Contouring

Contouring

The relative position of points in a plane 

are represented by a map. The value

of the map is even more if the relief 

(variation in the elevation of earth’s 

surface) is also included along with their 

relative positions. 

There are two methods by which the 

conformation of the ground may be 

presented on a map. 

(a) By delineating the surface slopes by 

shading, intended to given a impression 

of relative relief. The relative elevations 

of the points are not indicated in this 

case.

(b) By plotting the contour lines (imaginary 

line passing through points of equal 

elevations) on maps. These lines are 

arrange such that the form of the 

earth’s surface can be portrayed with 

greater accuracy and thoroughness, and 

can be readily be interpreted. 

Contours are used by engineers in a 

many ways.

 USE OF CONTOURS

(a) Proper and precise location of 

engineering works such as roads, 

canals, etc.

(b)In location of water supply, water 

distribution and to solve the problems 

of steam pollution.

(c)In planning and designing of dams, 

reservoirs, aqueducts, transmission 

lines, etc. 

(d)In selection of sites for new industrial 

plants. 

(e) Determining the indivisibility of 

stations.

(f) Determining the profile of the country 

along any direction.

. 

(g) To estimate the quantity of cutting, 

filling, and the capacity of reservoirs.

 DEFINITION OF CONTOUR

A contour may be defined as an imaginary 

line passing through points of equal 

elevation on the earth surface.

A contour line may also be defined as the 

intersection of a level surface with the 

surface of the earth.

Contour lines on a plan illustrate the 

topography of the ground. 

When the contours are drawn underwater, 

they are termed as submarine contours, 

fathoms or bathymetric curves. 

Note: Generally the contours are not 

visible on the grounds excepts in the case 

of shorelines.

CONTOUR INTERVAL

• The vertical distance between

consecutive contours is termed as

contour interval.

• It is desirable to have a constant

contour interval throughout the map.

• In special cases, a variable contour

interval may also be provided.

• A variable contour interval is, as far as

possible avoided since it gives a false

impression of the relative steepness of

the ground in different parts of the map.

• Generally contour intervals are taken 1 

to 15 m. 

• The smaller the contour interval, the 

more precisely the terrain relief is 

predicted on the plan.

• The contour interval depends upon the 

following factors. 

(i) Scale of the map 

(ii) Purpose of the map 

(iii) Nature of the Country

(iv) Time 

(v) Funds

1. Scale of the Map: 

If scale is small, the 

contour interval is kept large so that 

there is no overcrowding of the 

contours. On the other hand, if the scale 

of the map is large, the contour interval 

can be kept small.

2. Purpose of Map: 

The contour interval 

selected should be small so that the 

map serves the intended purpose, but at 

the same time it should not be too small 

otherwise the cost of the work would be 

too much. The contour interval should 

be kept small when the plan is required 

for the detailed design. 

3. Nature of Ground:

 For a flat ground, 

the contour interval is small, but for a 

steep slope, the contour interval is 

large. If the ground is broken, the 

contour interval is kept large so that the 

contours do not come too close to each 

other. 

4. Time:

 Contour interval is kept large 

when time is less. 

5. Funds:

 Contour interval is kept large 

when funds as less. 

 CHARACTERISTICS OF CONTOUR LINES

1. All the points of a contour line have the 

same elevation. The elevations of the 

contours are shown either by inserting 

the figure in a break in the respective 

contour or printed close to the contour. 

When no value is represented, it 

indicates a flat terrain. A zero meter 

contour line represents the coat line. 

2. Two contour lines do not intersect each 

other except in the cases of an 

overhanging cliff or a cave penetrating a 

hillside. 

3. A contour line is a closed curve. They 

may close either on the map or outside 

the map, it depends on the topography.

4. Equally spaced contour represent a 

uniform slope and contours that are 

well apart represents a gentle slope.

5. A set of close contours with higher 

figures inside and lower figures outside 

indicate a hillock, whereas in case of 

depressions & lakes, etc. the lower 

figures are inside and the higher figures 

are outside.

6. A watershed or bridge line (line joining 

the highest points of a series of hills) 

and the thalweg or valley line (line join 

the lowest points of a valley) cross the 

contours at right angles.

7. Irregular contours represent an uneven 

ground surface.

8. The direction of the steepest slope is 

along the shortest distance between the 

contours. 

At a point the direction of the steepest 

slope on a contour is, at right angles to 

the contour

9.Two contour lines having same 

elevations cannot unite and continue as 

one line. Similarly a single contour, 

cannot put into two lines.

 VALLEY LINE AND RIDGE LINE 

• The slopes of ravine intersect along a 

line referred as the axis of the ravine, 

the line of discharge, or a valley line in 

case of a valley. 

• Contour part of a ravine is a ridge-a 

convex form of terrain gradually 

declining in one direction.

• Two ravines are generally separated by 

a more or less pronounced ridge. 

• The line along which the slopes interest 

is referred as the axis of ridge, the 

watershed or watershed line. The 

watershed line is generally wavy. 

 SADDLE 

• The lowest points on the watershed are 

known as passes. 

• Pass is narrow this low land passing 

through high mountains on either sides. 

• Sometimes this narrow low land is cut 

back by the streams. This steep-sided 

depression is called as a COL. When this 

depression is broad and low, it is 

known as saddle. 

• A part of the land in form of tongue, 

which cuts out from a hilly are/is called 

as spur. 

• The contours are similar to that of a 

valley, with a difference that here the 

counter values decrease towards the 

vee. 

• A high land, having flat narrow top with 

steep (scrap) slope on one side and 

gentle (dip) slope on the other side is 

called as escarpment. 

 CLIFF

Cliff are the steep rock faces along the sea 

coast and may be vertical where the 

contour lines coincide with each other, an 

overhanging cliff where the contour lines 

intersect each other.