Abstract
The grid method for estimating reservoir capacity was modified by introducing 3 additional numerical techniques for estimating unknown water depths by interpolation. Estimations of the known volume of a physical model of a dam reservoir showed than the modified grid method was more accurate than published classical methods or the original grid method, with maximal errors of approximately 10 per cent using Newton and Lagrangian polynomials and 24 per cent using the cubic spline technique. Use of the Lagrangian method is recommended as it was easier to use than the Newton method. The cubic spline method should not be used when spacing between measured ranges was irregular.