The graph portion of Orient is used to plot data on a triangular Point-Girdle-Random (PGR) eigenvalue plot. It is particularly useful in map domain analysis, where domains are defined in the Map portion of the program, or input with the data file.
Given the orientation matrix eigenvectors e1, e2, and e3 for n data points, where e1 >= e2 >= e3, the following are defined (Vollmer 1989):
| Point | P = (e1 - e2)/n |
| Girdle | G = 2(e2 - e3)/n |
| Random | R = 3e3/n |
| Cylindricity | C = P + G |
these have the property that:
P + G + R = 1
and form the basis of a triangular plot. Cylindrical data sets plot near the top of the graph, along the P-G join, point or cluster distributions plot near the upper left (P), girdle distributions plot near the upper right (G), and random or uniformly distributed data will plot near the bottom of the graph (R). Below is a plot of bedding plane poles from a cylindrical fold, and the corresponding PGR plot. These indicate a well defined girdle with a distinct maximum.
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For comparison, the plot of ice fabric axes below shows a much more scattered distribution, and plots nearer to the bottom of the PGR graph.
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