Use equal area Mollweide projection for computing fraction of overlap…

… between inventory and model grid cells (#4)

emiproc/grids.py

@@ -743,18 +743,19 @@ def intersected_cells(self, corners):
             # The inventory cell lies outside the cosmo grid
             return []
 
-        # Here we assume a flat earth. The error is less than 1% for typical
-        # grid sizes over europe. Since we're interested in the ratio of areas,
-        # we can calculate in degrees^2.
+        molly = ccrs.Mollweide()
+        corners = molly.transform_points(self.projection,corners[:,0],corners[:,1])
         inv_cell = Polygon(corners)
 
+
         intersections = []
         # make sure we iterate only over valid gridpoint indices
         for i in range(max(0, lon_idx_min), min(self.nx, lon_idx_max)):
             for j in range(max(0, lat_idx_min), min(self.ny, lat_idx_max)):
-                corners = list(zip(*self.cell_corners(i, j)))
+                corners = np.array(list(zip(*self.cell_corners(i, j))))
+                corners = molly.transform_points(self.projection,corners[:,0],corners[:,1])
+
                 cosmo_cell = Polygon(corners)
-
                 if cosmo_cell.intersects(inv_cell):
                     overlap = cosmo_cell.intersection(inv_cell)
                     intersections.append((i, j, overlap.area / inv_cell.area))