A precise definition of residual gravity is given and in a form most suitable for numerical application. Some pains are taken to show that the residual-gravity picture one obtains depends on the method used in obtaining it and that there are many ways in which this may be done. Several examples from actual work on developed oil fields are given in the form of figures and maps to bear out the above statement. Conclusions are drawn as to the relative order of sensitivity of several factors entering into the computations in determining a given residual picture. Factors considered in this paper are (1) size of unit figure, (2) kind (or shape) of unit figure, (3) density of reference net; that is, number of reference points per unit area, and (4) sensitivity of reference points to lateral displacement on gravity map.