This module makes stereo plots of axial, spherical data (e.g. strike-dip measurements in structural geology), and performs the Bingham test for uniform distribution.
Three data formats can be used, all using the geographic angle convention (degrees, clockwise from north):
- Trend (azimuth) and plunge (angle down from the horizontal) for axial data
- Dip azimuth and dip angle (down from the horizontal) for planes. The pole (normal vector) of the plane is plotted.
- Strike and dip for planes, using the right-hand rule convention with the dip down to the right from the strike. The pole to the plane is plotted.
Density contouring is based on a modified Kamb method algorithm by Vollmer (1995). Both equal area (Schmidt) and equal angle (Wulff) projections are available. Projections are to the lower hemisphere. Density estimates can use an inverse area, inverse area squared or exponential law, giving progressively higher smoothing.
The Bingham test for uniform distribution of axial data can be used to test for preferred direction (Bingham 1974; Mardia & Jupp 2000, p. 232-233). An adjusted S statistic is computed according to Jupp (2001). See the Past manual for details. The reported p value for uniformity is estimated using the chi-squared distribution. A small p (e.g. p<0.05) means significantly non-uniform distribution.
References
Bingham, C. 1974. An antipodally symmetric distribution on the sphere. Annals of Statistics 2:1201-1225.
Jupp, P.E. 2001. Modifications of the Rayleigh and Bingham tests for uniformity of directions. Journal of Multivariate Analysis 77:1-20.
Mardia, K.V. & Jupp, P.E. 2000. Directional Statistics. John Wiley & Sons.
Vollmer, F.W. 1995. C program for automatic contouring of spherical orientation data using a modified Kamb method. Computers & Geosciences 21:31-49.