Added example of finding an axial plane

examples/axial_plane.py

@@ -0,0 +1,55 @@
+"""
+Illustrates fitting an axial plane to two clusters of dip measurements.
+
+In this case, we're faking it by using Anglier's fault orientation data,
+but pretend these were bedding dips in two limbs of a fold instead of fault
+orientations.
+
+The steps mimic what you'd do graphically:
+
+    1. Find the centers of the two modes of the bedding measurements
+    2. Fit a girdle to them to find the plunge axis of the fold
+    3. Find the midpoint along that girdle between the two centers
+    4. The axial plane will be the girdle that fits the midpoint and plunge
+       axis of the fold.
+"""
+import matplotlib.pyplot as plt
+import mplstereonet
+
+import parse_angelier_data
+
+# Load data from Angelier, 1979
+strike, dip, rake = parse_angelier_data.load()
+
+# Plot the raw data and contour it:
+fig, ax = mplstereonet.subplots()
+ax.density_contour(strike, dip, rake, measurement='rakes', cmap='gist_earth',
+                    sigma=1.5)
+ax.rake(strike, dip, rake, marker='.', color='black')
+
+# Find the two modes
+centers = mplstereonet.kmeans(strike, dip, rake, num=2, measurement='rakes')
+strike_cent, dip_cent = mplstereonet.geographic2pole(*zip(*centers))
+ax.pole(strike_cent, dip_cent, 'ro', ms=12)
+
+# Fit a girdle to the two modes
+# The pole of this plane will be the plunge of the fold axis
+axis_s, axis_d = mplstereonet.fit_girdle(*zip(*centers), measurement='radians')
+ax.plane(axis_s, axis_d, color='green')
+ax.pole(axis_s, axis_d, color='green', marker='o', ms=15)
+
+# Now we'll find the midpoint. We could project the centers as rakes on the
+# plane we just fit, but it's easier to get their mean vector instead.
+mid, _ = mplstereonet.find_mean_vector(*zip(*centers), measurement='radians')
+midx, midy = mplstereonet.line(*mid)
+
+# Now let's find the axial plane by fitting another girdle to the midpoint
+# and the pole of the plunge axis.
+xp, yp = mplstereonet.pole(axis_s, axis_d)
+
+x, y = [xp[0], midx], [yp[0], midy]
+axial_s, axial_dip = mplstereonet.fit_girdle(x, y, measurement='radians')
+
+ax.plane(axial_s, axial_dip, color='lightblue', lw=3)
+
+plt.show()