Gay (1971) described the application of three-dimensional (3-D) viewing to geophysical data, principally contoured magnetic and gravity data. Subsequently, the method has been extensively applied to actual physical data, such as structural contour maps, isopach maps, and topography, both subaerial and submarine (e.g., Gay, 1981). These types of data have the same dimensions in the vertical and horizontal directions, i.e., physical distance. The question has therefore arisen many times over the years as to the mathematical relationship between the vertical and horizontal scales in the stereo model--what is the actual vertical exaggeration of a stereo view and how does one go about measuring it, or controlling it in case one is constructing a stereo pair? This question was not addressed in the 1971 paper, because there is no "true" vertical scale for gravity or magnetic maps, and hence no "exaggeration" (different dimensions in vertical and horizontal directions).