Merge pull request #55 from mathewcohle/fix/parallel-arc-dist

Fix calculation of ParallelArcDist
TUW-GEO · May 31, 2018 · 06452de · 06452de
2 parents f52b897 + d05bc8c
commit 06452de
Show file tree

Hide file tree

Showing 2 changed files with 9 additions and 3 deletions.
diff --git a/pygeogrids/geodetic_datum.py b/pygeogrids/geodetic_datum.py
@@ -199,7 +199,7 @@ def EllN(self, lat):
         Parameters
         ----------
         lat : numpy.array, list or float
-            Geodatic latitudes of the points in the grid
+            Geodatic latitudes of the points in the grid in degrees
 
         Returns
         -------
@@ -270,10 +270,9 @@ def ParallelArcDist(self, lat, lon1, lon2):
         dist : np.array, float
             Parallel arc distance
         """
-        lat = np.deg2rad(lat)
         lon1 = np.deg2rad(lon1)
         lon2 = np.deg2rad(lon2)
-        return self.EllN(lat) * np.cos(lat) * (lon2 - lon1)
+        return self.EllN(lat) * np.cos(np.deg2rad(lat)) * (lon2 - lon1)
 
     def MeridianArcDist(self, lat1, lat2):
         """

diff --git a/tests/test_geodatic_datum.py b/tests/test_geodatic_datum.py
@@ -65,6 +65,13 @@ def test_ParallelArcDist(self):
         dist = self.datum.ParallelArcDist(0., 0., 360.)
         nptest.assert_almost_equal(dist, self.datum.geod.a * 2 * np.pi)
 
+        lat, lon1, lon2 = 45., -5., 5.
+        __, __, great_circle_dist = self.datum.geod.inv(lon1, lat, lon2, lat)
+        parallel_dist = self.datum.ParallelArcDist(lat, lon1, lon2)
+
+        assert great_circle_dist < parallel_dist, \
+            (great_circle_dist, parallel_dist)
+
 
 if __name__ == "__main__":
     unittest.main()