Contour

A map represents the relative positions of points in a plan. The value of the map is enhanced if the variation in the elevation of the earth’s surface is also included along with their relative position in a plan. There are two methods by which the conformation of the ground may be presented on a map. One way is by delineating the surface slopes by hachure shading, etc., intended to give an impression of relative relief. The relative elevations of the points are not indicated in such a case. To a layman this method of portraying relief is very simple and legible and is very commonly used for geographical mapping. The other way, which is usually employed in plans for engineering purposes, is plotting the contour lines on maps. These lines are so arranged that the form of the earth’s surface can be portrayed with greater accuracy and thoroughness and can readily be interpreted (Figure 27.14).

A contour may be defined as an imaginary line passing through the points of equal elevation. All the points on any one contour line have the same elevation above the datum surface and the contour may thus be defined as the line of same level. When the contours are drawn underwater, they are termed as submarine contours, fathoms or bathymetric curves. This is the best method of representation of features such as hills, depressions and undulations on a two-dimensional paper.

Contours are used in a variety of ways. Some of the engineering uses of contours are as follows:

1. With the help of a contour map, proper and precise location of engineering works such as roads and canals can be decided.
2. In the location of water supply, water distribution and to solve the problems of stream pollution, etc.
3. Planning and designing of dams, reservoirs, transmission lines, etc.
4. To select suitable sites for new industrial plants.
5. To ascertain the inter-visibility of stations.
6. To ascertain the profile of the land along any direction.
7. To estimate the quantity of cutting, filling and capacity of reservoirs.

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.

Figure 27.14 Contours