(fyafat&t 25 



STEREOGRAPHIC 

 PROBLEMS 



Hugh McClellan 



Stereographic projection is an ancient and useful method for graphically 

 solving geometrical problems in three dimensions. It has been widely used by 

 crystallographers, but has been generally neglected by geologists; however, 

 there is increasing evidence that geologists more and more are coming to 

 appreciate the speed and convenience of the stereographic method, which can 

 be applied to many problems of structural geology. 



FIRST PRINCIPLES Figure 25-1 shows the bottom half of a 



sphere, through the center of which a plane 

 Wulff Stereographic Net has been passed. The strike is northwesterly, 



and the dip is about 40 degrees southwesterly. 

 The plane traces a great circle on the sphere, and points on the great circle have 

 been projected to the zenith of the sphere. Dots indicate where these lines pene- 

 trate the equatorial plane along the arc of a circle. In like manner a family of 

 planes, all with the same strike, but varying in dip, may be projected through 

 the equatorial plane, producing the meridional arcs of the stereographic net 

 (fig. 25-2). 



Vertical planes that may be passed through the hemisphere perpendicular 

 to the north-south axis trace small circles on the spherical surface. These small 



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