In graphical method, Interpolation is done with help of a tracing paper. Sometimes tracing cloth is also used.

The same contour appears on eir sides of a ridge or valley, for highest horizontal plane that interests ridge must cut it on both sides. The same is true in case of a lower horizontal plane that cuts a valley.

The vertical distance between any two consecutive contours is called a contour interval. The contour interval is kept constant for a contour plan.

Contour maps are useful to determine Determination of intervisibility between two points, tracing of contour gradient and location of route, calculation of resorvoir capacity. It is also useful to draw section.

A closed contour line with one or more higher ones inside to represent a hill. Similarly, closed contour line with one or more lower ones inside it indicates a depression without an outlet.

The contour plan may be used to calculate capacity of a reservoir. Contour maps are also useful to determine Determination of intervisibility between two points, tracing of contour gradient and location of route, calculation of reservoir capacity.

If scale is small, contour interval should be large. If scale is large, contour interval should be small.

Estimation method is extremely rough and is used for small scale work only. The positions of contour points between guide points are located by estimation.

Contour gradient is a line lying throughout on surface of ground and preserving a constant inclination to horizontal. If inclination of such a line is given, its direction from a point may be easily located eir on map or on ground.

Interpolation of contours is process of spacing contours proportionately between plotted ground points established by indirect methods.