Four tecbniques are treated. The technique using the area and the perimeter is very easy but considerable error can be made and commonly the technique using grid is used. Particularly, simaar technique using computer makes possible to represent the slope at each point of the grid and to compare with other data. The tehcnnique using vector product, with which a cuuved surface can be measured directly by segmenting by triangles, is useful but it represent considerable error where the relief is rugged.