocene basalt (Breckenridge
and others, 1997).
Coarser gravels are found centrally in the valley, and
finer sands and gravels are near the margins. Some of the
sands and gravels are classified as eddy and pendant bar
deposits (Breckenridge and Othberg, 1998a and 1998b).
The high-energy depositional environment resulted in
cross-bedded gravel deposits with intercalated layers
of finer sands and clays. Discriminating these sand
and gravel layers with gravity methods is impractical,
but their existence is visible in local gravel pits. The
gravel structure is further complicated by the occasional
occurrence of clast cementation (Breckenridge and
others, 1997). A primarily calcium carbonate cement
varies in development, from minimal clast rinds to
near complete matrix filling. The cement was found
to have fine amounts of angular silica, perhaps carried
downward by water infiltrating through surface ash. The
coatings, which must have originally formed in the zone
of a fluctuating water table, are now found in the vadose
Evidence described by Pardee (1942) included
severely scoured constrictions in the lake basin, huge bars
of current-transported debris, and giant ripple marks with
heights of 16 m and spacings of 160 m. Bretz and others
(1956) and Bretz (1959) provided revisions to his original
work that suggested several flood episodes. Chambers
(1971, 1984) followed with detailed descriptions of the
sedimentation cycles of the rhythmically bedded floor
sediments in the Clark Fork valley. He interpreted them
as evidence of multiple filling and flooding sequences.
Present work is looking at whether sedimentary
variations between rythmite beds represent distinct
flood events or are merely products of different energy
levels from a single event, as well as at the timing of
each depositional feature (Breckenridge and Othberg,
zone. The cemented gravels and intercalated fine beds
provide complicated hydrologic characteristics in the
aquifer that may adversely affect hydrologic modeling.
from Cady (1976) and Purves (1969) that complemented
ours. Results from other geophysical investigations also
helped in clarifying the subsurface conditions and in
developing the model.
The gravels are mantled with thin volcanic ash and
loess deposits. The distribution of these eolian sediments
varies greatly and is difficult to quantify. Deposits seldom
reach a meter in thickness and in most places only a few
centimeters (R.M. Breckenridge and K.L. Othberg, oral
commun., 1998). Some of the material is thought to
percolate into the matrix of the underlying gravels. The
surficial geologic mapping of Breckenridge and Othberg
(1998a and 1998b) did not include ash and loess.
SEISMIC SURVEYS
Newcomb and others (1953) completed two seismic
refraction surveys near Spokane, Washington, in May
and June of 1951 to locate the base of the glacial outwash
aquifer. This geophysical investigation of the Rathdrum
Prairie aquifer obtained stratigraphic and hydrologic
information about the Spokane Valley. The study also
sought to determine the type of material underlying the
aquifer and to locate the bedrock of the ancestral valley.
One survey trended north-south across the Spokane
Valley east of the Idaho-Washington border; the other
trended east-west across the Hillyard trough, north of
Spokane. The survey near the Washington-Idaho border
(Figure 4) provided the primary bedrock “ties” for the
gravity modeling in our report.
PREVIOUS GEOPHYSICAL
INVESTIGATIONS
Geophysical data from two previous studies have
been included in our research to create a more complete
model of the subsurface geology of the Rathdrum
Prairie. We incorporated 484 gravity measurements
Figure 4. Locations of geophysical work performed on the Rathdrum Prairie.
Newcomb and others (1953) based their interpretation
on limited reverse seismic refraction data. Their few
refractions indicated a V-shaped valley with a maximum
aquifer thickness of about 480 m where glacial outwash
contacts the Latah Formation. The contact corresponds to
an elevation of about 530 m and occurs with the inference
of Pardee and Bryan (1926) about bedrock depth in this
region. Newcomb and others (1953) interpreted five
subsurface units from their refractions: (1) soil and subsoil
of the glacial outwash, (2) unsaturated glacial outwash,
(3) saturated glacial outwash, (4) Latah Formation with
intercalated igneous rocks, and (5) granitic rock. The
velocity obtained for the Latah Formation was so high
that Newcomb and others (1953) reasonably presumed
that basalt sills and dikes were present. These sills and
dikes have also been found in the Latah Creek vicinity
(Pardee and Bryan, 1925) and in the highway cuts
southeast of Coeur d’Alene (Conners, 1976). Newcomb
and others (1953) were limited by the technology of
their time in the amount of data that could be obtained
and interpreted. The geologic interfaces shown in their
model were based on only a few data points.
Seismic reflections on the Rathdrum Prairie by Gerstel
and Palmer (1994) followed the lines of Newcomb and
others (1953) on Idaho Road (Washington) so that results
of the two studies could be compared (Figure 4). (Because
Idaho Road lies in both Washington and Idaho, its
location will be identified with the state name following
in parentheses.) Their unconventional technique used
a single point, zero-offset shot-receiver procedure and
a pneumatic acoustic source. They claimed to find
an undulating bedrock surface 160 m deep, with an
average bedrock-sediment interface at about 475 m
above sea level. Their interpretation revealed a much
more U-shaped valley floor than previously thought
(Figure 5). The velocities used for reflection processing
were obtained from cross-hole seismic techniques and
Figure 5. Seismic reflection profile (top) and interpretation (bottom) of Gerstel and Palmer (1994). An unconventional single point
receiver technique, with a pneumatic acoustic source, was used.
were unavailable for us to compare with the refraction
velocities measured by Newcomb and others (1953).
of the aquifer, he claimed that the basement configuration
appeared to be a fluvially dissected erosional valley
with erosional terraces composed of either a complex
basement or basalt.
GRAVITY SURVEYS
Purves (1969) never numerically modeled his data
because of the difficulty in running computations before
the common use of computers. Instead, he relied on the
simple supposition that, in a relatively simple geologic
system, the Bouguer gravity field will generally mimic
the underlying bedrock shape. The data from his study
were used extensively in our study and provide the
effective starting point for our investigation.
Bonini (1963) conducted the first regional gravity
study of Idaho and produced a reconnaissance Bouguer
anomaly map of the state using over 1,200 observations.
The study was part of one on gravity anomalies, isostatic
equilibrium, and tectonic features of the northwestern
United States. Bonini correlated the Idaho batholith with
negative Bouguer anomalies (as low as -236 mGals),
the Snake River Plain with a broad gravity high, and
the Columbia River Plateau as a relative gravity high.
In northern Idaho and western Montana, he identified a
modest variation in the Bouguer anomaly that generally
followed a north-south trend. Bonini concluded that the
pattern reflected the broad structural fabric and variations
in density values of different members of the Belt
Supergroup rocks. The survey was not detailed enough
to identify individual features or units that the gravity
variations were attributed to. Though no subsurface
geological modeling was performed, Bonini (1963) laid
the groundwork for future gravity work in Idaho.
In 1969, Hammond (1974) began a gravity survey
and subsequent remodeling of the northern Rathdrum
Prairie. This U.S. Geological Survey study, undertaken
in cooperation with the then Idaho Department of Water
Administration, evaluated previous estimates of (1) the
quantity of underflow moving toward the Rathdrum
Prairie from the Athol area across a line extending about
10 km northwestward from Chilco and (2) the quantity
of water being recharged to the aquifer by Lake Pend
Oreille. A detailed gravity survey was conducted to
define the configuration of the bedrock surface and to
calculate the thickness of fill from the southern end of
Lake Pend Oreille south to the three Chilco channels,
which were the focus of the study. Unfortunately, details
about the gravity data reduction and modeling were not
included in the report. Depth and Bouguer anomaly
contours produced by Hammond (1974) only overlap
the very northern section of the region encompassed by
our study, but show a trend that continues to Lake Pend
Oreille.
Purves (1969) performed an extensive gravity survey
of the Spokane Valley-Rathdrum Prairie to identify
subsurface stratigraphic units, possible areas of underflow
impedance, and glacial conditions. Over 743 gravity
measurements were taken on 16 profiles in Idaho and
Washington. All standard corrections, including terrain,
were made, and measurements were tied to the extended
gravity control network of North America established
by Wollard and Behrendt (1961). The survey by Purves
(1969) provides the starting point for our study, and
selected data from that work are modeled with ours.
GRAVITY DATA COLLECTION
Our investigation combines gravity data from three
main sources: Purves (1969); Cady and Meyer (1976),
which includes the data of Bonini (1963) and Hammond
(1974); and our own measurements taken in 1997. Data
collected for our study was carefully planned to fill
gaps in the coverage of previous studies. The combined
result is an extensive data set that allows for more
comprehensive modeling and interpretation. The three
data sets are assimilated as a final numerical adjustment.
Principal facts for all gravity data are given in Table 1.
A significant finding of Purves (1969) was the
existence of a probable west-northwest trending
subsurface drainage divide within the Rathdrum Prairie
basin about 3.2 km west of the Washington-Idaho state
line. He suggests that this drainage divide was produced
by the terminal position of the maximum glacial advance
of a proposed Hayden Lake lobe and the recognized Pend
Oreille lobe. He claimed that the subsurface configuration
east of the divide, in the Rathdrum Prairie, appeared to
be dominantly influenced by glacial erosive processes,
with the U-shaped trough filled almost exclusively with
glacial material and minimal basalt. He cites irregular
geohydrological evidence near the proposed flow divide
that suggests the ultimate glacial terminus existed west
of this divide. His survey was inconclusive about the
existence of any Latah Formation in the Rathdrum Prairie
section. West of this divide, in the Spokane Valley section
PURVES DATA
In 1966, Purves (1969) collected over 743 gravity
measurements on transects of the Rathdrum Prairie and
Spokane Valley. Of these, 276 stations on 5 profiles were
used in our study (Figure 4). Though Purves did the
Table 1. Principal Gravity Station Data.
Field descriptions:
Count
Point_ID
DD_lat
DD_long
Easting
Northing
Elevation
Obs_Grav
C.B.A._1
C.B.A._2
Count
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
Point_ID
AB285
AG601
AG602
AG603
AG604
AG605
AG606
AG607
AG608
AG609
AH101
AH102
AH103
AH104
AH105
AH106
AH107
AH108
AH109
AH110
AH111
AH112
AH113
AH114
AH115
AH116
AH117
AH118
AH119
AH120
AH121
AH122
AH123
AH124
AH125
AH126
AH127
AH128
AH129
AH130
AH131
DD_lat
47.8022
47.6770
47.6773
47.6773
47.6773
47.6772
47.6772
47.6772
47.6772
47.6769
47.7593
47.7593
47.7594
47.7593
47.7593
47.7593
47.7592
47.7592
47.7592
47.7592
47.7592
47.7591
47.7590
47.7590
47.7590
47.7590
47.7589
47.7589
47.7589
47.7590
47.7590
47.7590
47.7590
47.7590
47.7589
47.7589
47.7589
47.7588
47.7588
47.7587
47.7587
Arbitrarily assigned.
Descriptive identification for each point. Alpha prefix describes data source.
A = Adema, P = Purves (1969), D = Cady and Meyer (1976a)
Degree-decimal latitude, WGS84 geiod.
Degree-decimal longitude, WGS84 geiod.
UTM zone 11 easting value, presented in meters.
UTM zone 11 northing value, presented in meters.
Elevation above MSL, presented in meters.
Observed station gravity, presented in mGal. Corrected only for drift.
Bouguer Anomaly, ρ=2.67 kg/m3.
Residual Bouguer anomaly, ρ=2.67 kg/m3, regional trend correction of 0.11 mGal per km east.
DD_long
-116.9156
-116.7979
-116.7914
-116.7881
-116.7805
-116.7762
-116.7723
-116.7673
-116.7618
-116.7567
-116.9584
-116.9533
-116.9479
-116.9428
-116.9370
-116.9316
-116.9264
-116.9211
-116.9159
-116.9128
-116.9075
-116.9038
-116.9007
-116.8930
-116.8885
-116.8839
-116.8788
-116.8722
-116.8669
-116.8621
-116.8564
-116.8506
-116.8441
-116.8390
-116.8334
-116.8295
-116.8257
-116.8213
-116.8172
-116.8133
-116.8085
Easting
506,323.36
515,179.26
515,658.67
515,908.83
516,471.92
516,805.57
517,097.47
517,472.87
517,889.71
518,265.29
503,122.67
503,497.46
503,913.80
504,288.48
504,725.73
505,121.23
505,516.74
505,912.25
506,307.76
506,536.89
506,932.40
507,202.98
507,432.00
508,014.91
508,347.95
508,702.01
509,076.50
509,576.21
509,971.72
510,325.78
510,763.04
511,199.98
511,678.98
512,074.48
512,490.71
512,782.31
513,052.89
513,385.93
513,698.25
513,989.74
514,343.81
Northing
5,294,103.18
5,280,195.86
5,280,227.97
5,280,228.65
5,280,230.43
5,280,231.24
5,280,232.29
5,280,233.17
5,280,234.41
5,280,204.81
5,289,315.06
5,289,315.33
5,289,346.43
5,289,315.92
5,289,316.28
5,289,316.57
5,289,316.87
5,289,317.46
5,289,317.76
5,289,318.11
5,289,318.40
5,289,318.81
5,289,288.39
5,289,288.96
5,289,289.47
5,289,290.01
5,289,290.57
5,289,291.33
5,289,291.93
5,289,292.78
5,289,293.44
5,289,294.40
5,289,295.13
5,289,296.04
5,289,296.97
5,289,297.41
5,289,298.13
5,289,298.94
5,289,299.72
5,289,269.38
5,289,270.23
10
Elevation
665.1
651.7
651.0
659.1
667.8
668.7
669.0
665.5
665.3
662.4
659.9
661.7
662.0
662.4
661.1
667.5
675.3
679.4
684.4
682.2
679.7
680.9
683.0
685.9
688.5
689.6
688.4
687.3
686.6
687.5
689.5
692.2
692.9
697.5
697.6
699.2
701.0
702.0
702.0
701.0
700.4
Obs Grav
980,649.10
980,632.59
980,631.27
980,629.50
980,627.90
980,627.50
980,627.70
980,628.60
980,629.14
980,630.56
980,643.52
980,642.02
980,641.51
980,641.16
980,641.81
980,640.33
980,637.82
980,636.44
980,635.13
980,635.39
980,635.15
980,634.33
980,633.45
980,632.25
980,631.63
980,631.19
980,630.92
980,630.57
980,630.34
980,629.51
980,628.31
980,627.49
980,627.21
980,626.04
980,625.15
980,624.51
980,623.76
980,623.47
980,623.40
980,623.48
980,623.20
C.B.A. 1
-92.38
-100.83
-102.33
-102.51
-102.42
-102.66
-102.38
-102.13
-101.59
-100.65
-95.81
-96.97
-97.44
-97.72
-97.33
-97.56
-98.55
-99.15
-99.47
-99.65
-100.40
-100.98
-101.45
-102.08
-102.16
-102.41
-102.93
-103.49
-103.86
-104.53
-105.33
-105.64
-105.77
-106.04
-106.90
-107.22
-107.62
-107.70
-107.77
-107.88
-108.26
C.B.A. 2
-4.42
-3.13
-4.10
-4.01
-3.30
-3.18
-2.57
-1.90
-0.91
0.45
-11.38
-12.12
-12.13
-12.00
-11.13
-10.93
-11.48
-11.64
-11.53
-11.46
-11.78
-12.06
-12.28
-12.26
-11.98
-11.84
-11.94
-11.95
-11.89
-12.17
-12.49
-12.32
-11.92
-11.76
-12.16
-12.16
-12.26
-11.98
-11.70
-11.49
-11.48
Count
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
Point_ID
AH132
AH133
AH134
AH135
AH136
AH137
AH138
AH139
AH140
AH141
AH142
AH143
AH144
AH145
AH146
AH147
AM301
AM302
AM303
AM304
AM305
AM306
AM307
AM308
AN501
AN502
AN503
AN504
AN505
AN506
AN507
AN508
AN509
AN510
AN511
AP001
AP002
AP004
AP005
AP006
AP007
AP008
AP009
AP010
AP011
AP012
AP013
AP014
AP015
AP016
AP017
AP018
AP019
AP020
AP021
AP022
AP023
AP024
AP025
DD_lat
47.7587
47.7587
47.7587
47.7586
47.7587
47.7587
47.7587
47.7587
47.7587
47.7586
47.7587
47.7585
47.7585
47.7585
47.7586
47.7586
47.7767
47.7729
47.7681
47.7638
47.7604
47.7569
47.7544
47.7508
47.6860
47.6860
47.6860
47.6859
47.6859
47.6858
47.6859
47.6858
47.6858
47.6858
47.6859
47.7021
47.7845
47.7813
47.7784
47.7755
47.7720
47.7693
47.7655
47.7634
47.7606
47.7585
47.7557
47.7530
47.7500
47.7471
47.7447
47.7416
47.7386
47.7359
47.7331
47.7301
47.7269
47.7242
47.7214
DD_long
-116.8050
-116.7996
-116.7937
-116.7913
-116.7884
-116.7856
-116.7825
-116.7786
-116.7754
-116.7724
-116.7681
-116.7654
-116.7630
-116.7607
-116.7577
-116.7566
-116.7781
-116.7782
-116.7783
-116.7783
-116.7784
-116.7784
-116.7783
-116.7783
-116.7937
-116.7913
-116.7883
-116.7855
-116.7811
-116.7778
-116.7738
-116.7714
-116.7686
-116.7661
-116.7631
-116.9498
-116.9369
-116.9370
-116.9370
-116.9371
-116.9371
-116.9372
-116.9373
-116.9373
-116.9373
-116.9373
-116.9373
-116.9373
-116.9373
-116.9373
-116.9373
-116.9372
-116.9372
-116.9372
-116.9372
-116.9372
-116.9371
-116.9371
-116.9372
Easting
514,614.39
515,009.89
515,467.86
515,634.53
515,863.36
516,071.47
516,300.61
516,591.91
516,841.77
517,049.88
517,383.22
517,591.34
517,757.71
517,924.38
518,153.52
518,236.70
516,628.05
516,608.22
516,609.69
516,611.08
516,612.38
516,613.49
516,614.02
516,615.32
515,489.46
515,656.13
515,885.27
516,093.88
516,427.54
516,677.69
516,969.60
517,157.30
517,365.71
517,553.41
517,782.55
503,772.11
504,723.30
504,723.60
504,723.91
504,703.31
504,703.70
504,703.81
504,704.20
504,704.53
504,704.64
504,704.79
504,705.20
504,705.31
504,705.62
504,706.04
504,706.26
504,706.56
504,706.56
504,706.97
504,707.08
504,707.38
504,707.69
504,708.10
504,708.21
Northing
5,289,270.94
5,289,272.15
5,289,273.15
5,289,273.71
5,289,274.36
5,289,274.98
5,289,275.63
5,289,276.37
5,289,277.06
5,289,277.68
5,289,278.80
5,289,279.42
5,289,279.67
5,289,280.22
5,289,280.88
5,289,281.31
5,291,283.22
5,290,850.81
5,290,326.11
5,289,862.96
5,289,461.67
5,289,091.15
5,288,813.26
5,288,411.96
5,281,215.57
5,281,216.13
5,281,216.78
5,281,186.32
5,281,187.13
5,281,188.12
5,281,188.87
5,281,189.46
5,281,190.08
5,281,190.67
5,281,191.32
5,282,986.70
5,292,125.66
5,291,786.21
5,291,446.47
5,291,137.77
5,290,736.47
5,290,427.81
5,290,026.21
5,289,779.40
5,289,470.73
5,289,254.39
5,288,945.72
5,288,637.05
5,288,297.61
5,287,988.64
5,287,710.75
5,287,371.31
5,287,031.56
5,286,722.90
5,286,414.23
5,286,074.79
5,285,735.04
5,285,426.38
5,285,117.71
11
Elevation
700.7
698.5
699.3
699.2
697.2
694.8
693.2
695.0
693.7
689.0
690.3
698.2
699.4
699.4
699.3
699.3
700.8
698.3
700.2
697.8
698.1
691.2
689.5
687.3
669.5
672.2
675.4
677.3
676.9
675.5
674.8
669.4
666.1
666.3
664.8
649.4
660.0
662.6
664.1
662.7
668.7
668.8
666.5
666.3
661.5
661.1
660.0
660.9
668.1
670.6
674.1
684.3
681.2
679.7
679.4
678.3
676.0
674.6
672.4
Obs Grav
980,622.81
980,623.14
980,622.91
980,623.02
980,623.54
980,624.05
980,624.16
980,623.77
980,624.17
980,624.96
980,624.54
980,622.89
980,622.93
980,623.00
980,623.21
980,623.15
980,622.32
980,622.57
980,622.20
980,622.97
980,623.26
980,624.60
980,624.93
980,626.23
980,627.95
980,626.59
980,625.29
980,624.80
980,625.58
980,626.37
980,627.58
980,629.11
980,630.07
980,630.46
980,630.48
980,641.74
980,648.02
980,646.32
980,644.99
980,644.34
980,641.75
980,640.96
980,640.79
980,640.75
980,642.05
980,641.78
980,641.24
980,640.01
980,637.45
980,636.76
980,636.48
980,634.42
980,634.89
980,635.15
980,635.02
980,634.92
980,634.99
980,635.12
980,635.47
C.B.A. 1
-108.60
-108.68
-108.74
-108.63
-108.51
-108.46
-108.65
-108.65
-108.50
-108.58
-108.73
-108.79
-108.50
-108.42
-108.19
-108.22
-110.62
-110.51
-110.09
-109.39
-108.71
-108.39
-108.16
-106.94
-102.80
-103.68
-104.37
-104.48
-103.78
-103.26
-102.14
-101.64
-101.28
-100.81
-101.00
-93.81
-93.37
-94.35
-95.15
-95.85
-96.99
-97.53
-97.84
-97.72
-97.12
-97.28
-97.79
-98.60
-99.49
-99.45
-98.80
-98.62
-98.49
-98.27
-98.21
-98.24
-98.32
-98.20
-98.02
C.B.A. 2
-11.53
-11.17
-10.72
-10.43
-10.06
-9.78
-9.72
-9.40
-8.98
-8.82
-8.61
-8.44
-7.97
-7.70
-7.23
-7.16
-11.33
-11.24
-10.82
-10.12
-9.44
-9.12
-8.88
-7.66
-4.76
-5.46
-5.90
-5.78
-4.71
-3.91
-2.48
-1.77
-1.18
-0.50
-0.44
-8.66
-7.18
-8.15
-8.96
-9.67
-10.81
-11.35
-11.67
-11.55
-10.95
-11.11
-11.62
-12.42
-13.32
-13.27
-12.62
-12.44
-12.31
-12.09
-12.03
-12.06
-12.14
-12.02
-11.84
Count
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
Point_ID
AP026
AP027
AP028
AP029
AP030
AP031
AP032
AP033
AP034
AP035
AP036
AP037
AP038
AP039
AP040
AP041
AR401
AR402
AR403
AR404
AS201
AS202
AS203
AS204
AS205
AS206
AS207
AS208
AS209
AS210
AS211
AS212
AS213
AS214
AS215
AS216
AS217
AV701
AV702
AV703
AV704
AV705
AV706
AV707
AV708
AV709
D7928
D7942
D7961
D7962
D7972
D7977
D7984
D7985
D7987
D7992
D8006
D8010
D8012
DD_lat
47.7184
47.7157
47.7132
47.7108
47.7080
47.7047
47.7021
47.6982
47.6958
47.7885
47.7909
47.7939
47.7956
47.7969
47.7994
47.8025
47.7480
47.7534
47.7586
47.7661
47.7824
47.7800
47.7779
47.7758
47.7735
47.7707
47.7685
47.7670
47.7654
47.7633
47.7611
47.7587
47.7568
47.7542
47.7518
47.7490
47.7478
47.6734
47.6765
47.6837
47.6914
47.6968
47.7030
47.7098
47.7159
47.7225
47.6267
47.6392
47.6467
47.6468
47.6573
47.6583
47.6663
47.6663
47.6668
47.6698
47.6763
47.6787
47.6798
DD_long
-116.9371
-116.9371
-116.9371
-116.9372
-116.9371
-116.9369
-116.9371
-116.9375
-116.9382
-116.9369
-116.9369
-116.9369
-116.9369
-116.9368
-116.9368
-116.9367
-116.7973
-116.7973
-116.7972
-116.7971
-116.7648
-116.7648
-116.7648
-116.7648
-116.7647
-116.7648
-116.7648
-116.7648
-116.7649
-116.7650
-116.7650
-116.7651
-116.7652
-116.7654
-116.7653
-116.7652
-116.7652
-116.7861
-116.7857
-116.7858
-116.7856
-116.7857
-116.7856
-116.7862
-116.7866
-116.7863
-116.7667
-116.9137
-116.8883
-116.8877
-117.0398
-116.9138
-116.7702
-116.8633
-116.8298
-117.0917
-116.8107
-116.8168
-116.8048
Easting
504,708.52
504,708.74
504,708.96
504,709.17
504,709.59
504,730.49
504,709.99
504,689.55
504,627.30
504,723.13
504,722.80
504,722.48
504,722.23
504,742.90
504,742.67
504,742.66
515,200.39
515,198.84
515,197.28
515,195.09
517,624.93
517,625.76
517,626.40
517,627.14
517,627.78
517,628.81
517,629.45
517,630.01
517,609.84
517,610.59
517,611.22
517,612.06
517,592.09
517,592.92
517,593.75
517,594.47
517,594.94
516,055.96
516,076.08
516,073.80
516,092.15
516,069.89
516,088.59
516,044.95
516,001.38
516,020.19
517,531.48
506,489.17
508,386.83
508,428.58
497,017.34
506,466.09
517,247.00
510,260.46
512,783.82
493,118.46
514,199.41
513,760.92
514,657.19
Northing
5,284,777.96
5,284,500.07
5,284,222.18
5,283,944.60
5,283,635.63
5,283,265.44
5,282,987.52
5,282,555.11
5,282,277.43
5,292,588.81
5,292,835.62
5,293,175.37
5,293,360.62
5,293,514.84
5,293,792.73
5,294,132.48
5,288,099.33
5,288,685.88
5,289,272.43
5,290,106.10
5,291,934.36
5,291,656.47
5,291,409.66
5,291,193.62
5,290,946.51
5,290,637.85
5,290,390.73
5,290,205.47
5,290,020.18
5,289,804.15
5,289,557.34
5,289,279.45
5,289,063.08
5,288,785.19
5,288,507.60
5,288,198.63
5,288,075.23
5,279,797.11
5,280,136.59
5,280,939.17
5,281,803.65
5,282,390.17
5,283,100.47
5,283,841.14
5,284,520.26
5,285,261.33
5,274,614.81
5,275,981.08
5,276,817.03
5,276,817.09
5,277,985.06
5,278,111.24
5,279,028.63
5,279,011.92
5,279,048.05
5,279,377.43
5,280,131.60
5,280,377.75
5,280,503.43
12
Elevation
671.1
669.1
667.5
663.5
659.9
664.2
657.0
663.5
670.4
656.6
658.9
665.0
687.2
722.7
724.5
723.5
694.7
696.9
699.2
702.5
736.6
734.9
729.3
729.3
722.4
705.8
702.0
699.8
698.3
698.9
699.0
699.2
697.7
696.9
686.0
685.0
724.7
652.0
661.3
670.7
677.2
680.2
684.0
680.5
683.3
681.0
680.2
723.7
677.7
677.8
681.1
810.9
648.2
748.4
651.2
634.2
675.9
707.6
654.6
Obs Grav
980,635.55
980,635.84
980,636.30
980,637.45
980,638.80
980,638.12
980,639.98
980,638.78
980,637.16
980,649.20
980,649.48
980,649.47
980,645.22
980,637.10
980,637.52
980,639.20
980,624.71
980,623.83
980,622.92
980,622.71
980,617.25
980,617.23
980,618.25
980,617.78
980,618.45
980,621.43
980,622.09
980,622.57
980,622.85
980,622.60
980,622.55
980,622.72
980,623.14
980,623.50
980,625.78
980,626.25
980,618.42
980,632.79
980,629.53
980,626.49
980,624.40
980,624.03
980,623.86
980,624.55
980,624.66
980,625.28
980,624.38
980,625.19
980,633.69
980,633.88
980,637.50
980,607.50
980,635.19
980,618.75
980,638.32
980,646.50
980,628.88
980,625.00
980,633.32
C.B.A. 1
-97.89
-97.72
-97.30
-96.67
-95.69
-95.15
-94.31
-93.64
-93.41
-93.11
-92.51
-91.40
-91.50
-92.50
-92.34
-91.07
-106.87
-107.81
-108.75
-109.00
-109.09
-109.27
-109.17
-109.44
-109.88
-109.92
-109.82
-109.62
-109.49
-109.41
-109.21
-108.78
-108.48
-108.02
-107.54
-106.41
-106.90
-100.26
-101.99
-103.88
-105.41
-105.68
-105.68
-106.30
-106.19
-106.61
-98.44
-88.97
-90.30
-90.14
-86.96
-91.62
-97.51
-94.56
-93.55
-89.56
-99.27
-97.15
-99.53
C.B.A. 2
-11.71
-11.54
-11.12
-10.49
-9.51
-8.95
-8.13
-7.48
-7.32
-6.91
-6.31
-5.21
-5.30
-6.28
-6.13
-4.86
-9.15
-10.09
-11.03
-11.29
-8.70
-8.88
-8.78
-9.05
-9.49
-9.53
-9.42
-9.23
-9.12
-9.03
-8.84
-8.40
-8.12
-7.67
-7.19
-6.06
-6.54
-1.60
-3.31
-5.20
-6.71
-7.01
-6.98
-7.65
-7.59
-7.98
1.85
-0.83
-0.07
0.14
-9.24
-3.50
2.46
-2.27
1.52
-16.13
-2.65
-1.02
-2.41
Count
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
Point_ID
D8013
D8015
D8021
D8026
D8027
D8032
D8037
D8038
D8044
D8045
D8047
D8050
D8051
D8052
D8053
D8058
D8059
D8068
D8070
D8073
D8074
D8075
D8076
D8086
D8087
D8089
D8091
D8094
D8095
D8096
D8097
D8098
D8100
D8102
D8104
D8108
D8109
D8111
D8116
D8117
D8124
D8125
D8126
D8127
D8128
D8129
D8130
D8131
D8132
D8133
D8142
D8143
D8147
D8150
D8153
D8156
D8157
D8158
D8159
DD_lat
47.6803
47.6818
47.6837
47.6848
47.6863
47.6868
47.6878
47.6887
47.6908
47.6908
47.6917
47.6928
47.6933
47.6933
47.6943
47.6963
47.6963
47.7017
47.7018
47.7033
47.7033
47.7053
47.7053
47.7098
47.7098
47.7108
47.7117
47.7133
47.7148
47.7148
47.7153
47.7153
47.7158
47.7163
47.7167
47.7187
47.7187
47.7193
47.7228
47.7232
47.7298
47.7302
47.7302
47.7303
47.7303
47.7303
47.7303
47.7308
47.7308
47.7308
47.7353
47.7358
47.7383
47.7398
47.7417
47.7442
47.7442
47.7443
47.7447
DD_long
-116.8248
-116.8052
-116.8067
-116.7798
-116.8117
-116.7948
-116.7913
-116.7848
-116.7748
-116.7798
-116.9138
-116.7637
-116.7682
-116.7717
-116.8718
-116.8923
-116.9783
-116.8103
-116.7867
-116.7848
-116.9483
-116.8698
-116.8498
-116.8917
-116.9598
-116.9167
-116.8933
-116.9798
-116.9567
-117.0000
-116.8498
-116.9418
-116.8067
-117.0223
-116.9783
-116.9578
-116.9583
-116.8288
-116.9577
-117.0223
-116.9733
-116.7647
-116.8923
-116.8718
-116.8068
-116.8288
-116.9583
-116.8498
-116.9133
-117.0223
-117.0223
-116.9578
-116.9478
-116.9903
-116.9578
-117.0677
-117.0677
-116.7942
-116.8717
Easting
513,155.84
514,615.01
514,510.06
516,532.21
514,134.25
515,405.89
515,655.58
516,155.63
516,905.49
516,530.09
506,461.78
517,738.40
517,383.96
517,133.79
509,629.88
508,087.14
501,625.87
514,234.22
516,005.77
516,151.17
503,876.16
509,773.58
511,274.25
508,126.60
503,021.58
506,251.28
508,001.23
501,521.12
503,250.28
499,999.98
511,271.85
504,375.47
514,501.32
498,333.30
501,625.15
503,166.82
503,125.07
512,833.49
503,166.65
498,333.33
501,999.66
517,642.50
508,081.80
509,623.30
514,497.35
512,830.92
503,124.56
511,268.62
506,498.78
498,333.58
498,333.83
503,165.78
503,915.43
500,728.77
503,165.36
494,918.88
494,918.88
515,430.41
509,620.53
Northing
5,280,561.48
5,280,719.40
5,280,935.27
5,281,064.19
5,281,243.07
5,281,276.99
5,281,401.39
5,281,495.38
5,281,744.25
5,281,743.37
5,281,815.84
5,281,963.07
5,282,023.47
5,282,022.79
5,282,098.24
5,282,342.71
5,282,337.47
5,282,941.34
5,282,945.86
5,283,131.34
5,283,110.26
5,283,333.43
5,283,336.01
5,283,824.85
5,283,819.85
5,283,946.02
5,284,040.70
5,284,220.70
5,284,375.67
5,284,374.70
5,284,447.57
5,284,438.02
5,284,516.46
5,284,560.17
5,284,591.07
5,284,807.92
5,284,807.86
5,284,882.92
5,285,271.08
5,285,331.99
5,286,042.34
5,286,130.37
5,286,109.16
5,286,111.50
5,286,121.64
5,286,117.89
5,286,104.38
5,286,176.16
5,286,168.92
5,286,165.67
5,286,659.60
5,286,721.78
5,287,000.20
5,287,153.20
5,287,370.19
5,287,649.58
5,287,649.58
5,287,667.60
5,287,716.99
13
Elevation
735.9
647.9
655.8
663.1
647.9
661.6
673.8
673.8
671.9
672.2
648.8
658.5
661.6
672.2
648.8
648.5
749.0
664.6
682.9
656.1
648.5
656.4
660.7
673.2
648.8
665.8
669.2
656.4
661.3
647.6
688.1
662.2
671.9
639.3
651.8
662.9
664.0
687.2
673.2
642.4
651.8
683.8
676.5
678.6
687.8
693.3
664.6
693.9
674.4
643.9
643.9
659.4
678.3
649.0
655.5
651.2
651.2
684.1
683.5
Obs Grav
980,621.00
980,634.32
980,633.44
980,629.00
980,634.13
980,629.50
980,625.19
980,624.38
980,625.94
980,625.19
980,639.50
980,630.75
980,629.00
980,626.63
980,638.88
980,639.69
980,623.44
980,631.32
980,623.63
980,633.00
980,641.38
980,633.88
980,630.69
980,631.38
980,642.69
980,634.07
980,632.00
980,641.69
980,639.13
980,643.00
980,623.44
980,637.57
980,625.19
980,646.25
980,642.13
980,638.88
980,638.44
980,623.57
980,636.44
980,645.32
980,640.25
980,624.07
980,630.82
980,628.69
980,625.75
980,625.38
980,638.50
980,625.32
980,634.44
980,647.07
980,648.44
980,639.32
980,635.57
980,643.25
980,640.69
980,649.75
980,649.75
980,626.07
980,628.13
C.B.A. 1
-95.92
-99.81
-99.00
-102.81
-96.62
-102.89
-104.96
-105.85
-104.79
-105.51
-95.08
-101.86
-103.70
-104.22
-95.79
-95.65
-91.64
-101.89
-106.03
-101.23
-94.59
-101.12
-103.51
-100.58
-93.97
-99.68
-100.63
-94.01
-95.75
-94.62
-106.50
-97.27
-107.90
-93.17
-94.84
-96.14
-96.36
-106.93
-96.95
-94.17
-98.01
-107.57
-102.74
-104.48
-105.59
-104.93
-97.32
-104.93
-99.58
-92.82
-91.83
-98.02
-98.31
-96.45
-97.96
-89.83
-89.83
-107.13
-105.41
C.B.A. 2
-0.45
-2.74
-2.04
-3.62
-0.08
-4.94
-6.74
-7.08
-5.19
-6.32
-6.97
-1.35
-3.57
-4.37
-4.20
-5.75
-8.85
-5.23
-7.42
-2.46
-9.32
-9.37
-10.11
-10.64
-9.64
-11.80
-10.83
-11.34
-11.18
-13.62
-13.10
-11.45
-10.95
-14.00
-12.06
-11.66
-11.92
-11.81
-12.46
-15.00
-14.81
-7.16
-12.85
-12.89
-8.64
-9.82
-12.88
-11.53
-11.43
-13.65
-12.66
-13.54
-13.01
-14.65
-13.48
-14.42
-14.42
-9.16
-13.82
Count
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
Point_ID
D8162
D8163
D8164
D8165
D8166
D8167
D8170
D8181
D8182
D8185
D8186
D8187
D8191
D8193
D8194
D8196
D8197
D8199
D8200
D8201
D8203
D8205
D8206
D8207
D8215
D8216
D8217
D8218
D8223
D8224
D8225
D8227
D8228
D8229
D8233
D8236
D8237
D8243
D8244
D8247
D8249
D8250
D8256
D8257
D8265
D8266
D8267
D8268
D8269
D8270
D8271
D8272
D8275
D8283
D8284
D8285
D8286
D8287
D8291
DD_lat
47.7448
47.7448
47.7448
47.7448
47.7448
47.7448
47.7452
47.7503
47.7503
47.7517
47.7517
47.7522
47.7563
47.7577
47.7583
47.7588
47.7588
47.7593
47.7597
47.7598
47.7633
47.7652
47.7652
47.7652
47.7697
47.7698
47.7718
47.7718
47.7737
47.7738
47.7738
47.7742
47.7743
47.7743
47.7757
47.7767
47.7767
47.7803
47.7803
47.7817
47.7828
47.7828
47.7858
47.7858
47.7883
47.7883
47.7883
47.7883
47.7883
47.7883
47.7883
47.7883
47.7887
47.7937
47.7937
47.7947
47.7947
47.7947
47.7957
DD_long
-116.8072
-116.8287
-116.8498
-116.8923
-116.9138
-116.9367
-116.9578
-117.0078
-117.0078
-117.0683
-117.0683
-116.9583
-116.9583
-117.0937
-116.7898
-117.0617
-117.0617
-116.8718
-116.9367
-116.9583
-116.9358
-117.0118
-117.0118
-117.0118
-116.9348
-116.7638
-117.0618
-117.0618
-116.8498
-116.8717
-116.8933
-116.9358
-116.9148
-116.9363
-116.8092
-116.9658
-116.9658
-117.0383
-117.0383
-116.9363
-116.9797
-116.9797
-116.9417
-116.9417
-116.7638
-116.7853
-116.8068
-116.8283
-116.8502
-116.8713
-116.8933
-116.9363
-116.9148
-116.9268
-116.9268
-117.0617
-117.0617
-117.0617
-117.1223
Easting
514,451.62
512,848.27
511,265.63
508,079.64
506,455.26
504,747.70
503,165.28
499,416.95
499,416.95
494,877.99
494,877.99
503,122.95
503,122.78
492,984.24
515,759.35
495,378.46
495,378.46
509,617.77
504,746.36
503,122.59
504,808.44
499,125.84
499,125.84
499,125.84
504,891.12
517,691.75
495,379.58
495,379.58
511,259.47
509,615.10
507,991.94
504,807.57
506,389.30
504,765.83
514,297.35
502,559.84
502,559.84
497,128.37
497,128.37
504,765.16
501,518.96
501,518.96
504,369.48
504,369.48
517,685.44
516,083.31
514,481.18
512,858.33
511,214.45
509,633.34
507,989.46
504,764.76
506,387.43
505,492.36
505,492.36
495,381.45
495,381.45
495,381.45
490,846.63
Northing
5,287,727.06
5,287,723.10
5,287,719.79
5,287,714.65
5,287,712.49
5,287,710.81
5,287,771.49
5,288,326.46
5,288,326.46
5,288,483.20
5,288,483.20
5,288,543.24
5,289,006.39
5,289,164.08
5,289,243.12
5,289,285.33
5,289,285.33
5,289,322.17
5,289,378.16
5,289,376.92
5,289,779.55
5,289,993.67
5,289,993.67
5,289,993.67
5,290,489.64
5,290,514.54
5,290,705.57
5,290,705.57
5,290,930.76
5,290,958.43
5,290,955.97
5,290,983.44
5,290,984.93
5,290,983.38
5,291,184.31
5,291,259.76
5,291,259.76
5,291,661.35
5,291,661.35
5,291,817.05
5,291,938.58
5,291,938.58
5,292,279.61
5,292,279.61
5,292,582.86
5,292,578.30
5,292,574.04
5,292,570.37
5,292,566.96
5,292,563.95
5,292,561.46
5,292,557.79
5,292,590.42
5,293,145.15
5,293,145.15
5,293,267.85
5,293,267.85
5,293,267.85
5,293,396.87
14
Elevation
705.8
706.4
689.6
681.4
680.5
671.0
658.2
647.5
647.5
649.7
649.7
658.8
659.4
654.6
693.3
650.6
650.6
686.3
660.3
658.8
665.2
670.4
670.4
670.5
668.0
702.1
651.2
651.2
694.8
687.5
686.3
664.0
676.8
664.0
704.5
652.4
652.4
668.6
668.6
663.4
732.6
732.6
657.6
657.6
741.1
704.2
703.0
697.5
693.3
696.9
678.9
657.3
664.3
662.8
662.8
732.0
732.0
732.0
718.0
Obs Grav
980,623.82
980,624.88
980,626.57
980,631.00
980,633.07
980,636.13
980,640.38
980,647.75
980,647.75
980,650.38
980,650.38
980,641.32
980,641.75
980,653.57
980,623.57
980,652.57
980,652.57
980,630.50
980,641.63
980,643.50
980,640.50
980,646.63
980,646.63
980,646.82
980,640.75
980,621.75
980,654.00
980,654.00
980,628.25
980,631.82
980,633.94
980,642.75
980,637.32
980,642.88
980,624.07
980,649.88
980,649.88
980,650.13
980,650.13
980,646.07
980,635.50
980,635.50
980,648.13
980,648.13
980,615.63
980,624.25
980,625.82
980,629.63
980,631.44
980,631.50
980,638.57
980,649.32
980,643.94
980,648.75
980,648.75
980,637.63
980,637.63
980,637.63
980,642.88
C.B.A. 1
-105.20
-104.04
-105.78
-102.95
-101.04
-99.74
-98.06
-93.06
-93.06
-90.09
-90.09
-97.63
-97.44
-86.03
-109.19
-88.00
-88.00
-103.80
-97.70
-96.10
-98.19
-90.88
-90.88
-90.67
-97.95
-110.25
-87.49
-87.49
-105.68
-103.53
-101.61
-97.09
-100.09
-96.97
-108.13
-91.95
-91.95
-88.65
-88.65
-94.46
-91.53
-91.53
-93.77
-93.77
-110.46
-109.09
-107.80
-105.07
-104.07
-103.27
-99.64
-92.85
-97.04
-92.75
-92.75
-89.27
-89.27
-89.27
-89.08
C.B.A. 2
-8.31
-8.91
-12.39
-13.06
-12.94
-13.51
-13.57
-12.70
-12.70
-14.72
-14.72
-13.19
-13.00
-12.75
-10.85
-12.08
-12.08
-12.22
-11.47
-11.67
-11.90
-10.84
-10.84
-10.63
-11.57
-9.79
-11.58
-11.58
-12.29
-11.95
-11.82
-10.81
-12.07
-10.73
-11.40
-8.13
-8.13
-10.81
-10.81
-8.22
-8.86
-8.86
-7.96
-7.96
-10.01
-10.39
-10.87
-9.93
-10.73
-11.68
-9.85
-6.61
-9.02
-5.70
-5.70
-13.35
-13.35
-13.35
-18.15
Count
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
Point_ID
D8292
D8293
D8294
D8296
D8298
D8299
D8304
D8305
D8309
D8310
D8311
D8312
D8313
D8314
D8315
D8316
D8317
D8318
D8322
D8323
D8330
D8331
D8335
D8338
D8344
D8345
D8347
D8348
D8349
D8350
D8351
D8352
D8353
D8354
D8355
D8356
D8360
D8361
D8364
D8374
D8380
D8381
D8382
D8391
D8398
D8420
D8421
D8424
D8426
D8427
D8428
D8429
D8430
D8431
D8432
D8433
D8434
D8437
D8440
DD_lat
47.7958
47.7958
47.7958
47.7958
47.7963
47.7968
47.7968
47.8023
47.8023
47.8028
47.8028
47.8028
47.8028
47.8028
47.8028
47.8028
47.8028
47.8028
47.8028
47.8048
47.8048
47.8103
47.8103
47.8117
47.8128
47.8133
47.8133
47.8173
47.8173
47.8173
47.8173
47.8173
47.8173
47.8173
47.8173
47.8178
47.8178
47.8192
47.8192
47.8247
47.8283
47.8318
47.8318
47.8318
47.8373
47.8462
47.8593
47.8597
47.8607
47.8608
47.8608
47.8608
47.8608
47.8612
47.8613
47.8613
47.8613
47.8613
47.8643
DD_long
-116.9598
-116.9598
-116.9797
-116.9797
-116.9383
-116.9798
-116.9798
-116.9148
-116.9148
-116.7637
-116.8068
-116.8068
-116.8283
-116.8283
-116.8503
-116.8503
-116.8717
-116.8717
-116.9352
-116.9363
-116.9363
-116.8993
-116.8993
-116.8967
-117.0088
-116.9468
-116.9468
-116.8068
-116.8068
-116.8282
-116.8282
-116.8498
-116.8498
-116.8713
-116.8713
-116.7867
-116.7867
-116.8848
-116.8848
-116.7747
-116.7698
-116.8067
-116.8713
-116.8713
-117.0908
-116.8068
-116.7792
-116.7797
-116.7638
-116.7502
-116.7637
-116.7708
-116.8068
-116.8018
-116.7863
-116.7918
-116.7963
-116.7963
-116.8067
Easting
503,016.60
503,016.60
501,518.67
501,518.67
504,618.36
501,518.70
501,518.70
506,385.83
506,385.83
517,701.24
514,477.17
514,477.17
512,854.62
512,854.62
511,211.36
511,211.36
509,609.83
509,609.83
504,846.38
504,763.05
504,763.05
507,549.34
507,549.34
507,736.38
499,334.57
503,992.82
503,992.82
514,473.16
514,473.16
512,851.22
512,851.22
511,250.00
511,250.00
509,628.06
509,628.06
515,970.09
515,970.09
508,629.40
508,629.40
516,861.97
517,235.14
514,469.14
509,625.26
509,625.26
493,202.59
514,465.11
516,518.50
516,476.67
517,660.58
518,678.90
517,681.60
517,141.37
514,460.89
514,834.87
515,977.62
515,582.72
515,229.57
515,229.57
514,460.07
Northing
5,293,390.34
5,293,390.34
5,293,389.59
5,293,389.59
5,293,453.10
5,293,482.22
5,293,482.22
5,294,102.97
5,294,102.97
5,294,188.38
5,294,179.53
5,294,179.53
5,294,175.55
5,294,175.55
5,294,172.14
5,294,172.14
5,294,169.41
5,294,169.41
5,294,163.41
5,294,379.32
5,294,379.32
5,294,999.96
5,294,999.96
5,295,154.43
5,295,272.72
5,295,335.84
5,295,335.84
5,295,784.71
5,295,784.71
5,295,781.03
5,295,781.03
5,295,777.69
5,295,777.69
5,295,774.63
5,295,774.63
5,295,850.35
5,295,850.35
5,295,989.15
5,295,989.15
5,296,624.74
5,297,027.21
5,297,390.19
5,297,380.11
5,297,380.11
5,297,993.59
5,298,995.68
5,300,452.23
5,300,513.72
5,300,640.74
5,300,643.81
5,300,640.78
5,300,639.04
5,300,631.94
5,300,663.58
5,300,697.61
5,300,696.40
5,300,695.57
5,300,695.57
5,301,002.46
15
Elevation
718.0
772.2
772.2
703.3
648.2
648.2
664.0
664.0
738.1
701.5
701.5
704.5
704.5
689.9
689.9
680.8
680.8
723.1
723.1
723.1
669.2
669.2
674.1
679.9
735.9
735.9
703.9
703.9
686.9
686.9
683.2
683.2
676.2
676.2
707.9
707.9
684.1
684.1
750.9
705.2
687.2
693.6
693.6
732.6
693.9
695.4
694.5
694.5
699.4
694.5
695.4
706.4
703.9
694.5
701.8
703.0
703.0
710.9
742.0
Obs Grav
980,638.82
980,638.82
980,630.19
980,630.19
980,640.25
980,650.63
980,650.63
980,649.13
980,649.13
980,618.44
980,628.63
980,628.63
980,630.00
980,630.00
980,633.69
980,633.69
980,639.75
980,639.75
980,639.19
980,639.00
980,639.00
980,648.38
980,648.38
980,647.19
980,648.88
980,637.88
980,637.88
980,630.07
980,630.07
980,636.00
980,636.00
980,640.07
980,640.07
980,645.00
980,645.00
980,627.07
980,627.07
980,645.82
980,645.82
980,617.94
980,627.50
980,636.32
980,645.38
980,645.38
980,640.82
980,638.57
980,641.69
980,641.44
980,640.00
980,635.88
980,640.13
980,640.38
980,639.82
980,640.19
980,642.25
980,640.07
980,639.94
980,639.69
980,640.44
C.B.A. 1
-89.08
-89.90
-89.90
-93.38
-83.29
-83.29
-92.59
-92.59
-109.55
-106.57
-106.57
-104.62
-104.62
-103.72
-103.72
-99.37
-99.37
-91.05
-91.19
-91.19
-92.93
-92.93
-93.35
-89.52
-90.01
-90.01
-105.95
-105.95
-103.32
-103.32
-99.91
-99.91
-96.13
-96.13
-108.06
-108.06
-93.58
-93.58
-109.17
-108.21
-104.25
-93.32
-93.32
-86.52
-101.98
-99.61
-100.08
-101.63
-104.69
-101.51
-101.10
-99.60
-99.75
-99.50
-100.28
-100.19
-100.44
-98.40
-91.13
C.B.A. 2
-4.76
-5.58
-7.23
-10.71
2.79
-0.62
-9.92
-4.57
-21.53
-6.10
-9.64
-7.70
-9.48
-8.58
-10.39
-6.04
-7.80
0.52
-4.86
-4.95
-6.69
-3.62
-4.05
-0.01
-9.74
-4.61
-20.55
-9.02
-6.40
-8.18
-4.77
-6.53
-2.75
-4.54
-16.47
-9.49
4.98
-3.09
-18.68
-8.67
-4.29
3.60
-1.73
5.07
-28.46
-2.70
-0.91
-2.51
-4.26
0.04
-0.65
0.26
-2.84
-2.18
-1.71
-2.04
-2.68
-0.65
5.78
Count
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
Point_ID
D8445
D8446
D8447
D8448
D8449
D8450
D8452
D8453
D8454
D8455
D8456
D8460
D8462
D8463
D8467
D8468
D8470
PCR01
PCR02
PCR03
PCR04
PCR05
PCR06
PCR07
PCR08
PCR09
PCR10
PCR11
PCR12
PCR13
PCR14
PCR15
PCR16
PCR17
PCR18
PCR19
PCR20
PCR21
PCR22
PCR23
PCR24
PCR25
PCR26
PCR27
PCR28
PCR29
PCR30
PCR31
PCR32
PCR33
PCR34
PCR35
PCR36
PCR37
PCR38
PHL01
PHL02
PHL03
PHL04
DD_lat
47.8663
47.8678
47.8678
47.8678
47.8678
47.8678
47.8678
47.8682
47.8682
47.8682
47.8682
47.8682
47.8683
47.8688
47.8698
47.8728
47.8728
47.7578
47.7533
47.7519
47.7506
47.7492
47.7475
47.7461
47.7447
47.7433
47.7419
47.7403
47.7389
47.7375
47.7361
47.7344
47.7331
47.7317
47.7303
47.7289
47.7272
47.7258
47.7244
47.7231
47.7217
47.7203
47.7186
47.7172
47.7161
47.7144
47.7131
47.7114
47.7100
47.7086
47.7072
47.7061
47.7061
47.7042
47.7031
47.7508
47.7522
47.7536
47.7550
DD_long
-116.8572
-116.8487
-116.8333
-116.8387
-116.8387
-116.8483
-116.8533
-116.8068
-116.8222
-116.8237
-116.8282
-116.8433
-116.8152
-116.7998
-116.8068
-116.7697
-116.7923
-116.9899
-116.9902
-116.9902
-116.9902
-116.9899
-116.9899
-116.9899
-116.9902
-116.9902
-116.9899
-116.9899
-116.9899
-116.9899
-116.9899
-116.9899
-116.9899
-116.9899
-116.9899
-116.9899
-116.9899
-116.9899
-116.9899
-116.9899
-116.9899
-116.9899
-116.9899
-116.9896
-116.9899
-116.9899
-116.9899
-116.9899
-116.9899
-116.9896
-116.9896
-116.9899
-116.9904
-116.9902
-116.9899
-117.0063
-117.0063
-117.0063
-117.0063
Easting
510,678.35
511,322.14
512,464.78
512,069.88
512,069.88
511,342.86
510,968.98
514,458.98
513,295.62
513,191.71
512,838.56
511,716.64
513,814.84
514,978.11
514,458.61
517,220.44
515,537.87
514,457.21
500,749.38
500,728.71
500,728.77
500,728.82
500,749.60
500,749.55
500,749.60
500,728.94
500,728.99
500,749.76
500,749.71
500,749.77
500,749.82
500,749.88
500,749.82
500,749.87
500,749.93
500,749.99
500,749.73
500,749.98
500,750.04
500,750.09
500,749.84
500,749.89
500,749.95
500,749.89
500,770.97
500,750.11
500,750.06
500,750.11
500,750.05
500,750.11
500,770.88
500,770.93
500,750.07
500,729.35
500,729.49
500,750.38
499,521.08
499,521.02
499,521.28
Northing
5,301,241.11
5,301,396.57
5,301,398.91
5,301,398.31
5,301,398.31
5,301,396.60
5,301,396.03
5,301,465.30
5,301,462.63
5,301,462.47
5,301,461.63
5,301,459.32
5,301,463.72
5,301,528.55
5,301,619.79
5,301,967.07
5,301,962.08
5,302,082.94
5,289,160.02
5,288,666.06
5,288,511.58
5,288,357.40
5,288,202.94
5,288,017.68
5,287,863.50
5,287,708.98
5,287,554.80
5,287,400.35
5,287,215.09
5,287,060.60
5,286,906.42
5,286,751.93
5,286,566.67
5,286,412.49
5,286,258.01
5,286,103.52
5,285,949.34
5,285,764.08
5,285,609.59
5,285,455.41
5,285,300.92
5,285,146.74
5,284,992.26
5,284,806.99
5,284,652.54
5,284,529.11
5,284,343.84
5,284,189.66
5,284,004.40
5,283,849.92
5,283,695.76
5,283,541.28
5,283,417.84
5,283,417.81
5,283,201.78
5,283,078.10
5,288,388.17
5,288,542.65
5,288,696.84
16
Elevation
724.1
715.8
719.5
719.5
724.1
726.8
706.4
703.6
703.6
707.3
714.3
703.9
705.8
706.4
704.8
708.2
707.3
642.7
640.9
645.4
646.3
645.7
647.0
647.0
647.6
648.2
647.6
647.6
648.8
648.2
644.5
644.5
645.7
648.2
650.0
650.9
652.4
652.4
650.9
648.8
646.3
645.4
645.4
645.7
643.3
645.7
646.7
645.4
644.5
644.2
644.8
642.4
639.3
637.2
629.9
648.8
649.7
651.8
662.2
Obs Grav
980,641.19
980,640.75
980,640.38
980,639.75
980,639.94
980,640.57
980,642.57
980,642.50
980,643.69
980,643.69
980,641.94
980,641.25
980,644.57
980,640.13
980,642.44
980,641.32
980,641.13
980,626.61
980,627.87
980,629.48
980,630.51
980,631.19
980,632.18
980,632.86
980,633.24
980,633.55
980,633.23
980,633.74
980,634.45
980,634.85
980,634.60
980,635.08
980,635.48
980,695.76
980,637.15
980,637.61
980,638.03
980,638.17
980,637.77
980,637.21
980,636.53
980,636.01
980,635.63
980,634.86
980,634.25
980,633.93
980,633.73
980,633.60
980,633.41
980,633.07
980,633.40
980,633.29
980,632.59
980,631.61
980,630.48
980,630.02
980,629.95
980,630.13
980,632.26
C.B.A. 1
-95.55
-97.70
-97.57
-97.38
-95.73
-93.16
-97.55
-96.83
-96.82
-97.74
-96.94
-95.94
-100.09
-97.74
-99.43
-98.99
-96.22
-91.74
-93.09
-93.57
-94.29
-94.99
-95.58
-96.14
-96.25
-96.30
-95.98
-96.35
-96.66
-97.07
-97.50
-97.85
-97.86
-98.26
-98.32
-98.45
-98.40
-98.41
-98.22
-98.00
-97.71
-97.25
-96.73
-95.76
-95.56
-94.56
-94.04
-94.04
-93.93
-93.51
-93.55
-93.85
-93.83
-93.18
-93.56
-93.13
-92.99
-93.10
-92.75
C.B.A. 2
-2.81
-4.24
-2.86
-3.10
-1.45
0.32
-4.48
0.08
-1.19
-2.23
-1.82
-2.05
-3.90
-0.26
-2.53
0.95
1.88
5.17
-11.27
-11.77
-12.48
-13.19
-13.76
-14.31
-14.43
-14.50
-14.18
-14.53
-14.84
-15.24
-15.68
-16.03
-16.04
-16.43
-16.50
-16.62
-16.57
-16.59
-16.39
-16.17
-15.89
-15.42
-14.90
-13.93
-13.71
-12.74
-12.21
-12.22
-12.10
-11.68
-11.70
-12.00
-12.00
-11.38
-11.76
-11.31
-12.51
-12.62
-12.27
Count
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
Point_ID
PHL05
PHL06
PHL07
PHL08
PHL09
PHL10
PHL11
PHL12
PHL13
PHL14
PHL15
PHL16
PHL17
PHL18
PHL19
PHL20
PHL21
PHL22
PHL23
PHL24
PHL25
PHL26
PHL27
PHL28
PHL29
PHL30
PHL31
PHL32
PHR01
PHR02
PHR03
PHR04
PHR05
PHR06
PHR07
PHR08
PHR09
PHR10
PHR11
PHR12
PHR13
PHR14
PHR15
PHR16
PHR17
PHR18
PHR19
PHR20
PHR21
PHR22
PHR23
PHR24
PHR25
PHR26
PHR27
PHR28
PIH01
PIH02
PIH03
DD_lat
47.7564
47.7581
47.7594
47.7608
47.7619
47.7628
47.7636
47.7644
47.7650
47.7658
47.7669
47.7672
47.7672
47.7658
47.7644
47.7636
47.7636
47.7636
47.7636
47.7631
47.7619
47.7603
47.7589
47.7575
47.7558
47.7539
47.7522
47.7508
47.7106
47.7092
47.7075
47.7058
47.7042
47.7022
47.7006
47.6989
47.6975
47.6961
47.6947
47.6933
47.6919
47.6906
47.6892
47.6878
47.6864
47.6850
47.6822
47.6806
47.6792
47.6778
47.6764
47.6750
47.6736
47.6722
47.6714
47.6697
47.8103
47.8086
47.8072
DD_long
-117.0066
-117.0066
-117.0066
-117.0066
-117.0068
-117.0087
-117.0104
-117.0120
-117.0139
-117.0158
-117.0175
-117.0191
-117.0224
-117.0224
-117.0224
-117.0224
-117.0202
-117.0180
-117.0150
-117.0137
-117.0120
-117.0120
-117.0120
-117.0117
-117.0117
-117.0117
-117.0117
-117.0117
-117.1107
-117.1107
-117.1107
-117.1107
-117.1107
-117.1107
-117.1107
-117.1107
-117.1107
-117.1107
-117.1107
-117.1107
-117.1104
-117.1104
-117.1104
-117.1104
-117.1104
-117.1104
-117.1101
-117.1101
-117.1101
-117.1101
-117.1101
-117.1101
-117.1098
-117.1098
-117.1098
-117.1098
-116.8926
-116.8926
-116.8926
Easting
499,521.22
499,500.44
499,500.50
499,500.44
499,500.38
499,500.52
499,354.60
499,229.70
499,104.79
498,959.37
498,813.44
498,688.65
498,563.83
498,313.97
498,314.02
498,313.78
498,313.75
498,480.42
498,647.09
498,875.93
498,979.92
499,105.02
499,104.96
499,104.71
499,125.49
499,125.74
499,125.57
499,125.52
499,125.57
491,685.93
491,685.67
491,685.31
491,685.26
491,684.89
491,684.73
491,684.36
491,684.00
491,683.75
491,683.50
491,683.55
491,683.30
491,724.79
491,724.54
491,724.29
491,724.04
491,723.79
491,723.54
491,744.06
491,743.69
491,743.44
491,743.19
491,742.94
491,742.69
491,763.46
491,763.21
491,763.18
491,762.82
508,048.76
508,049.01
Northing
5,288,851.32
5,289,005.78
5,289,190.73
5,289,345.22
5,289,499.70
5,289,623.11
5,289,715.82
5,289,808.26
5,289,901.01
5,289,962.65
5,290,055.36
5,290,178.88
5,290,209.77
5,290,210.00
5,290,055.52
5,289,901.03
5,289,808.70
5,289,808.65
5,289,808.60
5,289,808.34
5,289,746.64
5,289,623.12
5,289,437.86
5,289,283.67
5,289,129.22
5,288,943.96
5,288,727.92
5,288,542.66
5,288,388.18
5,283,917.51
5,283,763.32
5,283,578.06
5,283,392.80
5,283,207.54
5,282,991.50
5,282,806.24
5,282,620.98
5,282,466.80
5,282,312.31
5,282,157.83
5,282,003.64
5,281,849.22
5,281,694.73
5,281,540.55
5,281,386.07
5,281,231.58
5,281,077.40
5,280,768.46
5,280,583.50
5,280,429.01
5,280,274.52
5,280,120.34
5,279,965.86
5,279,811.40
5,279,657.22
5,279,564.59
5,279,379.33
5,295,000.42
5,294,815.16
17
Elevation
664.9
670.7
674.7
674.7
671.3
670.7
671.3
671.3
671.7
668.6
668.6
669.2
675.6
681.7
684.8
687.2
676.8
671.7
676.2
677.7
682.3
685.1
685.1
680.2
666.5
662.8
653.0
650.6
636.9
635.7
634.2
633.5
632.9
632.3
631.7
631.1
630.2
629.3
629.0
629.6
628.4
627.7
627.4
626.5
625.3
620.4
619.5
618.3
621.0
622.9
621.7
623.5
629.0
630.5
631.4
633.2
671.7
666.5
666.8
Obs Grav
980,632.44
980,633.81
980,634.32
980,633.86
980,632.15
980,631.64
980,631.31
980,631.26
980,631.83
980,631.41
980,631.36
980,631.08
980,632.18
980,632.94
980,633.05
980,633.74
980,632.37
980,632.21
980,633.71
980,633.97
980,635.39
980,636.58
980,637.00
980,636.22
980,632.83
980,631.78
980,629.59
980,629.14
980,624.02
980,623.57
980,623.23
980,623.62
980,624.00
980,624.54
980,624.98
980,625.62
980,626.00
980,626.45
980,626.86
980,627.61
980,627.97
980,628.30
980,628.49
980,628.55
980,628.36
980,627.46
980,626.60
980,626.48
980,626.93
980,627.37
980,627.41
980,627.79
980,628.74
980,629.17
980,629.18
980,629.17
980,630.15
980,629.77
980,630.73
C.B.A. 1
-92.44
-92.63
-92.31
-91.93
-91.08
-90.90
-90.55
-90.58
-91.15
-91.50
-91.54
-91.14
-90.82
-90.02
-89.33
-89.40
-90.35
-91.34
-91.81
-91.61
-91.86
-92.31
-92.62
-92.84
-92.34
-91.86
-91.71
-91.66
-85.98
-85.68
-85.55
-85.98
-86.37
-86.93
-87.39
-88.04
-88.51
-89.09
-89.45
-89.95
-90.43
-90.83
-90.87
-90.99
-90.92
-90.99
-90.05
-90.08
-89.79
-89.69
-89.88
-89.72
-89.34
-89.31
-88.98
-88.44
-93.72
-94.36
-95.13
C.B.A. 2
-11.96
-12.18
-11.86
-11.48
-10.63
-10.45
-10.26
-10.43
-11.14
-11.64
-11.85
-11.59
-11.40
-10.87
-10.19
-10.25
-11.20
-12.01
-12.30
-11.85
-11.98
-12.29
-12.61
-12.83
-12.30
-11.82
-11.67
-11.63
-5.94
-13.83
-13.69
-14.13
-14.52
-15.08
-15.54
-16.19
-16.66
-17.23
-17.60
-18.10
-18.58
-18.93
-18.97
-19.09
-19.02
-19.09
-18.15
-18.16
-17.87
-17.77
-17.96
-17.81
-17.42
-17.37
-17.04
-16.50
-21.78
-4.51
-5.28
Count
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
Point_ID
PIH04
PIH05
PIH06
PIH07
PIH08
PIH09
PIH10
PIH11
PIH12
PIH13
PIH14
PIH15
PIH16
PIH17
PIH18
PIH19
PIH20
PIH21
PIH22
PIH23
PIH24
PIH25
PIH26
PIH27
PIH28
PIH29
PIH30
PIH31
PIH32
PIH33
PIH34
PIH35
PIH36
PIH37
PIH38
PIH39
PIH40
PIH41
PIH42
PIH43
PIH44
PIH45
PIH46
PIH47
PIH48
PIH49
PIH50
PIH51
PIH52
PIH53
PIH54
PIH55
PIH56
PIH57
PIH58
PIH59
PIH60
PIH61
PIH62
DD_lat
47.8058
47.8044
47.8031
47.8017
47.8000
47.7986
47.7972
47.7958
47.7944
47.7928
47.7914
47.7900
47.7883
47.7869
47.7856
47.7842
47.7828
47.7814
47.7797
47.7783
47.7769
47.7756
47.7739
47.7725
47.7711
47.7697
47.7683
47.7667
47.7653
47.7639
47.7625
47.7611
47.7594
47.7581
47.7567
47.7553
47.7539
47.7522
47.7508
47.7494
47.7481
47.7467
47.7453
47.7436
47.7422
47.7408
47.7394
47.7381
47.7364
47.7350
47.7336
47.7322
47.7306
47.7292
47.7278
47.7264
47.7250
47.7233
47.7219
DD_long
-116.8926
-116.8926
-116.8926
-116.8926
-116.8926
-116.8926
-116.8926
-116.8926
-116.8926
-116.8926
-116.8926
-116.8926
-116.8926
-116.8926
-116.8929
-116.8929
-116.8929
-116.8929
-116.8929
-116.8929
-116.8929
-116.8929
-116.8929
-116.8929
-116.8929
-116.8929
-116.8929
-116.8929
-116.8929
-116.8929
-116.8929
-116.8929
-116.8929
-116.8929
-116.8929
-116.8929
-116.8929
-116.8929
-116.8929
-116.8932
-116.8932
-116.8932
-116.8932
-116.8932
-116.8932
-116.8932
-116.8932
-116.8932
-116.8932
-116.8932
-116.8932
-116.8932
-116.8932
-116.8932
-116.8932
-116.8932
-116.8932
-116.8932
-116.8932
Easting
508,049.07
508,049.43
508,049.49
508,049.85
508,049.91
508,050.16
508,050.52
508,050.58
508,050.94
508,051.00
508,051.25
508,051.61
508,051.67
508,051.92
508,052.28
508,031.62
508,031.98
508,032.04
508,032.40
508,032.65
508,032.70
508,033.07
508,033.13
508,033.38
508,033.73
508,033.79
508,034.15
508,034.21
508,034.46
508,034.82
508,034.88
508,035.24
508,035.29
508,035.54
508,035.90
508,035.96
508,036.32
508,036.38
508,036.63
508,036.98
507,995.60
507,995.65
507,996.02
507,996.07
507,996.32
507,996.68
507,996.74
507,997.09
507,997.15
507,997.71
507,997.76
507,998.12
507,998.17
507,998.42
507,998.78
507,998.84
507,999.19
507,999.25
507,999.50
Northing
5,294,660.97
5,294,506.49
5,294,352.31
5,294,197.82
5,294,043.34
5,293,858.07
5,293,703.90
5,293,549.41
5,293,395.23
5,293,240.74
5,293,055.48
5,292,901.30
5,292,746.82
5,292,561.56
5,292,407.07
5,292,252.86
5,292,098.38
5,291,944.19
5,291,789.71
5,291,604.45
5,291,450.27
5,291,295.78
5,291,141.30
5,290,956.04
5,290,801.86
5,290,647.37
5,290,493.19
5,290,338.70
5,290,153.44
5,289,998.96
5,289,844.78
5,289,690.29
5,289,536.11
5,289,350.85
5,289,196.37
5,289,042.18
5,288,887.70
5,288,733.21
5,288,547.95
5,288,393.77
5,288,239.23
5,288,085.04
5,287,930.56
5,287,776.38
5,287,591.12
5,287,436.63
5,287,282.15
5,287,127.97
5,286,973.48
5,286,788.22
5,286,634.04
5,286,479.56
5,286,325.38
5,286,140.11
5,285,985.63
5,285,831.14
5,285,676.97
5,285,522.48
5,285,337.22
18
Elevation
668.0
669.8
671.3
673.5
672.6
673.8
674.7
669.5
672.0
672.0
672.3
674.4
678.1
681.4
684.8
686.3
687.8
688.1
688.1
688.1
686.9
686.0
685.1
683.5
681.7
682.6
682.6
683.2
683.2
683.8
682.9
680.8
681.1
683.8
685.1
684.8
683.8
682.0
681.1
680.5
679.0
679.0
679.0
679.0
679.0
679.0
678.7
678.1
677.4
676.5
675.0
674.7
673.8
673.2
672.9
672.9
672.9
673.2
672.6
Obs Grav
980,631.91
980,633.07
980,633.60
980,634.49
980,635.19
980,635.65
980,636.09
980,636.39
980,636.61
980,636.97
980,637.35
980,638.35
980,639.46
980,640.43
980,641.18
980,641.78
980,642.39
980,642.85
980,643.32
980,643.65
980,643.73
980,643.84
980,643.84
980,643.67
980,643.74
980,644.10
980,644.40
980,644.73
980,645.12
980,645.32
980,645.24
980,645.14
980,645.26
980,645.75
980,645.91
980,646.06
980,646.00
980,645.78
980,645.78
980,645.93
980,646.36
980,646.43
980,646.66
980,646.73
980,646.99
980,647.15
980,647.37
980,647.39
980,647.50
980,647.50
980,647.32
980,647.26
980,647.12
980,646.80
980,646.68
980,646.60
980,646.42
980,646.36
980,646.24
C.B.A. 1
-95.90
-96.53
-96.59
-96.88
-97.65
-97.70
-97.81
-99.14
-98.68
-98.91
-99.12
-99.52
-99.71
-99.83
-99.72
-99.86
-100.00
-100.27
-100.61
-100.81
-101.02
-101.22
-101.30
-101.33
-101.68
-101.71
-101.75
-101.95
-102.21
-102.15
-102.15
-102.39
-102.51
-102.05
-101.81
-101.90
-101.91
-101.98
-102.05
-102.20
-102.84
-102.79
-102.88
-102.82
-102.95
-102.97
-103.13
-103.40
-103.27
-104.04
-103.37
-103.25
-103.18
-102.87
-102.68
-102.48
-102.16
-101.86
-101.73
C.B.A. 2
-6.05
-6.67
-6.74
-7.02
-7.79
-7.85
-7.96
-9.28
-8.82
-9.06
-9.26
-9.67
-9.86
-9.97
-9.86
-10.03
-10.17
-10.44
-10.77
-10.97
-11.19
-11.38
-11.46
-11.50
-11.85
-11.88
-11.91
-12.11
-12.37
-12.31
-12.31
-12.55
-12.67
-12.21
-11.97
-12.06
-12.07
-12.14
-12.21
-12.36
-13.05
-12.99
-13.09
-13.02
-13.15
-13.18
-13.33
-13.60
-13.47
-14.24
-13.57
-13.45
-13.38
-13.07
-12.89
-12.68
-12.36
-12.06
-11.93
Count
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
Point_ID
PIH63
PIH64
PIH65
PIH66
PIH67
PIH68
PIH69
PIH70
PIH71
PIH72
PIH73
PIH74
PIH75
PIH76
PIH77
PIH78
PIH79
PIH80
PIH81
PIR01
PIR02
PIR03
PIR04
PIR05
PIR06
PIR07
PIR08
PIR09
PIR10
PIR11
PIR12
PIR13
PIR14
PIR15
PIR16
PIR17
PIR18
PIR19
PIR20
PIR21
PIR22
PIR23
PIR24
PIR25
PIR26
PIR27
PIR28
PIR29
PIR30
PIR31
PIR32
PIR33
PIR34
PMA01
PMA02
PMA03
PMA04
PMA05
PMA06
DD_lat
47.7206
47.7192
47.7178
47.7161
47.7147
47.7133
47.7122
47.7114
47.7103
47.7094
47.7086
47.7072
47.7061
47.7044
47.7031
47.7017
47.6997
47.6983
47.6967
47.7439
47.7422
47.7408
47.7394
47.7381
47.7364
47.7353
47.7336
47.7322
47.7308
47.7294
47.7278
47.7261
47.7247
47.7233
47.7217
47.7203
47.7186
47.7172
47.7156
47.7142
47.7125
47.7111
47.7094
47.7081
47.7067
47.7050
47.7036
47.7019
47.7003
47.6967
47.6953
47.6936
47.6925
47.6697
47.6706
47.6708
47.6714
47.6714
47.6714
DD_long
-116.8932
-116.8932
-116.8932
-116.8932
-116.8932
-116.8932
-116.8934
-116.8937
-116.8940
-116.8943
-116.8934
-116.8929
-116.8929
-116.8929
-116.8929
-116.8929
-116.8926
-116.8929
-116.8929
-117.0470
-117.0464
-117.0464
-117.0464
-117.0464
-117.0464
-117.0464
-117.0464
-117.0464
-117.0464
-117.0464
-117.0462
-117.0464
-117.0462
-117.0462
-117.0462
-117.0462
-117.0462
-117.0462
-117.0462
-117.0462
-117.0462
-117.0462
-117.0462
-117.0462
-117.0462
-117.0462
-117.0462
-117.0462
-117.0462
-117.0470
-117.0467
-117.0467
-117.0464
-117.1098
-117.1074
-117.1052
-117.1033
-117.1011
-117.0986
Easting
507,999.85
507,999.91
508,000.27
508,000.32
508,000.57
508,000.93
508,000.98
508,001.15
507,980.40
507,959.85
507,939.10
508,001.84
508,043.64
508,043.80
508,044.05
508,044.41
508,044.46
508,065.62
508,044.95
508,045.20
496,480.69
496,522.38
496,522.13
496,522.19
496,521.94
496,521.88
496,521.74
496,521.69
496,521.75
496,521.50
496,521.55
496,542.21
496,521.13
496,541.91
496,541.96
496,541.60
496,541.66
496,541.60
496,541.35
496,541.29
496,541.35
496,541.29
496,541.04
496,540.98
496,540.73
496,540.78
496,540.73
496,540.48
496,540.42
496,540.36
496,477.67
496,498.44
496,498.08
496,518.96
491,762.82
491,929.83
492,096.61
492,242.79
492,409.46
Northing
5,285,183.04
5,285,028.55
5,284,874.37
5,284,719.89
5,284,534.63
5,284,380.14
5,284,225.96
5,284,102.55
5,284,009.89
5,283,886.15
5,283,793.49
5,283,700.96
5,283,546.84
5,283,423.43
5,283,238.17
5,283,083.69
5,282,929.20
5,282,713.20
5,282,558.98
5,282,373.72
5,287,617.52
5,287,432.33
5,287,277.84
5,287,123.35
5,286,969.17
5,286,783.91
5,286,660.50
5,286,475.24
5,286,320.76
5,286,166.27
5,286,012.09
5,285,826.86
5,285,641.56
5,285,487.11
5,285,332.93
5,285,147.66
5,284,993.18
5,284,807.92
5,284,653.74
5,284,468.48
5,284,313.99
5,284,128.73
5,283,974.55
5,283,789.29
5,283,634.80
5,283,480.62
5,283,295.36
5,283,140.87
5,282,955.61
5,282,770.35
5,282,369.26
5,282,214.81
5,282,029.54
5,281,906.17
5,279,379.33
5,279,471.60
5,279,502.33
5,279,563.80
5,279,563.74
19
Elevation
672.0
670.7
668.9
668.0
666.5
666.5
670.7
672.3
670.7
669.2
669.8
667.7
664.0
661.6
663.7
662.8
662.2
658.5
649.4
665.5
665.5
665.5
657.3
649.4
647.0
643.6
643.6
642.4
642.4
642.4
642.1
641.8
640.5
640.5
638.7
635.4
636.0
634.8
634.8
636.3
637.5
637.5
638.4
639.3
639.3
639.6
639.6
639.3
638.4
628.7
630.2
629.6
630.2
631.4
631.4
630.8
630.8
631.7
632.3
Obs Grav
980,646.05
980,645.56
980,645.12
980,644.94
980,645.05
980,645.36
980,646.17
980,646.38
980,646.03
980,645.25
980,645.28
980,644.44
980,643.01
980,641.71
980,641.46
980,641.47
980,641.36
980,640.51
980,638.25
980,631.25
980,630.76
980,630.91
980,629.98
980,628.22
980,627.99
980,627.97
980,628.48
980,628.64
980,629.07
980,629.53
980,629.71
980,629.84
980,629.72
980,630.00
980,629.73
980,629.49
980,629.66
980,629.84
980,630.13
980,630.27
980,630.34
980,630.13
980,630.02
980,629.88
980,629.78
980,629.51
980,629.07
980,628.83
980,628.34
980,626.10
980,626.61
980,626.50
980,626.37
980,629.14
980,629.60
980,629.27
980,629.04
980,629.94
980,630.62
C.B.A. 1
-101.52
-101.14
-100.95
-100.83
-101.14
-101.32
-101.05
-100.78
-100.65
-100.07
-99.78
-99.29
-98.49
-97.57
-96.67
-96.73
-96.60
-96.43
-96.08
-89.71
-89.12
-89.15
-89.96
-89.87
-90.03
-90.63
-91.03
-91.30
-91.61
-91.94
-92.06
-92.12
-92.15
-92.31
-92.33
-92.70
-92.60
-92.92
-93.07
-92.74
-92.40
-92.24
-91.52
-91.03
-90.77
-90.29
-89.82
-89.49
-89.05
-88.64
-88.67
-88.56
-88.18
-88.81
-89.33
-89.20
-89.01
-89.71
-90.25
C.B.A. 2
-11.72
-11.34
-11.15
-11.03
-11.34
-11.52
-11.25
-10.98
-10.87
-10.32
-10.05
-9.48
-8.64
-7.72
-6.82
-6.88
-6.75
-6.56
-6.23
0.14
-11.99
-11.97
-12.78
-12.69
-12.85
-13.46
-13.85
-14.13
-14.43
-14.76
-14.88
-14.93
-14.97
-15.11
-15.13
-15.50
-15.41
-15.73
-15.88
-15.54
-15.21
-15.05
-14.32
-13.84
-13.58
-13.10
-12.63
-12.30
-11.86
-11.45
-11.54
-11.42
-11.03
-11.64
-17.39
-17.07
-16.71
-17.25
-17.60
Count
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
Point_ID
PMA07
PMA08
PMA09
PMA10
PMA11
PMA12
PMA13
PMA14
PMA15
PMA16
PMA17
PMA18
PMA19
PMA20
PMA21
PMA22
PMR01
PMR02
PMR03
PMR04
PMR05
PMR06
PMR07
PMR08
PMR09
PMR10
PMR11
PNL01
PNL02
PNL03
PNL04
PNL05
PNL06
PNL07
PNL08
PNL09
PNL10
PNL11
PNL12
PNL13
PNL14
PNL15
PNL16
PNL17
PNL18
PNL19
PNL20
PNL21
PNL22
PNL23
PNL24
PNL25
PNL26
PNL27
PNL28
PNL29
PNL30
PNL31
DD_lat
47.6714
47.6714
47.6714
47.6714
47.6714
47.6714
47.6714
47.6714
47.6714
47.6714
47.6714
47.6714
47.6714
47.6714
47.6714
47.6714
47.6700
47.6683
47.6669
47.6656
47.6642
47.6625
47.6611
47.6597
47.6583
47.6567
47.6556
47.7439
47.7439
47.7439
47.7439
47.7439
47.7439
47.7439
47.7439
47.7439
47.7439
47.7439
47.7456
47.7469
47.7483
47.7497
47.7511
47.7525
47.7539
47.7553
47.7569
47.7583
47.7597
47.7608
47.7622
47.7633
47.7644
47.7656
47.7669
47.7683
47.7700
47.7711
DD_long
-117.0964
-117.0943
-117.0921
-117.0902
-117.0883
-117.0861
-117.0842
-117.0817
-117.0795
-117.0773
-117.0754
-117.0732
-117.0710
-117.0689
-117.0667
-117.0653
-117.0885
-117.0885
-117.0885
-117.0885
-117.0885
-117.0885
-117.0885
-117.0885
-117.0885
-117.0885
-117.0885
-117.0470
-117.0489
-117.0511
-117.0533
-117.0555
-117.0574
-117.0593
-117.0615
-117.0637
-117.0656
-117.0678
-117.0678
-117.0678
-117.0678
-117.0678
-117.0678
-117.0678
-117.0680
-117.0680
-117.0680
-117.0680
-117.0689
-117.0699
-117.0710
-117.0721
-117.0732
-117.0743
-117.0754
-117.0757
-117.0762
-117.0765
Easting
492,597.16
492,764.14
492,930.82
493,076.77
493,222.73
493,368.68
493,535.66
493,681.61
493,869.31
494,035.99
494,202.97
494,348.92
494,494.87
494,661.55
494,828.53
494,995.20
495,099.72
493,347.70
493,347.65
493,347.40
493,347.14
493,346.89
493,346.83
493,346.58
493,346.33
493,346.38
493,346.01
493,345.87
496,480.69
496,335.04
496,168.37
496,001.70
495,835.33
495,689.37
495,564.44
495,397.77
495,231.40
495,085.44
494,919.08
494,919.13
494,919.38
494,919.33
494,919.57
494,919.52
494,919.77
494,898.99
494,899.24
494,899.30
494,899.54
494,837.33
494,753.98
494,671.04
494,587.69
494,504.64
494,442.31
494,359.07
494,317.58
494,297.22
Northing
5,279,563.42
5,279,563.37
5,279,563.01
5,279,562.93
5,279,562.54
5,279,562.46
5,279,562.41
5,279,562.02
5,279,562.00
5,279,561.64
5,279,561.60
5,279,561.51
5,279,561.43
5,279,561.07
5,279,561.02
5,279,560.97
5,279,560.83
5,279,408.25
5,279,222.99
5,279,068.50
5,278,914.32
5,278,759.83
5,278,574.57
5,278,420.39
5,278,265.90
5,278,111.42
5,277,926.46
5,277,802.75
5,287,617.52
5,287,617.61
5,287,617.66
5,287,617.71
5,287,617.76
5,287,617.84
5,287,617.96
5,287,618.31
5,287,618.37
5,287,618.45
5,287,618.50
5,287,803.76
5,287,958.25
5,288,112.43
5,288,266.92
5,288,421.40
5,288,575.59
5,288,730.04
5,288,884.53
5,289,069.48
5,289,223.97
5,289,378.36
5,289,501.95
5,289,656.31
5,289,779.89
5,289,903.48
5,290,027.09
5,290,181.45
5,290,335.87
5,290,521.11
20
Elevation
633.8
636.0
639.0
640.2
641.8
643.9
644.2
647.6
648.5
648.5
648.2
648.8
647.9
649.4
646.0
649.4
644.5
648.8
653.0
653.3
651.8
645.1
641.5
641.8
642.1
636.0
632.0
665.5
665.5
666.2
666.2
971.0
665.8
664.3
662.2
651.8
650.6
651.5
651.5
650.9
650.6
650.6
650.3
650.0
649.4
649.1
648.8
649.4
648.5
647.6
648.2
648.5
648.5
648.8
648.8
648.8
650.0
650.6
Obs Grav
980,631.41
980,632.19
980,633.00
980,633.70
980,633.78
980,633.89
980,633.55
980,633.59
980,633.30
980,632.97
980,632.50
980,631.66
980,630.85
980,631.16
980,630.49
980,630.97
980,634.42
980,635.64
980,636.66
980,637.08
980,637.23
980,635.89
980,635.11
980,635.42
980,635.39
980,634.18
980,632.97
980,631.24
980,631.38
980,631.51
980,631.41
980,631.68
980,631.81
980,632.06
980,631.50
980,628.89
980,628.29
980,628.21
980,628.23
980,627.95
980,627.71
980,627.63
980,627.46
980,627.37
980,627.06
980,626.78
980,626.62
980,626.46
980,626.13
980,625.99
980,625.89
980,625.79
980,625.34
980,625.15
980,625.08
980,624.52
980,624.17
980,623.34
C.B.A. 1
-90.70
-90.97
-91.10
-91.53
-91.27
-90.91
-90.49
-89.76
-89.27
-88.92
-88.52
-87.52
-86.77
-86.44
-86.51
-86.24
-91.19
-91.31
-91.25
-91.46
-91.81
-91.82
-91.72
-91.83
-91.58
-91.57
-91.11
-89.91
-90.09
-90.19
-90.10
-90.38
-90.57
-91.14
-91.05
-90.74
-90.41
-90.13
-90.15
-90.12
-90.08
-90.13
-90.16
-90.25
-90.20
-90.11
-90.15
-89.98
-89.97
-90.14
-90.01
-89.96
-89.64
-89.51
-89.56
-88.99
-88.41
-87.21
C.B.A. 2
-17.85
-17.93
-17.88
-18.14
-17.73
-17.20
-16.60
-15.71
-15.01
-14.48
-13.90
-12.74
-11.82
-11.31
-11.20
-10.75
-15.58
-17.63
-17.57
-17.78
-18.13
-18.14
-18.03
-18.15
-17.90
-17.88
-17.43
-16.23
-12.96
-13.22
-13.32
-13.78
-14.15
-14.89
-14.93
-14.80
-14.66
-14.53
-14.74
-14.71
-14.66
-14.72
-14.74
-14.84
-14.78
-14.72
-14.76
-14.59
-14.58
-14.82
-14.78
-14.82
-14.60
-14.56
-14.67
-14.19
-13.66
-12.48
standard gravity reductions to his data, we chose to work
directly from his original data to ensure that identical
corrections were applied to all observations. Purves’
original data and accompanying notes were provided
to us by the Washington State Department of Natural
Resources (Stephen Palmer, written commun., 1997).
Bouguer, and terrain were applied to standard gravity
measurements but could not be checked.
NEW OBSERVATIONS
In 1997, we undertook 146 gravity measurements
along three main transects in the Rathdrum Prairie as
well as at the mouths of Hayden Lake and Coeur d’Alene
Lake (Figure 4). The primary transects were along Idaho
Road (Idaho) and Hayden Avenue with five smaller
transects near the mouths of Hayden Lake and Coeur
d’Alene Lake.
This information included the following data: gravity
readings, elevations, times, locations, temperatures, base
readings, and meter constants. It also contained Purves’
notes on how he collected and recorded his findings.
He logged horizontal positions using an automobile’s
odometer and marked in tenths of a mile. He noted
vertical positions with an altimeter, and these were
verified daily against local benchmarks at numerous
places. He monitored elevation drift due to changes in
barometric pressure by allowing no more than a 0.03
percent disparity in elevations between local benchmarks.
His correction correlated to a vertical accuracy of ± 8 cm
(approx.). The horizontal locations were plotted by us
on a topographic map from the details of starting points
for each line. We digitally sampled the plotted points to
obtain coordinates for each gravity station. Therefore,
the accuracy of the station positions can be no better than
1/10 of a mile according to the odometer. Because of the
Rathdrum Prairie’s relatively consistent trends and the
substantial amount of additional data being considered,
we are confident that Purves’ locations are sufficient for
our study. We have no reason to suspect inconsistent
levels of precision or accuracy in the data.
Measurements were read with a Lacoste and Romberg
model G gravity meter (no. 1069). A standard base plate
was used. At least two measurements were taken for each
station, with the requirement that they agree to within
0.01 mGal. These were read at the base station at least
three times a day to monitor instrument drift. The base
station was a U.S. Geological Survey first-order, secondclass benchmark—designation P285 on the Rathdrum
7.5-minute quadrangle, near the corner of Idaho Highway
53 and Greensferry Road, about 0.6 km from Rathdrum.
Principal facts for the benchmark are given in Table 2.
All gravity measurements are thought to be precise to at
least 0.01 mGal before correction.
Coordinates and elevations for each gravity station
were obtained using the differential Global Positioning
System (GPS) technique. Leica SR 399 receivers
were used, with the previously described benchmark
as a coordinate tie to the geodetic system. With these
techniques, horizontal and vertical accuracy is at least
± 2 cm (J.S. Oldow, oral commun., 1997). Normally, a
2-cm-vertical variation should result in no more than a
0.01 mGal gravity variation. Measurements were
recorded on level road surfaces, with the GPS receiver
and gravity meter at the same elevation.
CADY AND MEYER DATA
In their geologic study of uranium deposits in the
region, Cady and Meyer (1976a) compiled the principal
facts for 2,077 gravity stations around Spokane,
Washington. From these data, they constructed a Bouguer
gravity anomaly map of the Okanagan, Sandpoint,
Ritzville, and Spokane 1° x 2° quadrangles (Cady and
Meyer, 1976b). The principal facts for the stations were
obtained from various sources, including USGS openfile reports and the U.S. Department of Defense’s gravity
library. Data from investigations by Bonini (1963) and
Hammond (1975) were included; data from Purves
(1969) were not. The Cady and Meyer (1976a) principal
facts were obtained in digital form from Hittelman and
others (1994), a CD-ROM collection of significant
gravity data sets.
The field survey was designed with two goals: to cover
previously uncovered sections of the Rathdrum Prairie,
and to enable a meaningful comparison and check of
Purves’ (1969) data. The Idaho Road (Idaho) profile fits
between two profiles of Purves (1969), Corbin Road and
Idaho Highway 41. These three profiles, on the western
side of the aquifer, are constrained to the north and south
by bedrock exposure, which reduces the uncertainty
of the profile’s geometry at these locations. The Idaho
Road (Idaho) profile provides a meaningful comparison
with the data of Purves (1969), indirectly verifying their
validity. The Hayden Avenue profile is oriented east-west
and designed to tie the north-south profiles and improve
the model of the east-west trend. The difficulty, to be
discussed later, is its orientation with the regional trend
We used 206 stations from this data set (Figure 4).
They are spaced generally one per square mile over
the study area to complement the profiles measured by
Purves (1969) and us. Unfortunately, raw gravity data
were unavailable. Corrections for drift, latitude, free-air,
21
Table 2. Principal Facts for Primary Benchmark.
The NGS Data Sheet
DATABASE = Sybase ,PROGRAM = datasheet, VERSION = 5.70
Starting Datasheet Retrieval.
1
National Geodetic Survey, Retrieval Date = NOVEMBER 29, 1998
SV0371 *****************************************************************
SV0371 DESIGNATION - P 285
SV0371 PID
- SV0371
SV0371 STATE/COUNTY- ID/KOOTENAI
SV0371 USGS QUAD - RATHDRUM (1986)
SV0371
SV0371
*CURRENT SURVEY CONTROL
SV0371 ___________________________________________________________________
SV0371* NAD 83(1986)- 47 48 08. (N) 116 54 56. (W) SCALED
SV0371* NAVD 88 665.103 (meters) 2182.09 (feet) ADJUSTED
SV0371 ___________________________________________________________________
SV0371 GEOID HEIGHT-17.41 (meters)
GEOID96
SV0371 DYNAMIC HT 665.142 (meters) 2182.22 (feet) COMP
SV0371 MODELED GRAV- 980,649.1 (mgal)
NAVD 88
SV0371
SV0371 VERT ORDER - FIRST CLASS II
SV0371
SV0371.The horizontal coordinates were scaled from a topographic map and have
SV0371.an estimated accuracy of +/- 6 seconds.
SV0371
SV0371.The orthometric height was determined by differential leveling
SV0371.and adjusted by the National Geodetic Survey in June 1991.
SV0371
SV0371.The geoid height was determined by GEOID96.
SV0371
SV0371.The dynamic height is computed by dividing the NAVD 88
SV0371.geopotential number by the normal gravity value computed on the
SV0371.Geodetic Reference System of 1980 (GRS 80) ellipsoid at 45
SV0371.degrees latitude (G = 980.6199 gals.).
SV0371
SV0371.The modeled gravity was interpolated from observed gravity values.
SV0371
SV0371;
North
East Units Estimated Accuracy
SV0371;SPC ID W - 682,440. 712,690. MT (+/- 180 meters Scaled)
SV0371
SV0371
SUPERSEDED SURVEY CONTROL
SV0371
SV0371 NGVD 29 663.931 (m)
2178.25 (f) ADJ UNCH 1 2
SV0371
SV0371.Superseded values are not recommended for survey control.
SV0371.NGS no longer adjusts projects to the NAD 27 or NGVD 29 datums.
SV0371.See file format.dat to determine how the superseded data were derived.
SV0371
SV0371_MARKER: DD = SURVEY DISK
SV0371_SETTING: 7 = SET IN TOP OF CONCRETE MONUMENT (ROUND)
SV0371_STAMPING: P 285 1944 P.C. 395+90.8 40.00
SV0371_STABILITY: C = MAY HOLD, BUT OF TYPE COMMONLY SUBJECT TO
SV0371+STABILITY: SURFACE MOTION
SV0371
22
SV0371 HISTORY - Date Condition
Recov. By
SV0371 HISTORY - 1944 MONUMENTED
IDDT
SV0371 HISTORY - 1944 GOOD
NGS
SV0371 HISTORY - 1980 GOOD
SV0371
SV0371
STATION DESCRIPTION
SV0371
SV0371’DESCRIBED BY NATIONAL GEODETIC SURVEY 1944
SV0371’1.0 MI SW FROM RATHDRUM.
SV0371’1.0 MILE SOUTHWEST ALONG STATE HIGHWAY NO. 53 FROM THE NORTHERN
SV0371’PACIFIC RAILROAD STATION AT RATHDRUM, ABOUT 0.1 MILE NORTHWEST OF THE
SV0371’TRACK, 89 FEET WEST OF CENTER-LINE OF A SIDE ROAD LEADING SOUTH AND
SV0371’ACROSS THE TRACK, 40 FEET SOUTHEAST OF CENTER-LINE OF HIGHWAY, 2 FEET
SV0371’NORTHEAST OF REFERENCE POST. A IDAHO STATE HIGHWAY BRONZE
SV0371’RIGHT-OF-WAY DISK SET IN TOP OF A CONCRETE POST PROJECTING ABOUT 0.4
SV0371’FOOT ABOVE THE GROUND.
SV0371
SV0371
STATION RECOVERY (1980)
SV0371
SV0371’RECOVERED 1980
SV0371’RECOVERED IN GOOD CONDITION.
of the gravity field and its location over a rather distinct
non-two-dimensional basin. Typical spacing was about
300 m between gravity stations. This spacing was based
on an analysis of the profiles presented in Purves (1969).
Small-scale variations identified in Bouguer gravity
profiles of Purves (1969) are equally prominent when
sampled every 300 m, as opposed to the approximate
150-m spacing that he typically used. Predictive forward
modeling also confirmed the efficacy of a 300-m spacing
in identifying the bedrock-sediment interface that has
been presumed to exist at depths by previous authors
(Newcomb, 1953; Hammond, 1974; Gerstel and Palmer,
1994).
a relative sense and should not be confused with an
absolute gravity measurement. For our study, the tie to
the local standard was a final step of the data reduction.
The meter readings were converted to mGal with
a calibration table provided by Lacoste & Romberg
specific to our gravity meter G-1069 for a given gravity
meter temperature. Purves (1969) used a standard
Worden gravity meter and had to multiply readings by the
meter constant of 0.09869-mGal/scale division. Original
information on meter readings was not available for the
data from Cady and Meyer (1976a).
TIDAL AND INSTRUMENT DRIFT
CORRECTIONS
GRAVITY DATA REDUCTION
All instrumental readings in the field require a
correction to compensate for the effects of instrument
drift and earth tides. Instrument drift may occur with
the Lacoste & Romberg gravity meter even though
its integral parts are housed in a vacuum at a fixed
temperature. Readings are corrected for the tidal drift
caused by the variable forces applied to the earth by the
sun and the moon.
Because our survey is using gravity measurements
from the two previous studies, various reductions to all
these field measurements are necessary to properly model
and interpret the combined gravity data. Reductions
must be consistent for each data set in the final product
to ensure that artifacts from inconsistent reduction do not
appear as anomalies on the final gravity map.
INSTRUMENT CALIBRATION
CORRECTION
After the tidal drift was removed, instrument drift
was removed by calibrating the measurements to a fixed
value at the previously introduced benchmark P285.
This instrument drift was never greater than 0.10 mGal
over one day. Data from Cady and Meyer (1976a) were
received already corrected for drift, but with no way to
verify the accuracy of those calculations. Purves (1969)
Every gravity meter has a unique calibration function
that is determined shortly after its production. This
calibration allows readings to be converted to a milliGal
(mGal) scale. This scale is accurate, however, only in
23
properly corrected for combined tidal and instrument
drift values as much as 0.10 mGal/day.
could not be obtained. The DEM-based, automated
procedure is composed of three software routines: one
that combines and reorganizes local 1:24,000 DEMs, a
second that uses the reduced DEMs and any additional
data provided by the user (GPS points, etc.) to calculate
“inner-zone” terrain corrections, and a third that applies
the “outer-zone” terrain corrections from a specially
produced DEM that provides regional coverage.
TERRAIN CORRECTIONS
The Rathdrum Prairie is a relatively flat plain with
elevation variations generally less than 50 m. These local
variations in topography, as well as the surrounding hills,
have an impact on the gravity and must be considered
with a terrain correction procedure. The nearby hills
give an upward component of gravitational attraction
that counteracts a part of the downward pull exerted
by the rest of the earth. Conversely, the surrounding
valleys below station elevations, which can be modeled
as holes in the theoretically continuous slab that extends
to the datum, produce a smaller downward pull than is
calculated with the Bouguer correction.
The digital elevation models supplied by the USGS are
preprocessed for use by the terrain correction software.
A routine called DEMREAD, written by Alan H. Cogbill
at the Los Alamos National Laboratory, performs the
required algorithms that combine numerous DEMs into
one large, project specific DEM. The preprocessing
enhances the execution speed of the terrain correction
program and reduces the data storage space of the DEM
file for that program to about 30 percent of the ASCII
data size provided by the USGS. Table 3 lists the USGS
7.5-minute DEMs (30-m x 30-m data spacing cast on
a Universal Transverse Mercator projection) that were
input to DEMREAD for combination and reduction to
binary format. Surface coverage was required to a radius
of 2,000 m around each gravity station. This radius for
inner-zone corrections is suggested by Cogbill (1997) for
regions of nonextreme topographic relief. All of the DEMs
used are classified as level 1 or 2, correlating to a vertical
RMS error of 7-15 m or 3 m, respectively. As a check of
the DEMs’ vertical accuracy, twenty GPS measurements
were taken within 2.5 m of DEM data points. Assuming
that the GPS measurements were accurate in the vertical
direction within 0.05 m, the average vertical error of the
DEM points that were near the GPS measurements was
3.4 m. Although this variation appears large, it agrees
with the 3-m RMS error reported by the USGS for level
1 DEMs and is well below the 7- to 15-m RMS error
reported for level 2 DEMs.
In the past, these corrections followed the method
developed by Hammer (1939) that calculated the
gravitational effect of flat-topped, cylindrical sections
or sloping planes, both based on elevations manually
read from topographic maps. Other methods developed
from Hammer’s original work include one for improved
sloping planes (Sandberg, 1958) and another for
improved conical prisms (Olivier and Simard, 1981).
The accuracy of these methods depends on how closely
a particular geometrical model conforms to the actual
terrain near a gravity station. Today, data manipulation
by computers, combined with the availability of lowprice 30-m digital elevation models (DEMs) produced
by the U.S. Geological Survey’s (USGS) National
Cartographic Information Center, allows for a more
accurate and less arduous method of terrain correction.
Methods developed by Plouff (1966) and Cogbill (1990)
that use the exhaustive surface coverage provided by
DEMs have an accuracy that depends mostly on how well
the DEM represents the terrain near the gravity station.
Corrections calculated using the DEMs are potentially
more accurate than hand procedures because of the more
subjective manual methods.
The terrain corrections are decomposed into two
parts: inner-zone and outer-zone corrections. Inner-zone
corrections account for topographic variations very close
to the gravity stations (5 m) out to a specified distance
(2,000 m), whereas outer-zone corrections are based on
coarser terrain data, and broader surface fitting methods
are calculated. The inner-zone corrections use a procedure
explained in Cogbill (1990) that takes advantage of the
USGS 7.5-minute DEMs. The inner zone is further
divided into an inner zone (radius of 250 m) and medium
zone (radius 250-2,000 m). A continuously differentiable
surface is fit to the elevation data within the smaller inner
zone. This surface is integrated numerically to obtain that
portion of the overall terrain correction. Mathematically,
the algorithm effectively integrates a line element between
the station and the medium zone, repeated every 3
Because data from the three sources (ours; Purves,
1969; Cady and Meyer, 1976a) are to be compared with
each other and presented as one product, the same terrain
correction technique would ideally be applied uniformly.
This would avoid any inconsistencies between manual
estimation methods and automated DEM-based
techniques. Unfortunately, nonterrain corrected data
from Cady and Meyer (1976a) were unavailable and
therefore not re-corrected in relation to the other data
sets. Furthermore, parameters of the terrain corrections
performed on the data from Cady and Meyer (1976a),
including Hammer zones and correction densities,
24
Table 3. Digital Elevation Models.
USGS DEM
Athol, ID
Fernan Lake, ID
Greenacres, WA
Hayden Lake, ID
Hayden, ID
Liberty Lake, WA-ID
Mica Bay, ID
Mica Peak, ID
Coeur d’Alene, ID
Mt. Coeur d’Alene, ID
Bayview, ID
Newman Lake, ID
Post Falls, ID
Rathdrum, ID
Rockford Bay, ID
Spirit Lake East, ID
more than 18 km from each gravity station. Outer terrain
corrections ranged from approximately 0.17 mGal for
points in the central part of the prairie to 0.78 mGal for
points in the surrounding hills.
Level
1
2
2
1
2
2
1
1
1
2
1
1
2
2
1
1
LATITUDE AND ELEVATION
CORRECTIONS
Because the earth is not a perfect, nonrotating sphere
but has an equatorial bulge and significant rotation, the
effects of latitude must be considered so that gravity
adjustments can be compared. Centrifugal acceleration
due to rotation, maximum at the equator and minimum
at the poles, acts to oppose gravitational acceleration.
Conversely, polar flattening acts to increase gravity
at the poles by making the geoid closer to the earth’s
center of mass. The latitude adjustment is calculated by
differentiating the Geodetic Reference System (GRS
1967) formula.
degrees of azimuth. In the medium zone, the terrain effect
is calculated by numerically integrating the elevation
data using a rectangular integration rule. Cogbill (1990)
estimates that integration errors are always less than
0.001 mGal, but notes that the calculated corrections
can be no better than the elevation data used to represent
the terrain about each gravity station. The accuracy of
the calculated corrections is also limited by the inherent
uncertainty of the estimated mean terrain density.
Because gravity varies inversely with distance from
the center of the earth, it is necessary to apply the free-air
correction, which reduces all readings to a datum surface.
The correction is 0.3086 mGal/m (Dobrin, 1976).
BOUGUER CORRECTIONS
The Bouguer correction removes the effect of a
presumed infinite slab of material between the horizontal
plane of each station and a datum plane. The correction
factor is -0.112 mGal/m above the datum, assuming an
average density for crustal rocks of 2,670 kg-m-3 (Dobrin,
1976).
The inner-zone corrections from all three data sets were
conducted using the method of Cogbill (1990), through
the software package, INNERTC, which he developed.
The terrain corrections ranged from approximately 0.02
mGal in the proximal regions of the prairie to ± 1.20
mGal in the surrounding hills. A correction density (r) of
2.68 g-cm-3 was used.
By applying the reductions listed above to the field
measured data, the Bouguer anomaly is produced.
The Bouguer anomaly is the observed value of gravity
minus the theoretical value at the latitude and elevation
of the observation point. This allows for variations in
the Bouguer anomaly to be interpreted and modeled in
relation to variations in subsurface geologic features.
Outer-zone corrections were calculated using a
method developed by Plouff (1966) and modified by
Alan Cogbill in his software package, OUTERTC.
The correction procedure fits a multiquadric surface
to elevation data from USGS 3-arcsec DEMs, and it
calculates the effect of this surface on each gravity
measurement. A compilation of the entire set of USGS
3-arcsec DEMs was provided by Cogbill, with the
accompanying program MAPFILE that selectively
removes the nonrequired DEMs and formats the
remaining DEMs for input to OUTERTC. Outer-zone
corrections cover the distance from the outermost radius
used in INNERTC (4,000 m for our study), to a maximum
radius of 111 km. The standard maximum radius of
167 km would have been preferred, but significant
complications arise with the integration of Canadian
DEMs. A correction density of r=2670 kg-m-3 was used.
The earth’s curvature is incorporated for elevation data
The Bouguer anomaly map produced by our study
is shown in Figure 6. The Bouguer anomaly has a
fairly straightforward basinal appearance, except for a
significant overriding, east-west regional trend.
CORRELATION OF
DATA SETS
The data from all three sources must be properly
correlated. The data from Cady and Meyer (1976a) are
referenced to GRS 1967. The data from Purves (1969)
were referenced to the control network of North America
25
REMOVAL OF
REGIONAL TREND
established by Wollard and Behrendt (1961). No control
point was available for our study, so this correlation
is used to reference the data to GRS 1967. To insure
consistency between the three data sets, eight locations
were identified where gravity measurements were taken
for all three stidies. The correlations of Cady and Meyer
(1976a) to our data and to Purves (1969) are shown in
Figure 6. The data sets, as represented by these points, are
evidently well correlated and only require the addition
of a constant to reference our data and those of Purves
(1969) to the GRS 1967 system.
Any pattern seen on the Bouguer anomaly map is the
sum of the attractions of local sources and broader, more
distant regional sources. A regional trend that affects
Bouguer anomaly because of changes in crustal thickness
is apparent in Figure 7. This poses a difficult problem
to gravity modeling of the Spokane Valley-Rathdrum
Prairie aquifer. The thickening of the continental crust
beneath the northern Rocky Mountains (Winston and
Figure 6. Locations of eight gravity measurements overlap between the three data sets. By correlating these
points, it is possible to indirectly reference the data of Purves (1969) and ours to GRS 1967, to which the data of
Cady and Meyer (1976a) are referenced.
26
others, 1989; Harrison and others, 1972), east of the
study area, causes a regional decrease in the Bouguer
anomaly of 0.8 to 1.9 mGal per km eastward. This trend
can be clearly observed on a portion of the Bouguer
anomaly map of Idaho shown in Figure 8. Bankey and
others (1985) is a complete Bouguer anomaly map of
Idaho produced through a compilation of existing data
sources. The collection of data from Cady and Meyer
(1976a) and Hammond (1974) is that included on the part
covering the Rathdrum Prairie, but data sets peripheral to
the study area allow for an interpolation of the regional
trend. Ten measurements were averaged to estimate
a regional trend of 1.1 mGal per km east. Though this
correction is subjective, it appears to reasonably estimate
the effect of crustal thickening. The trend was simplified
to an E-W orientation. Variations in this correction will
significantly alter models of east-west oriented profiles,
such as the Hayden Avenue profile.
A Bouguer gravity map that is adjusted to compensate
for the regional trend is shown in Figure 9. Compared
with the Bouguer anomaly map in Figure 7, the trendadjusted gravity apparently has a basinlike appearance.
The method used respects the general geologic model
of the subsurface and maintains the simplicity and
reproducibility of the corrections.
GRAVITY DATA MODELING
The overall geologic model of the Rathdrum Prairie
has been described as an ancestral valley that was
filled with various sediments during the late Tertiary
and Quaternary periods. The basin-fill geology has
generally been inferred from Bouguer anomaly (Figure 7).
Unfortunately, this general impression cannot predict
the specific depths and morphologies of buried surfaces.
Figure 7. Bouguer anomaly map of the Rathdrum Prairie based on our data and those from Purves (1969) and Cady and Meyer (1976a).
27
Floods. Measured densities of the basement rocks range
from 2.64 to 2.80 g-cm-3, with an average of 2.67 g-cm-3
(Purves, 1969; Birch, 1942; Harrison and others, 1972).
The densities of the Columbia River basalt fall into a
very broad range from 2.78 g-cm-3 for vesicular samples
to 3.21 g-cm-3 for massive samples. Latah sediments
have a measured dry density of 1.09-1.62 g-cm-3 and a
saturated density of 2.13-2.42 (Hosterman, 1960; Purves,
1969). The gravels of the Missoula Floods vary widely
in their densities because of the influence of different
particle sizes on porosity, as well as the presence of any
cementation. By using a source rock density of 2.602.80 g-cm-3 and porosity ranging from 20 percent for
cemented gravels to 40 percent for coarse uncemented
gravels, a dry bulk density range of 1.56 to 2.24 g-cm-3
can be inferred. Given this dry density and porosity
range, the saturated gravel density range is calculated at
1.96 to 2.44 g-cm-3. Measuring the densities of surface
samples is quite simple, but extending these densities to
subsurface gravels should be done with caution.
The density information introduced above is difficult
to model. The contrast in density between bedrock
and basalt is very small, as is also that between Latah
sediments and flood gravels. Adding to this problem is
the uncertainty about the locations of basalt dikes and
remaining basalt deposits, and the location and quantity
of Latah sediments. A complex geologic environment
probably exists beneath the prairie, considering the
numerous reworking episodes that have occurred.
Differentiating the basalt deposits from bedrock or
the Latah sediments from gravels is nearly impossible
through a gravity model because an infinite number of
combinations may create the same signature. Because of
the locally complex setting and small density contrasts,
the geologic models have been simplified to elements
that will materially affect gravity signatures.
Figure 8. Bouguer gravity map of the Rathdrum Prairie showing a
regional trend of +1.1 mGal/km east.
With a detailed gravity survey, however, small variations
in the local gravity field can be accurately detected.
These anomalies can presumably be attributed to buried
geologic surfaces that can be readily modeled. The
limiting factor to the models is determining the densities
of the geologic units. Because the densities of surface
rocks can be accurately measured and mapped, doing
so provides a starting point for estimating subsurface
densities.
The rocks of the Belt Supergroup, Cretaceous
intrusions, other dikes and sills, and Miocene basalt have
all been combined for the model and are referred to as
bedrock, with a density of 2.67 g-cm-3. Latah sediments
and flood gravels are modeled as dry sediments with a
density of 1.7 g-cm-3, and the intermingled sands and
clays are modeled as saturated sediments with a density
of 2.1 g-cm-3. The justification for these densities is
twofold: They are well within the range of average
measured and calculated densities, and they provide the
most realistic model when the seismic refraction bedrock
tie of Newcomb and others (1953) is imposed. Obviously,
this will oversimplify the actual geology in many places,
but will allow a more realistic and probable test of the
proposed geologic model without the complication of
determining contacts between units of similar densities.
Numerous studies have documented the surface
densities of rocks in and around the Rathdrum Prairie.
The rocks can be generally categorized into four main
units: (1) basement rocks of the Belt Supergroup
and Cretaceous igneous intrusions, (2) Tertiary basalt,
(3) Latah sediments, and (4) gravels of the Missoula
28
Figure 9. Residual Bouguer gravity map of the Rathdrum Prairie.
The modeling densities used by Hammond (1974) could
not be procured for comparison.
the Rathdrum Prairie, because it gives some control over
the 3-dimensional subsurface structure that ultimately
defines the gravity field. The 2¾-dimensional modeling
technique normally overestimates basement depth, unless
the basin segment of the model is chosen to be narrower
in the direction normal to the profile than the actual
basin, in which case the depth would be underestimated.
Adding additional uncertainty to non-3-dimensional
models is the elevation gradient of the basement walls in
the basin. In less than three dimensions, it is impossible
to exactly and correctly identify the perpendicular extent
of each modeled profile segment.
The theoretical gravity field of five proposed crosssections was modeled using the software GM-SYS of
Northwest Geophysical Associates in Corvallis, Oregon.
It uses the methods of Talwani and others (1959) and
Talwani and Heirtzler (1964) to calculate the gravity
model response. A powerful feature of the program
is its capability to calculate 2¾-dimensional models
from the routines of Rasmussen and Pedersen (1979).
A 2¾-dimension model allows for the extension of
each discrete model along its perpendicular axis to a
user-defined interface where an infinite half-slab of the
same profile shape is modeled with user-defined density
values. This is useful in a basin-type structure, such as
Five profiles were modeled on the basis of trendadjusted Bouguer gravity; their locations are shown on
Figure 4. Two of the models, Hayden Avenue and Idaho
29
Road (Idaho), were based on our data. Three models
are based on data collected by Purves (1969) and recorrected as part of our study: Idaho Road (Washington),
Corbin Road, and Idaho Highway 41. The profiles and
calculated and measured gravity values are shown in
Figure 10(a-e). All four north-south profiles are shown at
the same scale for reliable comparison. The profiles are
based on surficial mapping by Breckenridge and Othberg
(1998a, 1998b) and other geologic and geophysical data
previously mentioned. The depth tie for these models is
the seismic refraction profile of Newcomb and others
(1953). The seismic reflection profile of Gerstel and
Palmer (1994) confirms the depths of Newcomb and
others (1953) in the chosen location. The deepest point
on Newcomb and others’ profile has been fixed as the
deepest point on the Idaho Road (Washington) model. No
existing wells penetrate to bedrock in the proximal part
of the prairie, most only extend a short distance beneath
the water table. The ground-water levels shown in the
models have been interpolated from well-inventory data.
Profile element extensions in the third dimension, for
2¾-dimensional modeling, were estimated individually
for each profile.
GRAVITY MODEL INTERPRETATION
The gravity profiles in the preceding section represent
the possible subsurface structure of the Rathdrum Prairie.
Understanding this morphology will aid in interpreting
the region’s geologic history. No attempt has been made
to identify specific hydrologic boundaries within the
sediments, as such divisions would be overwhelmingly
subjective because of the low density contrasts involved.
The models are based on the existing pool of geological
hypotheses for the region. The reversed seismic refraction
profile of Newcomb and others (1953) was the only
bedrock tie used for these data; therefore, the modeled
depths to bedrock are not definitive. Additionally,
Figure 10a. The Idaho Road (Washington) profile is at the western end of the Rathdrum Prairie. At this point, the valley appears to be generally
V-shaped, with a small bench on the southern edge of the profile. A deeper section in the middle may be the result of fluvial erosion. The valley is
only half as wide at this location, as it is 8 km to the west at Idaho Road (Idaho). Based on the refraction profile of Newcomb and others (1953),
the maximum depth to bedrock in this profile has been fixed to a depth of 152 m below the ground surface at the indicated position. The maximum
aquifer thickness is 216 m, and the lowest elevation of the bedrock-sediment interface is 426 m.
30
Figure 10b. The Corbin Road profile is 5 km east of the Idaho Road (Washington) profile. This profile is characterized by a generally smooth floor
and an apparent embedded valley on the north edge, with shallow benches on both sides of the profile. The southern edge shows a small depression
in the present location of the Spokane River. It is not possible to determine, through gravity modeling, how much of the subsurface marginal rocks
is Miocene basalt rimrock. The maximum thickness of the aquifer is 263 m and the lowest elevation of the bedrock-sediment interface 384 m.
Figure 10c. The Idaho Road (Idaho) profile is significantly wider than the two profiles to its west. It is characterized by what appears to be an incised
river channel in the center of the valley (Rathdrum River?) and a perched depression on the north side. The maximum sediment thickness is 332
m, and the lowest elevation of the bedrock-sediment interface is 337 m. This model does not conclusively prove or disprove that this location was
overridden by the Cordilleran ice sheet.
31
Figure 10d. The Idaho Highway 41 profile is the widest of the north-south profiles. The incised valley evident on the Idaho Road (Idaho) profile is
less apparent here. Instead, there is a significant feature on the southern side of the valley. The interface is characterized by a smooth, undulating
surface. Because of the wide, smooth profile, Cordilleran glaciation possibly did override this location. The maximum aquifer thickness is 356 m,
and lowest elevation of the bedrock-sediment interface is 319 m.
Figure 10e. The Hayden Avenue profile is the only east-west profile. This makes accurate modeling particularly difficult because of the east-west
regional trend, causing the shape of this profile to be strongly affected by the trend correction factor. The general bedrock surface has a smooth,
undulating character similar to the Idaho Highway 41 profile. Evidence of Pleistocene glaciation across this profile is inconclusive. The valley
appears to be deeper on the eastern side, but that may be an influence of the regional trend. The maximum aquifer thickness is 283 m, and the lowest
elevation of the bedrock-sediment interface is 395 m.
32
variations in sediment densities and anomalies within
the metamorphic and crystalline bedrock can cause
variations in the gravity field that would lead to grossly
misinterpreting the shown depths. Table 4 presents the
modeled aquifer thickness and bedrock elevations.
control provided by the seismic refraction of Newcomb
and others (1953). Bedrock ties act to constrain the
model. The valley at this point is relatively constricted
and V-shaped. This suggests that the ancestral valley was
subject to fluvial erosion. It is not possible to determine
whether a major glacial advance overrode this profile,
but it seems unlikely from the geomorphic evidence of
Breckenridge (1989) and Waitt and Thorson (1983). Along
this profile, Newcomb and others (1953) interpreted the
lower half of the aquifer as Latah sediments intercalated
with Miocene basalt.
Table 4. Modeled Aquifer Characteristics
Profile
Idaho Road (Washington)
Corbin Road
Idaho Road (Idaho)
Idaho Highway 41
Hayden Avenue
Maximum
Sediment
Thickness
(meters)
Lowest
Elevation of
Bedrock
(meters)
216
263
332
356
283
426
384
337
319
411
Five kilometers to the east is the Corbin Road profile.
This profile has a smooth bedrock surface with what
appears to be an incised stream channel on the north
side. This feature may be the remnant of a flow on the
north side of the valley during the Miocene when the
region was impounded by the Columbia River Basalt
Group. Similarly, the apparent hump may be a large
basalt deposit that was not eroded during the Pleistocene.
Shallow benches exist on both sides of the valley.
The data contained in Table 2 reveal an interesting
feature that was first presented by Purves (1969),
though he noticed it only in Bouguer anomaly profiles.
The elevation of the aquifer’s base rises significantly
beneath Corbin Road, effectively thinning the sediments
by about 80 m compared to the neighboring profiles.
Hypotheses vary for explaining this feature. Purves
(1969) suggests that the local bedrock high is evidence
that the advancement of the Pend Oreille lobe stopped
just east of the Idaho Road (Washington) profile. His
explanation is possible but not overwhelmingly evident
from these gravity data alone. A major basalt remnant
may exist in the locality of the bedrock high. It would
have had to survive Pleistocene erosion and would
have acted as a shielding mechanism from the Missoula
Floods for the Latah sediments to the west that were
identified by Newcomb and others (1953). The Latah
sediments may have been overridden by younger
basalt flows that slowed their erosion. A localized dike
or intrusion in the country rock could also explain the
feature. If such an event occurred and the intrusion was
a significantly higher density, the modeled feature could
be a misinterpretation. These very simplified hypotheses
are based on a feature that has only been identified with
gravity methods. Its existence should be studied further.
Five more kilometers to the east, the valley widens
significantly where the Idaho Road (Idaho) profile is
located. The bedrock surface shows a large feature
channel in the center of the profile. Perhaps this is the
location of the ancestral Rathdrum River. Modeling
such extreme variations in bedrock effectively with
gravity data is difficult because of the broad influence
that relatively deep features have on their surrounding
morphology. This may be the cause for the rise to the
right of the incised channel. Aside from the channel,
however, this profile has a consistent depth and a smooth
bedrock-sediment interface.
Two more kilometers to the east is the Idaho
Highway 41 profile. This incised stream channel is less
evident in the Idaho Highway 41 profile. The valley
is broad and smooth with few large variations in the
bedrock-sediment interface. Possibly the Pend Oreille
lobe did advance this far west, but evidence for or against
such an idea is not contained completely in this profile.
The Hayden Avenue profile was quite difficult to
model. It is oriented perpendicularly to the regional trend
and therefore significantly affected by any adjustments
to the trend correction. The valley appears to be deepest
on the eastern side and shallower on the west. This is
likely an effect in the difficulty of modeling a basin in
less than three dimensions. The profile forms an acute
angle with the north valley wall for about a kilometer
on the western end. The shallow bedrock on the north
side of the profile acts to increase the apparent gravity
on that end, which leads to a shallower modeled depth
Discussions of each profile are useful in attempting
to decipher the paleogeomorphology of the Rathdrum
Prairie. It should be reiterated that small-scale variations
in a modeled surface are a non-unique solution to a
complex physical situation. Many of these variations
could quite easily appear somewhat different in another
model based on the same data.
The most reliable of the models is the Idaho Road
(Washington) profile. This profile has the limited bedrock
33
to bedrock. This profile should be limited to the depth
to bedrock near the center of the profile, where the edge
effect is minimized.
Two primary factors limit the reliability of these
results. First, the models were created using 2¾
dimensional modeling techniques. Any model produced
with 3-dimensional techniques will be more accurate.
A 3-dimensional inversion of the basin would more
clearly define the aquifer boundaries. Second, only one
control point was used for our study. Future studies
must include two kinds of data gathering: (1) seismic
and gravity surveys to map the subsurface depths and
structure; and (2) well logs to distinguish geologic units
and confirm structure, rock densities, and areal and depth
measurements.
Analysis of the minimum elevations of the bedrocksediment interface suggests that a low point exists
between Idaho Road (Idaho) and Hayden Avenue,
as has been previously suggested by Purves (1969).
Unfortunately, the reliability of depths modeled becomes
less accurate when the profiles are further from the
location of the seismic refraction tie. Furthermore, the
adjustment for the regional trend is a notable subjective
action that would also affect this conclusion.
ACKNOWLEDGMENTS
CONCLUSION
This study was funded by the Idaho Geological
Survey. The gravity meter and GPS equipment necessary
to our research were provided by Dr. John Oldow of the
University of Idaho. Dr. Oldow provided insights on
the details of GPS and gravity surveying and training
in the use of the equipment. Dr. Stephen Palmer at the
Washington Department of Natural Resources provided
the original data of Purves (1969). Loudon Stanford at
the Idaho Geological Survey gave helpful advice on
manipulating USGS DEM and DRG data. The late Dan
Weisz was an excellent field assistant, and thanks should
also go to his parents, who allowed a GPS station to be
placed in their yard on the Rathdrum Prairie.
Understanding the thickness and subsurface geology
of the Rathdrum Prairie aquifer is critical to interpreting
the local and regional geologic history that will
ultimately lead to ensuring better aquifer management.
To that end, we have incorporated new gravity data with
a sizable existing gravity data set to create a broader
base from which to interpret the region’s subsurface
structure. From this information, we have developed the
geometry for each of the five modeled profiles as well
as the plausible depth and thickness relationships. The
results provide a much clearer picture of the aquifer’s
structure and extent.
The sediments that compose the Rathdrum Prairie
have a maximum thickness of about 356 m in the center
of the valley on Idaho Road (Idaho) between Post Falls
and Rathdrum. The aquifer thins to about 215 m only
10 km to the west at the Idaho-Washington state line.
The bedrock-sediment interface is generally smooth east
of Idaho Road (Idaho). From this point to the west, an
incised channel becomes apparent in the ancestral valley
floor. The lower boundary of the aquifer remains at a
generally consistent elevation across the study area, with
a shallow slope to the east. A low point in the elevation
of the bedrock-sediment interface is presumed to exist
between Idaho Road (Idaho) and Hayden Avenue.
This conclusion should be interpreted with caution,
as the regional trend correction ultimately determines
the magnitude of the interface’s slope in the east-west
direction.
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37

Gravity, Morphology, and Bedrock Depth of the Rathdrum Prairie

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