Contours- Purpose,Characteristics and Method of Plotting

Contours

It can be found in a topographical map, an imaginary line passing through points of equal elevations. It is one method to represent the features on earth such as mountains, depressions, undulations, etc.

Uses of Contours

1.       Contour map will help in designing dams, reservoirs, roads, canals,  etc.

2.       To select site for new projects.

3.       To check the inter visibility of stations.

4.       To determine the quantity of cutting, filling and can determine the capacity of reservoirs.

Contour Interval

                The vertical distance between consecutive contours is termed as contour interval. Generally the contour intervals are taken in the range of 1 to 15 m. The contour interval is inversely proportional to the scale of the map. When we have less time to complete a survey for a large area contour interval is kept larger.

Methods of Contouring

1.       Direct Method: In this method a series of points are located on the ground having same elevation. For a particular contour value the staff man is directed to move right or left until the required reading is obtained, this method is time consuming but it gives accurate result.

2.       Indirect Method: In Block Contouring the given area is divided into number of grids with a known interval and the staff reading is taken on the respective grid points to find the R.L values, by the method of interpolation the contour is plotted. In Radial contouring the same method is adopted but the R.L values are found on the radial lines running from the center point. This method is normally preferred on hilly areas.

   Characteristics of contour lines:

·      Steep slopes - contours are closely spaced

·     Gentle slopes - contours are less closely spaced

·   Valleys - contours form a V-shape pointing up the hill - these V's are always an indication of a     drainage path which could also be a stream or river

·     Ridges - contours form a V-shape pointing down the hill

·     Summits - contours forming circles

·     Depressions - are indicated by circular contour with lines radiating to the center

·     If the middle value is higher in a contour it means it is an elevation

·     If the middle value is lesser in an contour it means it is an depression