The graphical methods which have been suggested to date for the computation of topographic corrections and the effects of two- or three-dimensional geologic features involve mostly certain approximations. This makes it in many instances impossible to apply them to very steep topography or to abruptly changing subsurface structures. The approximations and the intricacies of the computations are due to the fact that either orthogonal or cylindrical coordinates are used. The author suggests a new method which is based on the use of spherical coordinates, the final formulae of which are very simple and which does not involve approximations.

For sections made through any sort of terrain or subsurface structure in 16 azimuths, six diagrams are required. The construction of these diagrams is described. They are applicable (1) to the computation of topographic corrections and of corrections for mine openings in underground torsion-balance work; (2) to computations of the influence of two- and three- dimensional subterranean features in torsion-balance interpretation; (3) to computations of the influence of such structures on the magnetometer.