Add grid_bounds_to_polygons

cf_xarray/__init__.py

@@ -9,4 +9,7 @@
 from .options import set_options  # noqa
 from .utils import _get_version
 
+from . import geometry  # noqa
+
+#
 __version__ = _get_version()

cf_xarray/geometry.py

@@ -438,3 +438,66 @@ def cf_to_lines(ds: xr.Dataset):
         geoms = np.where(np.diff(offset2) == 1, lines[offset2[:-1]], multilines)
 
     return xr.DataArray(geoms, dims=part_node_count.dims, coords=part_node_count.coords)
+
+
+def grid_to_polygons(ds: xr.Dataset) -> xr.DataArray:
+    """
+    Converts a regular 2D lat/lon grid to a 2D array of shapely polygons.
+
+    Modified from https://notebooksharing.space/view/c6c1f3a7d0c260724115eaa2bf78f3738b275f7f633c1558639e7bbd75b31456.
+
+    Parameters
+    ----------
+    ds : xr.Dataset
+        Dataset with "latitude" and "longitude" variables as well as their bounds variables.
+        1D "latitude" and "longitude" variables are supported. This function will automatically
+        broadcast them against each other.
+
+    Returns
+    -------
+    DataArray
+        DataArray with shapely polygon per grid cell.
+    """
+    import shapely
+
+    grid = ds.cf[["latitude", "longitude"]].load()
+    bounds = grid.cf.bounds
+    dims = grid.cf.dims
+
+    if "latitude" in dims or "longitude" in dims:
+        # for 1D lat, lon, this allows them to be
+        # broadcast against each other
+        grid = grid.reset_coords()
+
+    assert "latitude" in bounds
+    assert "longitude" in bounds
+    (lon_bounds,) = bounds["longitude"]
+    (lat_bounds,) = bounds["latitude"]
+
+    with xr.set_options(keep_attrs=True):
+        (points,) = xr.broadcast(grid)
+
+    bounds_dim = grid.cf.get_bounds_dim_name("latitude")
+    points = points.transpose(..., bounds_dim)
+    lonbnd = points[lon_bounds].data
+    latbnd = points[lat_bounds].data
+
+    if points.sizes[bounds_dim] == 2:
+        lonbnd = lonbnd[..., [0, 0, 1, 1]]
+        latbnd = latbnd[..., [0, 1, 1, 0]]
+
+    elif points.sizes[bounds_dim] != 4:
+        raise ValueError(
+            f"The size of the detected bounds or vertex dimension {bounds_dim} is not 2 or 4."
+        )
+
+    # geopandas needs this
+    mask = lonbnd[..., 0] >= 180
+    lonbnd[mask, :] = lonbnd[mask, :] - 360
+
+    polyarray = shapely.polygons(shapely.linearrings(lonbnd, latbnd))
+
+    # 'geometry' is a blessed name in geopandas.
+    boxes = points[lon_bounds][..., 0].copy(data=polyarray).rename("geometry")
+
+    return boxes

pyproject.toml

@@ -140,8 +140,7 @@ module=[
     "pint",
     "matplotlib",
     "pytest",
-    "shapely",
-    "shapely.geometry",
+    "shapely.*",
     "xarray.core.pycompat",
 ]
 ignore_missing_imports = true