Support for oceanic InterruptedGoodeHomolosine

- The ocean emphasis for the Interrupted Goode
  Homolosine projection (imh_o) requires proj
  version 7.1.0 or later.
- Option to select 'land' or 'ocean' emphasis
- Increaed n resolution from 31 points to 91 points
- Added docstring for class
- Added images of both land and ocean views

Author: jkrasting

docs/make_projection.py

@@ -41,6 +41,25 @@ def plate_carree_plot():
         ax.gridlines()
 
 
+def igh_plot():
+    import matplotlib.pyplot as plt
+    import cartopy.crs as ccrs
+
+    fig = plt.figure(figsize=(6.9228, 6))
+
+    ax1 = fig.add_subplot(2, 1, 1,
+                          projection=ccrs.InterruptedGoodeHomolosine(
+                              emphasis='land'))
+    ax1.coastlines(resolution='110m')
+    ax1.gridlines()
+
+    ax2 = fig.add_subplot(2, 1, 2,
+                          projection=ccrs.InterruptedGoodeHomolosine(
+                              central_longitude=-160, emphasis='ocean'))
+    ax2.coastlines(resolution='110m')
+    ax2.gridlines()
+
+
 def utm_plot():
     import matplotlib.pyplot as plt
     import cartopy.crs as ccrs
@@ -59,6 +78,7 @@ def utm_plot():
 
 MULTI_PLOT_CASES = {
     ccrs.PlateCarree: plate_carree_plot,
+    ccrs.InterruptedGoodeHomolosine: igh_plot,
     ccrs.UTM: utm_plot,
 }
 

docs/source/crs/projections.rst

@@ -274,10 +274,19 @@ InterruptedGoodeHomolosine
     import matplotlib.pyplot as plt
     import cartopy.crs as ccrs
 
-    plt.figure(figsize=(6.9228, 3))
-    ax = plt.axes(projection=ccrs.InterruptedGoodeHomolosine())
-    ax.coastlines(resolution='110m')
-    ax.gridlines()
+    fig = plt.figure(figsize=(6.9228, 6))
+
+    ax1 = fig.add_subplot(2, 1, 1,
+                          projection=ccrs.InterruptedGoodeHomolosine(
+                              emphasis='land'))
+    ax1.coastlines(resolution='110m')
+    ax1.gridlines()
+
+    ax2 = fig.add_subplot(2, 1, 2,
+                          projection=ccrs.InterruptedGoodeHomolosine(
+                              central_longitude=-160, emphasis='ocean'))
+    ax2.coastlines(resolution='110m')
+    ax2.gridlines()
 
 
 RotatedPole

lib/cartopy/crs.py

@@ -1930,17 +1930,59 @@ def transform_points(self, src_crs, x, y, z=None):
 
 
 class InterruptedGoodeHomolosine(Projection):
-    def __init__(self, central_longitude=0, globe=None):
-        proj4_params = [('proj', 'igh'), ('lon_0', central_longitude)]
-        super().__init__(proj4_params, globe=globe)
+    """
+    Composite equal-area projection empahsizing either land or
+    ocean features.
+
+    Original Reference:
+    Goode, J. P., 1925: The Homolosine Projection: A new device for
+        portraying the Earth's surface entire. Annals of the
+        Association of American Geographers, 15:3, 119-125,
+        DOI: 10.1080/00045602509356949
+
+    A central_longitude value of -160 is recommended for the oceanic view.
+
+    """
+    def __init__(self, central_longitude=0, globe=None, emphasis='land'):
+        """
+        Parameters
+        ----------
+        central_longitude: optional
+            The central longitude. Defaults to 0.
+        globe: :class:`cartopy.crs.Globe`, optional
+            If omitted, a default globe is created.
+        emphasis: optional
+            Options "land" and "ocean" are available. Defaults to "land"
+
+        """
+        if emphasis == 'land':
+            proj4_params = [('proj', 'igh'), ('lon_0', central_longitude)]
+            super().__init__(proj4_params,globe=globe)
+
+        elif emphasis == 'ocean':
+            if PROJ4_VERSION < (7, 1, 0):
+                _proj_ver = '.'.join(str(v) for v in PROJ4_VERSION)
+                raise ValueError('The Interrupted Goode Homolosine ocean '
+                                 'projection requires Proj version 7.1.0, '
+                                 'but you are using ' + _proj_ver)
+            proj4_params = [('proj', 'igh_o'), ('lon_0', central_longitude)]
+            super().__init__(proj4_params,globe=globe)
+
+        else:
+            msg = '`emphasis` needs to be either \'land\' or \'ocean\''
+            raise ValueError(msg)
 
         minlon, maxlon = self._determine_longitude_bounds(central_longitude)
         epsilon = 1e-10
 
         # Obtain boundary points
-        n = 31
-        top_interrupted_lons = (-40.0,)
-        bottom_interrupted_lons = (80.0, -20.0, -100.0)
+        n = 91
+        if emphasis == 'land':
+            top_interrupted_lons = (-40.0,)
+            bottom_interrupted_lons = (80.0, -20.0, -100.0)
+        elif emphasis == 'ocean':
+            top_interrupted_lons = (-90.0, 60.0)
+            bottom_interrupted_lons = (90.0, -60.0)
         lons = np.empty(
             (2 + 2 * len(top_interrupted_lons + bottom_interrupted_lons)) * n +
             1)