Slope Analysis - e
Earth shape and
analysis & Topographic
Grading Slope form
Topographic Contour map
Topographic contour map are composed of a series of line
that designate the elevation of the land above sea level. Each
line called contour line represents specific level (, elevation).
Contour Interval is the difference in elevation values between
two adjacent contours, so it indicates the elevation change from
one contour line to the next one. Contour Interval is the same
value for every elevation change in the same topography map.
c o n t o u r
Topographic area represented
by ABE, can be computed
using slope data read from
topographic contour maps.
Maps disregard slope in representing land area, therefore in
rough terrain, topographic area may be much greater
source of illustrate: Environmental Analysis by William M.Marsh page 31
Slope form and topographic map
Site Planning and Landuse Planning requires accurate slope
information which is available in form of topographic map and
aerial photography. The topographic contour map represents an
elevation data to show landform in two-dimentional drawing
Drawing illustrating the
technique of slope profiling
(Source of drawing: Marsh, William
M. Environmental Analysis for
Land use and Site Planning p. ..)
Graphical slope information can be generated by simple techniques from the contour map. Constructing a slope profile from
a topographic contour map is to plot the elevation of contour lines
on a two-dimentional graph.
• Contour interval: the height between each contour line.
• Cross section show specific section of that shape
• X axis represents distance, Y axis represents level change
Contours are set of lines of equal elevation above the same
reference plane, the number indicated on each line referenced
tothe DATUM PLANE, which is the level at “Mean Sea
Level”. A CONTOUR INTERVAL is the verticle distance
(Height) between contours.
Models are good mediums to give the feeling of the earth
form and study the natural drainage pattern
Basic elements that should show on
Landform of the ground
Group of existing trees
Surface water body
The model of Doi Intanon shows
landform of Royal project at Doi
Intanon, Chiengmai and the
surrounding area. which will help
site planner to understand landform,
drainage pattern, visual quality and
the overall elevation relationship of
the site and its surrounding
source: 3rd year Architectural
student Soa&D 2004
Slope profile shows earth form
To know the characteristics of contours is important for their
interpretation and essential to understand the nature of that landform.
A Uniform slope is sndicateed by evenly spaced contour plan.
A Ridge and Valley are shown similarity, but note that RIDGE
with the higher contour inside (contour line pointing down to lower
level), VALLEY with the lower contour inside (contour line pointing up to higher level)
If two sides of VALLEY contour lines are quite closed, it
CONVEX and CONCAVE landforms are the most common
landform found in nature and express the felling differently.
High point on SUMMIT or low point in a DEPRESSION
are indicated by spot elevation.
Existing contour lines are shown in dashed line, proposed
new Grade is shown in a set of solid lines.
Source of drawing: Untermann, Richard K.: Grade Easy
source of drawing: Untermann, Richard K: GRADE EASY
Topography map: contour
Read the contour line
1. Same elevation on one contour line and same level
change in between each contour line. (called contour interval)
2. Every contour closes on itself.
3. A summit or depression indicated by the elevation at
highest, lowest points.
4. Contour lines never cross.
5. Uniform sloping surface
6. Convex slope
7. Concave slope
9. Steep slope
10. Relatively level.
Natural drainage pattern : Water always flows in 90 degrees
direction of contour line. Always check the topography of overall
large scale drainage pattern besides the on-site topography.
Natural drainage pattern can be easily read from the topographic model to see how criticle the drainage factor is in each
particular area. In order to do a good site planning, we should try
to avoid to disturb or change the major natural drainage pattern
of the site.
Runoff water flows downhill
perpendicular to contour lines
Slope failures generally fall into 3 catagories
1. Grade: slopes that are too steep or too gentle for paricular
landuses and inclined by undercutting by construction processes
2. Erosion: slopes with steep inclinations, less vegetative
cover, loose soil materials and eroded caused by waves, streams,
3. Failure: slopes that are composed of weak, soft material
which have low bearing capacity and have tendency to mass
movement problem such as mudflow, slides, sinking which worsen
by increased or decreased ground water.
Mapping slope from Topography map
Slope compares the vertical distance to the horizontal distance. This comparison can be expresses as an angle, a percentage or as a ratio. the slope ratio is expressed by putting horizontal
distance firse and vertical distance second such as a 4:1 slope
means 1 metres of vertical rise in 4 metres of horizontal distance
Gradient percent = D/L*100
G= the gradient in percent
D= the distance in elevation between two points
L= the horizontal length between two points
Percent of slope = elevation(in depth, height)*100
percent slope G=D/L*100
Horizontal distance L=D/G *100
Verticle distance D=GL/100
Example of slope analysis
The topographic map of “Royal
Project at Doi Intanon, Chiengmai”
done on GIS shows contour lines,
Same project, the slope analysis was done to show
percent slope which will be factor to determine
constraints and opportunities of land development in
Slope analysis is the most basic crucial information in site
analysis to determine the appropriate location for site development
Slope analysis of 20.8 acre site.
Diagram shows the use of
graduated scale for mapping slopes
from a contour map.
Diagram show the use of graduated scale for mapping slope
analysis(percentage of slope) from a contour map.
Recommended suitable slope for each use concerning
possible cut-fill, mobility and drainage ability
Slope Ratio for suitable uses
Slope ratios are determined by constraints such as design
grade, soil stability, amount of planting, type of determined function, program and circulation.
Design height is determined by aesthetics and functional
requirement for each particular uses. Stability is related to angle
of repose of each natural material and soil. Slope stability is
also affected by water and wind.
Slope Criteria is suitable & recommended slope for each
use concerning mobility, possible cut-fill, maintenance and drainage ability. In addition, for some specific uses such as sport field,
tennis lawn, basketball field, we must try to follow the slope criteria to get the acceptable standard.
Angles of repose for various types of slope materials.
(Angles are given in degrees.)
• Maximum angle for earth material that it can be inclined, and it will fail if beyond.
A standard from: Marsh, William M. : Environmental Analysis for Land Use
and Site Planning P.56, Mcgraw-Hill, NY)
Designing a level plane on steeply
Designing a level plane on steeply sloping site.
Source: LANDSCAPE ARCHITECTURE, John Ormsbee Simands.
Avoid grading on the following conditions
(From Grade Easy by Richard Untermann)
1. Grading that results in radical loss of vegetation and
2. Grading that interrupts the natural drainage
3. Grading that results in aesthetic degradation
4. Grading on difficult slopes (excess of 25%)
5. Grading in environmental unique condition; floodplain,
6. Grading in areas effected by natural disaster; mudslides
or along earthquake fault lines
When not to
Grading : is the modification of existing land form. It is one
of the most important items in site planning, landscape design
and construction. Grading serves three main purposes:
1. To create a level to put something on; for a house, car,
sport play field, a hotel, etc.
2. To create circulation ways; road way, loading ramp, handicapped ramp, bicycle track, etc.
3. To create special effect and solve special problem ; make
a mound to hide parking lot or reduce sound, prevent erosion and
landslide, aesthetic or psychological purposes.
However, to make a land usable, all three purposes must
concern drainage factor and grading is essential to direct water
away from building and outdoor surfaces and conducted to drainage channel.
(see chapter 7 Drainage)
Manipulating contour line
To do the grading plan is to manipulate contour for that purpose. The grading plan should show both existing and revised
contours so we will see how much the grade has been changed.
souce of drawing: Brook, Gene R.,
Site Planning, Environmental
process and Development, p. 149153
Grading: making an area level to put
By cutting only
– the soil is stable
– require less cost for
– be able to do very
– cost to transport earth
from the site
By filling only
– easier to make a very
– good for low land,
flood problem area.
– Unstable earth, needs a
– Cost of transporting
earth into the site
– Soil cost
By the combination of
cut and fill
– balance earth in the
site, no transportation cost.
– Less earth cost
– Machine cost
– Must has good construction schedule and management
By the combination of
cut and fill: plan of slab on
Grading: making a proper slope for
Circulation routes should be level as possible.Two ways to
travel up slope terrain
(maximum slope for local street is 8%)
The normal grading method is
something between the two extremes
with roads or paths climbing and
Circulation routes crossing level country are relatively
easy to grade— providing
uniform surface and assuring
the roadway drains properly.
1. A uniform surface
means smoothing out the
bumps by spacing contours
2. Draining the level road
sometimes requires creating
artificial high and low points
for water flow direction.
These can be shown graphically with contours, spot
elevations, or sections.
Section through road with CROWN
Road sloped from side to side
The picture shows shortest
distance but results extensive
cutting and would have erosion
problem at the road bank.
The natural slope is
steep, the road crosses series
of contour will causes a very
steep bank since it needs
UNIFORM slope. We should
try to compromise the cut and
fill along the centerline.
To Design the road
centerline aligned with or
parallel to the contour lines
with minimal grading and
smoothly change in slope
from one contour line to
When grading by CUTTING , begin with the lowest
contour and work up, by
FILLING, begin with the top
contour and work down.
Contours are spaced
according to the gradient of
the proposed route typically
expressed in percent. For 10%
slope, one meter interval
contours would be spaced 10
The most common way if existing topography is NOT to
steep and the road is not too long, the method is to select the
contour in the middle of the road and cut half the contour above,
fill half of the contour below.
If cut/fill creates a steep bank, a heavy rainfall can cause
erosion. Proper retaining wall with good drain must be used on
the high side of the slope along with the consideration to separate road way into two levels.
Road crosses drainage swale, the problem concerning the
intersection between road and topographical depression, the use
of BOX CULVERT or BRIDGE would be chosen depend on the
length and depth of the swale and the image of the design.
Source of drawing: Brook, Gene
R., Site Planning, Environmental
process and Development p.149153
Source of drawing: Marsh, William M. Environmental Analysis for Land
use and Site Planning
Source of drawing: Untermann, Richard K.: Grade Easy
Source of drawing: Brook, Gene R., Site Planning, Environmental process
and Development p.149-153
1. Prof. Untermann, Richard K. ,Grade Easy,Department
of Landscape Architecture, University of Washington, Seattle
2. Brooks,Gene R.,Site Planning : Environmental process and development P.129-171, , Prentice Hall N.J. 07632
3. Boomkum, Decha, Site Planning p.83-106, Lecture
sheet , Chulalongkorn University, Thailand
4. Marsh, William M., Environment Analysis for Land
use and Site Planning, Mcgraw-Hill Company, NY.