How Spherical Projections are Used


Unlike mapping, we are not interested in projecting patterns on the sphere itself. Rather, we are interested in using the sphere to analyze angular relations between lines and planes.

Consider a two-dimensional analogue: a protractor. To measure the angle between two lines, we move the center of the circle to the intersection of the lines, note where the lines cross the circumference of the circle, and read off the angles. Note that:

Think of a spherical projection as a three-dimensional protractor.

Stereonet constructions are typically accurate to within 2 degrees because of tearing of the overlay at the pivot hole, parallax and plotting errors because the overlay is separated from the net, distortions in printing the net, and hand-eye coordination errors in plotting points and circles. With extra care most of these factors can be minimized to bring the accuracy to within a degree or so. The stereonet works because geologic structures can rarely be measured more precisely than within a couple of degrees or so. Like the slide rule, another effective graphic device, the stereonet is just accurate enough to serve its purpose.

If you attempt to mix stereonet results with those derived by more accurate methods, like calculation, the end result will be only as good as the stereonet accuracy.

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Created 28 December 1998, Last Update 16 March 1999
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