Contouring [Contour Line, Contour Interval and Horizontal Equivalent]

Contouring is, generally a leveling operation. The principal objective of contouring is to determine the points of the same reduced level (RL) on the ground. The types of equipment needed are the same for both leveling and contouring. The contour lines join the point of the same elevation directly or by the interpolation method. It shows the topographical features of the ground by comparing different contour lines of different elevations for a given closed area. Based on that topographical characteristic of the ground calculations for engineering projects are carried out.
Fig. Contour Lines

The main elements of Contouring are :

  1. Contour Line
  2. Contour Interval
  3. Horizontal Equivalent

Contour Line

Contour Line is defined as the line of intersection of the level surface with the ground surface. It as an imaginary line passing through points of equal or constant reduced levels (RL) on the ground surface.
For example, the contour of 98 m indicates that all the points on this line have a reduced level of 98 m. Similarly, for the contour of 99 m, all the points will have a reduced level of 99 m, and so on.


Contour Interval

Contour Interval is the vertical distance between any two consecutive contours as shown in the figure above. Suppose a map having contour lines of 100 m, 97 m, 94 m, and so on. The contour interval for the contour lines will be 3 m here. The interval in the contour line generally depends upon (i) The nature of the ground surface, (ii) The scale of the map prepared, and (iii) the purpose of the survey.

Horizontal Equivalent

The horizontal equivalent is the horizontal distance between any two continuous contour lines. It is not constant and varies according to the steepness of the ground. For steep slopes, the horizontal equivalent is narrow while for flatter slopes it is wide.

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