Photogeology Photogeology

Photogeology
Introduction to the interpretation of
aerial photographs
About Landslide studies
• Landslide studies can be organized into three phases
• 1) detection and classification of landslides
2) monitoring activity of existing landslides
3) analysis and prediction of slope failures in space
(spatial distribution) and time (temporal distribution).
• Remote sensing techniques can be and are often used
in all three stages of a landslide investigation.
Photogeology
• An aerial photo is an image obtained from
a big height (not less than 1000 meters)
and with particular features:
Focal distance fixed to infinite.
Frame of the negative (big dimensions).
Wide angle of lens opening (wide angle
lens).
Elements of a photo
• Fiducial marks
 Referring points that
can be observed in
every photo. In every
frame there is a point
for each corner, each
one is represented by
a point or a cross (x).
 Linking together
fiducial marks we
obtain the principal
point.
point
Elements of a photo
• Data strip
 Side of the frame
where are set
images of on board
instruments, that
are:
o Clock with data and
time.
o Altimeter with height
of flight.
o Bubble (spirit) level.
o Number of camera,
number of photo
and focal distance
of the lens.
Elements of a photo
• Bubble level: allow to control if the
plane is horizontal.
• Clock  allow to know the height
of the sun on the horizon and on
consequence the lenght of
shadows, that is used to find height
object
of objects: h = l . tg a
where:
h
- h is the height of the object
- l is how long is the shadow
- a is the incidence angle of light
rays.
a
l
Elements of a photo
• Altimeter  allow to
know the height of fly,
in meters (m.) or in feet
(ft.) (it is important to
control everytime unity
of measurement,
remembering that 1 ft.
= 0.3048 m.).
• Number of frame 
allows to order frames
in a recording
sequence.
Scale of photos
• There are two methods to find the scale:
1) With a known topographic map.
2) With an approximated topographic map,
knowing focal distance and height of fly.
Scale of photos
1)
Starting from a topographic detailed base: we choose
on the photo two points (related to two known points
on the topographic map) with a distance from one to
the other of at least 10 cm. and with the same height
(better if they are near to the principal point).
Considering df the distance measured on the photo, d
the distance on the map and S the scale of the map
we find:
x
d S
df
Where x is the scale of the frame.
Scale of photos
2) Starting from data obtainable from the map
(even if approximative) and from data obtained
from the photo we find:
x
c
Z 0 hm
Where:
x is the scale of the photo
c is the focal distance
Z0 is the height of fly referred to sea level
Hm medium altitude of the ground
Zm corrected height of fly
c
Zm
Strip
• A strip is a series of photos taken with
regular intervals with the same camera.
Every frame must have in common with
the previous an area of the 60% (overlap).
Moreover each strip have with the other
strips (sidelap) a common area of 15%.
• In this way the photographic covering of
the area is total.
Line of fly
• Considering two
adjacent frames of the
same strip, we have to
link principal points,
then for every photo we
find the point
corresponding to the
principal point of the
other, finally finding four
points: A, A’, B, B’
(Correspondent or
analog points). Linking
these points with a line
we find the line of fly.
fly
Line of fly
A
B’
A’
B
Relief displacement
r
• It is the
displacement of a
point on a photo
in comparison to
the real position
on the ground,
caused by an
effect created by
the relief.
n’
b’
a’
c
O
Zm
A
DZ
w
A’’
B
DR
R
Relief displacement
• We consider O the perspective center, n’ the principal point on the
frame and w the referring horizontal plane.
• Supposing that in the photo there would be a raised object (es. a
tower) the image of AB in the photo is a’b’. Considering similes
between AA’’B and A’’ON we will find:
r
r
Z (1)
Zm
Z
Zm
r
(2)
r
Where Dr is the displacement due to the relief, r the distance from the
principal point, DZ the height of the tower + height difference, Zm the
height of fly (respect to the referring plane).
• It results that:
 The relief displacement is proportional to the distance from the
principal point and to the difference of height and inversely proportional
to the height of fly.
 Using the relief displacement effect it is possible to find the height of
an object.
Stereoscopic view
• It is a system to
obtain a 3D vision of
one or more objects.
• Humans can have a
three dimensional
view only for little
distances: for long
ditances we must
use an artificial
stereoscopic vision,
observing two photos
with two different
points of view.
Charateristics of stereoscopic
photos
• They must cover part of the same area.
• The optical axle of the camera must
always lay on the same plane.
• Photos must have the same scale.
Stereoscopic exaggeration
• Effect that concern the view of the 3D
object that shows a raise of heights
(positive exaggeration: the scale of heights
is higher than the lenght one) or flattening
of the relief (negative exaggeration: the
vertical scale is lower of the horizontal
one).
Stereoscopes
• Instruments made to
visualize
photographies in 3D.
There are two kinds
of them:
• Pocket stereoscope.
• Mirror stereoscope.
Mirror stereoscope
• Device constructed to force
each eye to look essentially
straight down and along lines
that are parallel. Light rays
coming from photographs are
reflected first by surfacesilvered mirrors set at 45° to
the horizontal and then again
by small mirrors set parallel to
the larger ones. After reflection
by the small mirrors, the rays
are parallel to their original
direction, but are separated by
a distance determined by the
separation of small mirrors.
Use of stereoscope
• Place photos under the
stereoscope (according
to the line of fly) then
move them in order to
obtain that the distance
between the same point
in the left photo and in
the right photo is equal to
the instrumental base
one (normally 20-25 cm.).
• Now it is possible to see
the 3D image.
Interpretation of photos
• In the field of civil engineering it is useful to identify
geological and morphological conditions of the zone, in
particular to detect landslides and unstable slopes.
• It is necessary to identify parameters that represent
particular characteristics of the ground:
Parameters
Charateristics of the ground
Tone
Colour
Texture
Morphology
Drainage
Lithology
Density of drainage
Structures in general
Vegetation
Vegetation
Alignments
Disjunctive structures (faults,
fractures)
Interpretation of photos
• Tone:
Tone colour of objects, that is divided in very dark, dark,
medium grey, bright, very bright.
• Texture  micro changement in distribution of tones:
Eroded rocks
Dark colour, irregularly distributed.
Slightly carved rocks
Uniform bright colour.
Clastic rocks
Uniform tones (sandstones, etc), varied tones
(conglomerates, etc..).
Bedding
Alternation of bright and dark stripes.
Faults and fractures
Dark tone (higher concentration of humidity:
oxidation, etc..).
Oriented structures
(schystosity)
Series of bright and dark stripes (can also
highlight joints).
Vegetation
Stains that highlight > presence of water.
Interpretation of photos
• Draining surface and underground
hydrography (surface evidences).
• Classified from the shape given by water
courses (draining
draining pattern
pattern). It is divided in
erosional pattern (made by erosional
agents) and depositional pattern
(excavated inside floods).
• Foundamental patterns:
Interpretation of photos
• Dendritic:
Dendritic not
subjected to
structural
control; it is
found in thin,
homogeneous,
not permeable
materials.
Interpretation of photos
• Angular:
Angular
subjected to
strong structural
control by a
system of faults,
fractures or
joints;
sometimes the
control is due to
bedding.
Interpretation of photos
• Alignments  Continuous and
discontinuous traces that allow to detect
determined orientations on photos.
• It is possible to list different kinds of
alignments:
Interpretation of photos
1. Alignments
made by
straight lines:
- Alternation of
different
tones:
bedding.
- Insulated with
dark tone:
faults and
fractures.
Interpretation of photos
2.
a.
b.
c.
Alignments represented
by tracts of water
courses that fall in the
same straight line:
One long water course:
fault or fracture.
Short tracts of different
water courses that lay in
the same line:
dislocation or fracture.
Like point b but with
two systems of straight
lines: two systems of
faults or fractures.
A
C
B
Interpretation of photos
3.
-
-
Alignments made by
junction of different
morphological
characters:
Slope with straight
course: erosion due
to stratigraphic
condition or fault
surface.
One or more slopes
aligned with a water
course: dislocation.
Interpretation of photos
• The position of a bed in relation with
the horizontal is called an Attitude.
The position of a tilted bed can be
determined by the Dip and the
Strike.
• In the aspect of photo interpretation,
beds can be classified into three
groups on the basis of dip amount:
- Horizontal beds
- Medium dipping beds
- Steep and vertical dipping beds
Interpretation of photos
• The
appearance
of a
landscape
with
horizontal
and low
dipping beds
is that of a
CanyonMesa type.
• Hard beds
form
perpendicular
cliffs, softer
beds form
slopes.
Interpretation of photos
• The appearance of a
landscape with medium
dipping beds is that of a
Dipslope or Cuesta.
• It is an asymmetric ridge
where one slope is gentle,
long and agree with the dip
direction of the bed, the
other slope is short and
steep.
Interpretation of photos
• The appearance of a
landscape with steep and
vertical beds with sharp,
straight slightly curved
ridges with the two
opposing slopes dipping at
the same angles is that of a
Hogback.
• Vertical beds are strongly
eroded and often covered
by talus on both sides of
hogback ridges.
Practice
Example 1 - fault
•
Recognizable two
unities, A and B. A
shows a medium
grey tone and a
dendritic drainage
pattern. B shows a
bright grey tone and a
draining pattern
parallel to dendritic
with medium density.
It is possibile to see a
trace with SW-NE
trend, that represents
the line of a vertical
fault.
Practice
Example 2 - faulted
monoclinal
•
In this monoclinal it is
possibile to recognize three
lithological unities (starting
from the edge R, S and T).
•
R is resistant to erosion
(creates visible plateaux,
delimited by vertical slopes),
the tone is medium grey, the
drainage is dendritic.
•
S is less resitant to erosion
(gentle slopes), the tone is
bright, the pattern is parallel.
•
T is more resistant than S
and creates steep slopes,
the tone is bright grey.
•
Note sub-vertical faults that
cut the monoclinal in the
NE-SW direction.
Practice
Example 3 - weakly inclined
layers
•
It is possibile to
distinguish four unities: A,
B,C, D, dip to SW.
•
Starting downwards we
find A, resistant to
erosion, with bright tone
and passed by a series of
joints.
•
B is resistant and shows
a dark tone.
•
C is easily erodible , with
the dendritic drainage
pattern.
•
D is similar to B.
A
C
B
D
Practice
Example 4 - basaltic plateau
on sedimentary rocks
•
It is possibile to see two
unities: A and B.
•
A shows a bright to
middle grey tone, with a
drainage of dendritic kind,
tending to radial
centrifugal. Alignments
shows two systems of
faults. Note the volcanic
cone in A.
•
B shows beds resistant to
erosion and dip to west,
with more erodible levels
inside, with a mediumdark grey tone. The
pattern is high density
dendritic.
A
B
Practice
Example 5 - landslide
(Lecco)
• We can see
litotypes quite
erodible, of bright
tone, with parallel
draining pattern.
• Note tectonic
alignments.
• In the low left part it
is possibile to see a
landslide.
Practice
Example 6 - bedding (Bergamo)
•
It is possibile to note litotypes
with evident bedding and with
very bright tones.
•
We can recognize a deep
gravitative deformation of the
slope (DGPV), highlighted by
three great counterslopes.
The draining pattern is
parallel.
Practice
Example 7 - landslide
(Sondrio)
• The outcrop litho type
shows a medium-dark
grey tone, with an
angular hydro graphic
net, that follows the trend
of an alignment of faults
SW-NE.
• Note the presence of a
landslide.
Practice
Example 8 - alignment of faults
(Sondrio)
•
It is possibile to see a litotype
passed by joints (NE), while in
NW direction we can see an
alignment of faults. The tone
is medium to bright grey to
bright grey. The draining
pattern is nearly absent, but it
is possibile to see a dendritic
net on the SW side.
•
We can also note the
presence of glacial tongues.