Computation of Volume from Contours

Given figure shows a dam with full water level of 100 m and contours on upstream side. The capacity of reservoir to be found is nothing but volume of fill with water level at 100 m. The whole area lying within a contour line is found by planimeter. It can be noted that area to be measured is not between 2 consecutive contour lines.

Assume A0, A1, A2, ..., An be area of contours and h be contour interval. In that case from trapezoidal rule:

and by prismoidal rule:

Where there are n segments and n =even number.