An introduction to stereographic projections


Complete the following exercises in Marshak and Mitra by hand, using the equal angle stereonet provided to you in the back of M&M.

Marshak and Mitra problems

Page #

5-1: plotting planes


5-2: plotting lines

5-3: plane from lines, rake, angle between lineations


5-6: apparent dip from true dip

5-8: true dip from apparent dip



Now, solve these problems using one of the computer programs, found at

Marshak and Mitra problems

Page #

6-1: plotting planes and their poles

6-2: fold geometry


And the following problem…

In areas where structural measurements have been made it is often convenient to enter the data on equal-area (see pp. 157-62, 164-66, etc…in M&M) nets. The method of plotting is identical to that used with the equal-angle net, but the rectangular areas in the net occupy the same percentage of space so that points in the net can be counted and contoured for statistical purposes.

If the beds or foliations in a region have undergone cylindrical folding (pp. 157-59, 269 in M&M) and we plot their attitudes on the net, the planes tend to intersect at a point (or several points), called the b-axis; it coincides with the trend and plunge of the fold axis in the region.

This diagram, called a b-diagram (pp. 157-58, M&M), becomes very crowded when there are a large number of planar data. Hence it is generally more convenient to plot the poles to the planes, rather than the planes themselves. This is called a p-diagram (pp. 158-62 in M&M). If the fold is cylindrical, the points plotted will be distributed along a great circle on the stereonet. This arc, called a girdle (pg. 156 in M&M), has a pole normal to it, which is the fold axis.

Use the following array of data to construct b and p-diagrams (on different traces) and answer the following questions:

  1. How many fold axes are present?
  2. What is the trend and plunge of each fold axis?
  3. What is (are) the angle(s) between the fold axes? Remember, the angle between two planes is measured in the plane that includes both axes.

N45W, 80NE

N27E, 41SE

N50E, 44SE

310, 40SW

N3W, 55E

S75E, 60SW

277, 60SW

0, 50W

292, 70SW

N68E, 60SE

N60E, 70SE

N20E, 70NW

N25W, 60NE

335, 48SW

N82W, 44SW

S40W, 90

18, 42SE

N7E, 60NE

N65E, 44SE

S82W, 50SE