Full-featured Mercator CS

lib/iris/coord_systems.py

@@ -831,7 +831,9 @@ class Mercator(CoordSystem):
 
     grid_mapping_name = "mercator"
 
-    def __init__(self, longitude_of_projection_origin=0, ellipsoid=None):
+    def __init__(self, longitude_of_projection_origin=0.0, ellipsoid=None,
+                 min_latitude=-80.0, max_latitude=84.0,
+                 true_scale_lat=0.0):
         """
         Constructs a Mercator coord system.
 
@@ -840,17 +842,33 @@ def __init__(self, longitude_of_projection_origin=0, ellipsoid=None):
                     True longitude of planar origin in degrees.
             * ellipsoid
                     :class:`GeogCS` defining the ellipsoid.
+            * min_latitude
+                    the maximum southerly extent of the projection.
+                    Defaults to -80 degrees.
+            * max_latitude
+                    the maximum northerly extent of the projection.
+                    Defaults to 84 degrees.
+            * true_scale_lat
+                    the latitude where the scale is 1. Defaults to 0 degrees.
 
         """
-
-        #: True longitude of planar origin in degrees.
         self.longitude_of_projection_origin = longitude_of_projection_origin
-        #: Ellipsoid definition.
         self.ellipsoid = ellipsoid
+        self.min_latitude = min_latitude
+        self.max_latitude = max_latitude
+        self.true_scale_lat = true_scale_lat
 
     def __repr__(self):
-        res = "Mercator(longitude_of_projection_origin={!r}, ellipsoid={!r})"
-        return res.format(self.longitude_of_projection_origin, self.ellipsoid)
+        res = ("Mercator(longitude_of_projection_origin={!r}, "
+               "ellipsoid={!r}, "
+               "min_latitude={!r}, "
+               "max_latitude={!r}, "
+               "true_scale_lat={!r})")
+        return res.format(self.longitude_of_projection_origin,
+                          self.ellipsoid,
+                          self.min_latitude,
+                          self.max_latitude,
+                          self.true_scale_lat)
 
     def as_cartopy_crs(self):
         if self.ellipsoid is not None:
@@ -860,7 +878,10 @@ def as_cartopy_crs(self):
 
         return ccrs.Mercator(
             central_longitude=self.longitude_of_projection_origin,
-            globe=globe)
+            globe=globe,
+            min_latitude=self.min_latitude,
+            max_latitude=self.max_latitude,
+            latitude_true_scale=self.true_scale_lat)
 
     def as_cartopy_projection(self):
         return self.as_cartopy_crs()

lib/iris/tests/results/coord_systems/Mercator.xml

@@ -1,2 +1,2 @@
 <?xml version="1.0" ?>
-<mercator ellipsoid="GeogCS(semi_major_axis=6377563.396, semi_minor_axis=6356256.909)" longitude_of_projection_origin="90.0"/>
+<mercator ellipsoid="GeogCS(semi_major_axis=6377563.396, semi_minor_axis=6356256.909)" longitude_of_projection_origin="90.0" max_latitude="84.0" min_latitude="-80.0" true_scale_lat="0.0"/>

lib/iris/tests/test_coordsystem.py

@@ -33,17 +33,12 @@
 from iris.coord_systems import *
 
 
-
-
 def osgb():
     return TransverseMercator(latitude_of_projection_origin=49, longitude_of_central_meridian=-2,
                               false_easting=-400, false_northing=100,
                               scale_factor_at_central_meridian=0.9996012717,
                               ellipsoid=GeogCS(6377563.396, 6356256.909))
 
-def merc():
-    return Mercator(longitude_of_projection_origin=90.0,
-                    ellipsoid=GeogCS(6377563.396, 6356256.909))
 
 def stereo():
     return Stereographic(central_lat=-90, central_lon=-45,
@@ -391,58 +386,5 @@ def test_south_cutoff(self):
         self.assertEqual(ccrs.cutoff, 30)
 
 
-class Test_Mercator_construction(tests.IrisTest):
-    def test_merc(self):
-        tm = merc()
-        self.assertXMLElement(tm, ("coord_systems", "Mercator.xml"))
-
-
-class Test_Mercator_repr(tests.IrisTest):
-    def test_merc(self):
-        tm = merc()
-        expected = "Mercator(longitude_of_projection_origin=90.0, "\
-                   "ellipsoid=GeogCS(semi_major_axis=6377563.396, "\
-                   "semi_minor_axis=6356256.909))"
-        self.assertEqual(expected, repr(tm))
-
-
-class Test_Mercator_as_cartopy_crs(tests.IrisTest):
-    def test_as_cartopy_crs(self):
-        longitude_of_projection_origin = 90.0
-        ellipsoid = GeogCS(semi_major_axis=6377563.396,
-                           semi_minor_axis=6356256.909)
-
-        merc_cs = Mercator(
-            longitude_of_projection_origin,
-            ellipsoid=ellipsoid)
-
-        expected = ccrs.Mercator(
-            central_longitude=longitude_of_projection_origin,
-            globe=ccrs.Globe(semimajor_axis=6377563.396,
-                             semiminor_axis=6356256.909, ellipse=None))
-
-        res = merc_cs.as_cartopy_crs()
-        self.assertEqual(res, expected)
-
-
-class Test_Mercator_as_cartopy_projection(tests.IrisTest):
-    def test_as_cartopy_projection(self):
-        longitude_of_projection_origin = 90.0
-        ellipsoid = GeogCS(semi_major_axis=6377563.396,
-                           semi_minor_axis=6356256.909)
-
-        merc_cs = Mercator(
-            longitude_of_projection_origin,
-            ellipsoid=ellipsoid)
-
-        expected = ccrs.Mercator(
-            central_longitude=longitude_of_projection_origin,
-            globe=ccrs.Globe(semimajor_axis=6377563.396,
-                             semiminor_axis=6356256.909, ellipse=None))
-
-        res = merc_cs.as_cartopy_projection()
-        self.assertEqual(res, expected)
-
-
 if __name__ == "__main__":
     tests.main()

lib/iris/tests/unit/coord_systems/test_Mercator.py

@@ -0,0 +1,116 @@
+# (C) British Crown Copyright 2018, Met Office
+#
+# This file is part of Iris.
+#
+# Iris is free software: you can redistribute it and/or modify it under
+# the terms of the GNU Lesser General Public License as published by the
+# Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# Iris is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public License
+# along with Iris.  If not, see <http://www.gnu.org/licenses/>.
+"""Unit tests for the :class:`iris.coord_systems.Mercator` class."""
+
+from __future__ import (absolute_import, division, print_function)
+from six.moves import (filter, input, map, range, zip)  # noqa
+
+# Import iris.tests first so that some things can be initialised before
+# importing anything else.
+import iris.tests as tests
+
+import cartopy.crs as ccrs
+from iris.coord_systems import GeogCS, Mercator
+
+
+class Test_Mercator__basics(tests.IrisTest):
+    def setUp(self):
+        self.tm = Mercator(longitude_of_projection_origin=90.0,
+                           ellipsoid=GeogCS(6377563.396, 6356256.909))
+
+    def test_construction(self):
+        self.assertXMLElement(self.tm, ("coord_systems", "Mercator.xml"))
+
+    def test_repr(self):
+        expected = ("Mercator(longitude_of_projection_origin=90.0, "
+                    "ellipsoid=GeogCS(semi_major_axis=6377563.396, "
+                    "semi_minor_axis=6356256.909), "
+                    "min_latitude=-80.0, max_latitude=84.0, "
+                    "true_scale_lat=0.0)")
+        self.assertEqual(expected, repr(self.tm))
+
+
+class Test_Mercator__as_cartopy_crs(tests.IrisTest):
+    def test_simple(self):
+        # Check that a projection set up with all the defaults is correctly
+        # converted to a cartopy CRS.
+        merc_cs = Mercator()
+        res = merc_cs.as_cartopy_crs()
+        expected = ccrs.Mercator(globe=ccrs.Globe())
+        self.assertEqual(res, expected)
+
+    def test_extra_kwargs(self):
+        # Check that a projection with non-default values is correctly
+        # converted to a cartopy CRS.
+        longitude_of_projection_origin = 90.0
+        min_lat = -75.0
+        true_scale_lat = 14.0
+        ellipsoid = GeogCS(semi_major_axis=6377563.396,
+                           semi_minor_axis=6356256.909)
+
+        merc_cs = Mercator(
+            longitude_of_projection_origin,
+            ellipsoid=ellipsoid,
+            min_latitude=min_lat,
+            true_scale_lat=true_scale_lat)
+
+        expected = ccrs.Mercator(
+            central_longitude=longitude_of_projection_origin,
+            globe=ccrs.Globe(semimajor_axis=6377563.396,
+                             semiminor_axis=6356256.909, ellipse=None),
+            min_latitude=min_lat,
+            latitude_true_scale=true_scale_lat)
+
+        res = merc_cs.as_cartopy_crs()
+        self.assertEqual(res, expected)
+
+
+class Test_as_cartopy_projection(tests.IrisTest):
+    def test_simple(self):
+        # Check that a projection set up with all the defaults is correctly
+        # converted to a cartopy projection.
+        merc_cs = Mercator()
+        res = merc_cs.as_cartopy_projection()
+        expected = ccrs.Mercator(globe=ccrs.Globe())
+        self.assertEqual(res, expected)
+
+    def test_extra_kwargs(self):
+        longitude_of_projection_origin = 90.0
+        max_lat = 80.0
+        true_scale_lat = 14.0
+        ellipsoid = GeogCS(semi_major_axis=6377563.396,
+                           semi_minor_axis=6356256.909)
+
+        merc_cs = Mercator(
+            longitude_of_projection_origin,
+            ellipsoid=ellipsoid,
+            max_latitude=max_lat,
+            true_scale_lat=true_scale_lat)
+
+        expected = ccrs.Mercator(
+            central_longitude=longitude_of_projection_origin,
+            globe=ccrs.Globe(semimajor_axis=6377563.396,
+                             semiminor_axis=6356256.909, ellipse=None),
+            max_latitude=max_lat,
+            latitude_true_scale=true_scale_lat)
+
+        res = merc_cs.as_cartopy_projection()
+        self.assertEqual(res, expected)
+
+
+if __name__ == "__main__":
+    tests.main()