Scale threshold to Globe radius in more projections.

Fixes #1572, testing the example in the other projections as well.

lib/cartopy/crs.py

@@ -1638,9 +1638,15 @@ def __init__(self, central_longitude=0, false_easting=None,
                          false_northing=false_northing,
                          globe=globe)
 
+        # TODO: Let the globe return the semimajor axis always.
+        a = np.float(self.globe.semimajor_axis or WGS84_SEMIMAJOR_AXIS)
+        b = np.float(self.globe.semiminor_axis or a)
+
+        self._threshold = min(a, b) / 63.78137  # About 1e5 for defaults.
+
     @property
     def threshold(self):
-        return 1e5
+        return self._threshold
 
 
 class EckertI(_Eckert):
@@ -1810,9 +1816,15 @@ def __init__(self, central_longitude=0, globe=None,
                          false_northing=false_northing,
                          globe=globe)
 
+        # TODO: Let the globe return the semimajor axis always.
+        a = np.float(self.globe.semimajor_axis or WGS84_SEMIMAJOR_AXIS)
+        b = np.float(self.globe.semiminor_axis or a)
+
+        self._threshold = min(a, b) / 63.78137  # About 1e5 for defaults.
+
     @property
     def threshold(self):
-        return 1e5
+        return self._threshold
 
 
 class Robinson(_WarpedRectangularProjection):
@@ -2306,14 +2318,15 @@ def __init__(self, central_longitude=0.0, central_latitude=0.0,
         maxs = np.max(coords, axis=1)
         self._x_limits = mins[0], maxs[0]
         self._y_limits = mins[1], maxs[1]
+        self._threshold = min(a, b) / 63.78137  # About 1e5 for defaults.
 
     @property
     def boundary(self):
         return self._boundary
 
     @property
     def threshold(self):
-        return 1e5
+        return self._threshold
 
     @property
     def x_limits(self):