Allow regridding for projections in non-degree type units (#178)

* allow regridding for projections in non-degree type units

* fix tests

* add tests

* fix tests

* address review comments

esmf_regrid/schemes.py

@@ -191,10 +191,13 @@ def _cube_to_GridInfo(cube, center=False, resolution=None, mask=None):
             lat_bound_array = _contiguous_masked(lat.bounds, mask)
         # 2D coords must be AuxCoords, which do not have a circular attribute.
         circular = False
-    lon_bound_array = lon.units.convert(lon_bound_array, Unit("degrees"))
-    lat_bound_array = lat.units.convert(lat_bound_array, Unit("degrees"))
-    lon_points = lon.units.convert(lon.points, Unit("degrees"))
-    lat_points = lon.units.convert(lat.points, Unit("degrees"))
+    lon_points = lon.points
+    lat_points = lat.points
+    if crs is None:
+        lon_bound_array = lon.units.convert(lon_bound_array, Unit("degrees"))
+        lat_bound_array = lat.units.convert(lat_bound_array, Unit("degrees"))
+        lon_points = lon.units.convert(lon_points, Unit("degrees"))
+        lat_points = lon.units.convert(lat_points, Unit("degrees"))
     if resolution is None:
         grid_info = GridInfo(
             lon_points,

esmf_regrid/tests/unit/schemes/__init__.py

@@ -1,9 +1,11 @@
 """Unit tests for `esmf_regrid.schemes`."""
 
+from iris.coord_systems import OSGB
 import numpy as np
 from numpy import ma
 import pytest
 
+from esmf_regrid.schemes import ESMFAreaWeighted, ESMFBilinear, ESMFNearest
 from esmf_regrid.tests.unit.schemes.test__cube_to_GridInfo import _grid_cube
 from esmf_regrid.tests.unit.schemes.test__mesh_to_MeshInfo import (
     _gridlike_mesh,
@@ -166,3 +168,50 @@ def _test_mask_from_regridder(scheme, mask_keyword):
     np.testing.assert_allclose(
         getattr(rg_from_different, regridder_attr), mask_different
     )
+
+
+def _test_non_degree_crs(scheme):
+    """Test regridding scheme is compatible with coordinates with non-degree units."""
+    coord_system = OSGB()
+
+    # This definition comes from a small section of real user data.
+    n_lons_src = 2
+    n_lats_src = 3
+    lon_bounds = (-197500, -192500)
+    lat_bounds = (1247500, 1237500)
+    tm_cube = _grid_cube(
+        n_lons_src,
+        n_lats_src,
+        lon_bounds,
+        lat_bounds,
+        circular=False,
+        coord_system=coord_system,
+        standard_names=["projection_x_coordinate", "projection_y_coordinate"],
+        units="m",
+    )
+    data = np.arange(n_lats_src * n_lons_src).reshape([n_lats_src, n_lons_src])
+    tm_cube.data = data
+
+    n_lons_tgt = 12
+    n_lats_tgt = 14
+    lon_bounds_tgt = (-13, -12.8)
+    lat_bounds_tgt = (60.5, 60.7)
+    cube_tgt = _grid_cube(
+        n_lons_tgt, n_lats_tgt, lon_bounds_tgt, lat_bounds_tgt, circular=True
+    )
+
+    result = tm_cube.regrid(cube_tgt, scheme())
+
+    # Set expected results, this varies depending on the scheme.
+    if scheme is ESMFAreaWeighted:
+        expected_sum, expected_unmasked = 50.86147272655136, 21
+    elif scheme is ESMFBilinear:
+        expected_sum, expected_unmasked = 35.90837983047451, 13
+    elif scheme is ESMFNearest:
+        expected_sum, expected_unmasked = 490, 168
+
+    # Check that the data is as expected.
+    assert np.isclose(result.data.sum(), expected_sum)
+
+    # Check that the number of masked points is as expected.
+    assert (1 - result.data.mask).sum() == expected_unmasked

esmf_regrid/tests/unit/schemes/test_ESMFAreaWeighted.py

@@ -8,6 +8,7 @@
     _test_invalid_mdtol,
     _test_mask_from_init,
     _test_mask_from_regridder,
+    _test_non_degree_crs,
 )
 
 
@@ -62,3 +63,8 @@ def test_invalid_tgt_location():
     match = "For area weighted regridding, target location must be 'face'."
     with pytest.raises(ValueError, match=match):
         _ = ESMFAreaWeighted(tgt_location="node")
+
+
+def test_non_degree_crs():
+    """Test for coordinates with non-degree units."""
+    _test_non_degree_crs(ESMFAreaWeighted)

esmf_regrid/tests/unit/schemes/test_ESMFBilinear.py

@@ -8,6 +8,7 @@
     _test_invalid_mdtol,
     _test_mask_from_init,
     _test_mask_from_regridder,
+    _test_non_degree_crs,
 )
 
 
@@ -51,3 +52,8 @@ def test_mask_from_regridder(mask_keyword):
     Checks that use_src_mask and use_tgt_mask are passed down correctly.
     """
     _test_mask_from_regridder(ESMFBilinear, mask_keyword)
+
+
+def test_non_degree_crs():
+    """Test for coordinates with non-degree units."""
+    _test_non_degree_crs(ESMFBilinear)

esmf_regrid/tests/unit/schemes/test_ESMFNearest.py

@@ -8,6 +8,7 @@
 from esmf_regrid.tests.unit.schemes.__init__ import (
     _test_mask_from_init,
     _test_mask_from_regridder,
+    _test_non_degree_crs,
 )
 from esmf_regrid.tests.unit.schemes.test__cube_to_GridInfo import _grid_cube
 from esmf_regrid.tests.unit.schemes.test__mesh_to_MeshInfo import (
@@ -94,3 +95,8 @@ def test_mask_from_regridder(mask_keyword):
     Checks that use_src_mask and use_tgt_mask are passed down correctly.
     """
     _test_mask_from_regridder(ESMFNearest, mask_keyword)
+
+
+def test_non_degree_crs():
+    """Test for coordinates with non-degree units."""
+    _test_non_degree_crs(ESMFNearest)

esmf_regrid/tests/unit/schemes/test__cube_to_GridInfo.py

@@ -67,21 +67,23 @@ def _grid_cube(
     lat_outer_bounds,
     circular=False,
     coord_system=None,
+    standard_names=["longitude", "latitude"],
+    units="degrees",
 ):
     lon_points, lon_bounds = _generate_points_and_bounds(n_lons, lon_outer_bounds)
     lon = DimCoord(
         lon_points,
-        "longitude",
-        units="degrees",
+        standard_names[0],
+        units=units,
         bounds=lon_bounds,
         circular=circular,
         coord_system=coord_system,
     )
     lat_points, lat_bounds = _generate_points_and_bounds(n_lats, lat_outer_bounds)
     lat = DimCoord(
         lat_points,
-        "latitude",
-        units="degrees",
+        standard_names[1],
+        units=units,
         bounds=lat_bounds,
         coord_system=coord_system,
     )