1. Royal University of Bhutan
Jigme Namgyel Engineering College
Department of Civil Engineering & Surveying
Tutor : Phurba Tamang
Designation: Associate lecturer
Department of Civil Engineering and Surveying
Contouring
MODULE: SURVEYING
Spring Semester 2020
2. MID-TERM REVIEW, 2017
UNIT 7: Contouring
Introduction to Theodolite
Contouring is basically a levelling operation. The equipment are the same for levelling
and contouring. The main objective of contouring is to determine the points on the ground
having the same reduced level ( RL ). The contour lines join the points of same elevation
directly or by interpolation technique. It gives the topographical features of the ground.
Contours are used by engineers in variety of ways. Some of the engineering uses of
counters are as following.
• With the help of a contour map, proper and precise location of engineering works such as
roads, canals, etc. can be decided.
• In location of water supply and water distribution.
• Planning and designing of dams, reservoirs, aqueducts, transmission lines, etc.
• To select sites for new industrial plants.
• To ascertain the profile of the country along any direction.
• To estimate the quantity of cutting, filling and the capacity of reservoir.
3. MID-TERM REVIEW, 2017
Definitions
1. Contour: A contour may be define as an imaginary line passing through points of equal
elevation. A contour line may also be define as the intersection of a level surface with
the surface of the earth. For example, a contour of 100 m indicates that all the points
on the line have an RL of 100 m. Similarly, in a contour of 99 m, all points have an RL of
99 m and so on.
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Definitions
2. Contour Interval: The vertical distance between any two consecutive contours is known
as a counter interval. The interval depends upon nature of the ground, the sale of the map
and the purpose of the survey. Usually contour interval is kept large up to 2 m for projects
such as highways and railways, whereas it is kept as small as 0.5 m for measurement of
earth works, building sites, dams, etc. For a city surveys, a contour interval of 0.5 m may
be adopted and for more extended surveys such as geological survey, usually 6 to 15 m
are adopted.
Note: It should be remembered that the contour interval for a particular map is constant.
3. Horizontal Equivalent: The horizontal distance between any two consecutive contours is
known as horizontal equivalent. It is not constant. It varies according to the steepness of
the ground. For steep slopes the contour lines run close together and for flatter slopes
they are widely spaced.
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Uses of Contour Map
The following are the specific uses of the contour map:
1. The nature of the ground surface of a country can be understood by studying a contour
map. Hence, the possible route of communication between different places can be
demarcated.
2. A suitable site for economical alignment can be selected for any engineering project.
3. The capacity of a reservoir or the area of a catchment can be approximately computed.
4. The intervisibility or otherwise of different points can be established.
5. A suitable route for a given gradient can be marked on the map.
6. A section of the ground surface can be drawn in any direction from the contour map.
7. Quantities of earth can be approximately computed.
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Characteristics of contours
The contour lines in the following figure are closer near the top of a hill or high ground and
wide apart near the foot. This indicates a very steep slope towards the peak and a flatter
slope towards the foot. The contour interval is 5 m
Hill
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Characteristics of contours
In the figure, the contour lines are close near the bank of a pond or depression and wide
apart towards the center. This indicates a steep slope near the bank and a flatter slope at
the center.
Depression
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Characteristics of contours
In the figure, the contour lines cannot cross one another, except in the case of an
overhanging cliff. But the overlapping portion must be shown by a dotted line
Overhanging Cliff
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Characteristics of contours
When the higher values are inside the loop, it indicates a ridge line. Contour lines cross
ridge lines at right angles.
Ridge Line
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Characteristics of contours
When the lower values are inside the loop, it indicates a valley line. Contour lines cross
the valley line at right angles.
Valley line
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Characteristics of contours
A series of closed contours always indicates a depression or summit. The lower values
being inside the loop indicates a depression and the higher values being inside the loop
indicates a summit.
Depression Summit
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Characteristics of Contour Lines
The following are the characteristics of Contour Lines
1. All the points on a contour line have the same elevation. The elevations of the contours
are indicated either by inserting the figure in a break in the respective contour or
printed close to the contour. When no value is present, it indicates a flat terrain.
2. Two contour lines do not intersect each other except in the cases of an overhanging
cliff.
3. A contour line must close onto itself, not necessarily within the limits of a map.
4. Equally spaced contour represent a uniform slope and contours that are well apart
indicate a gentle slope.
5. A set of close contours with higher figures inside and lower figures outside indicate a
hillock, whereas in the case of depression, lakes, etc., the higher figures are outside
and lower figures are inside.
6. Irregular contours represent uneven ground.
