Comment on F. W. Sohont's paper “Transforming the stereographic map”

Abstract

The method described in this paper of transforming the zenith of a stereographic map is of importance in its bearing on the construction of such maps for the determination of the location of earthquake‐epicenters from seismograph‐records. (Although no mention is made in the paper, it is here assumed that its presentation at the joint meeting of two seismological groups envisaged this application of the method.) The stereographic projection is one of two among the group of azimuthal projections (the equidistant, or Postal's, being the other) that are the most appropriate for this purpose, as has been pointed out by a number of writers. As the geographic position of a given seismographic station is made to be the point of tangency of the plane of projection with the sphere, a different grid has to be constructed or computed for each station. In the case of the stereographic projection the grid may be constructed rather simply since the meridians and parallels are arcs of circles. Whether the method proposed in the present paper of deriving one grid from another by changing the zenith offers advantages in time‐saving over the conventional method of independent construction is a question it would be of interest to settle by drawing two grids for the same station by the two different methods.