Merge pull request #722 from davidhassell/ugrid-weights

Fix bug when calculating UGRID cell areas when non-spatial coordinates span the discrete axis

Author: davidhassell

Changelog.rst

@@ -3,6 +3,9 @@ version 3.17.0
 
 **2024-??-??**
 
+* Fix occasional bug when calculating UGRID cell areas when
+  non-spatial coordinates span the discrete axis
+  (https://github.com/NCAS-CMS/cf-python/issues/721)
 * New keyword parameter to `cf.Field.insert_dimension`:
   ``constructs`` (https://github.com/NCAS-CMS/cf-python/issues/719)
 * Added the ``cell_measures`` and ``coordinates`` keyword arguments to

cf/weights.py

@@ -1652,6 +1652,9 @@ def _geometry_ugrid_cells(cls, f, domain_axis, cell_type, auto=False):
         )
 
         for key, aux in auxiliary_coordinates_1d.items():
+            if str(aux.ctype) not in "XYZ":
+                continue
+
             aux_axis = f.get_data_axes(key)[0]
 
             ugrid = f.domain_topology(default=None, filter_by_axis=(aux_axis,))
@@ -1665,8 +1668,8 @@ def _geometry_ugrid_cells(cls, f, domain_axis, cell_type, auto=False):
             ):
                 continue
 
-            # Still here? Then this auxiliary coordinate has either UGRID
-            # or geometry cells.
+            # Still here? Then this X, Y, or Z auxiliary coordinate is
+            # for either UGRID or geometry cells.
             if aux.X:
                 aux_X = aux.copy()
                 x_axis = aux_axis
@@ -1701,7 +1704,7 @@ def _geometry_ugrid_cells(cls, f, domain_axis, cell_type, auto=False):
 
             raise ValueError(
                 "Can't create weights: X and Y cells span different domain "
-                "axes"
+                f"axes: {x_axis} != {y_axis}"
             )
 
         axis = x_axis