Add grid_bounds_to_polygons

cf_xarray/geometry.py

@@ -293,3 +293,58 @@ def cf_to_points(ds: xr.Dataset):
         j += n
 
     return xr.DataArray(geoms, dims=node_count.dims, coords=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().reset_coords()
+    bounds = ds.cf.bounds
+
+    assert "latitude" in bounds
+    assert "longitude" in bounds
+    (lon_bounds,) = bounds["longitude"]
+    (lat_bounds,) = bounds["latitude"]
+
+    (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:
+        # geopandas needs this
+        lonbnd = lonbnd[..., [0, 0, 1, 1]]
+        mask = lonbnd[..., 0] >= 180
+        lonbnd[mask, :] = lonbnd[mask, :] - 360
+        latbnd = latbnd[..., [0, 1, 1, 0]]
+
+    elif points.sizes[bounds_dim] == 4:
+        raise NotImplementedError
+    else:
+        raise ValueError(
+            f"The size of the detected bounds or vertex dimension {bounds_dim} is not 2 or 4."
+        )
+
+    polyarray = shapely.polygons(shapely.linearrings(lonbnd, latbnd))
+    boxes = points[lon_bounds][..., 0].copy(data=polyarray)
+
+    return boxes