Change .geobox to look for CRS via CF convention first

Order of resolution is something like:

- x.coords[x.grid_mapping].spatial_ref
- x.coords[<looks like crs coord>].spatial_ref
- x.{x,y,latitude,longitude}.attrs['crs']
- x.attrs['crs']

datacube/utils/xarray_geoextensions.py

@@ -25,10 +25,17 @@ def _norm_crs(crs):
         raise ValueError('Can not interpret {} as CRS'.format(type(crs)))
 
 
-def _get_crs(obj):
-    if not isinstance(obj, xarray.Dataset) and not isinstance(obj, xarray.DataArray):
-        raise ValueError('Can not get crs from {}'.format(type(obj)))
-
+def _get_crs_from_attrs(obj):
+    """ Looks for attribute named `crs` containing CRS string
+        1. Checks spatials coords attrs
+        2. Checks data variable attrs
+        3. Checks dataset attrs
+
+        Returns
+        =======
+        Content for `.attrs[crs]` usually it's a string
+        None if not present in any of the places listed above
+    """
     if isinstance(obj, xarray.Dataset):
         if len(obj.data_vars) > 0:
             data_array = next(iter(obj.data_vars.values()))
@@ -39,22 +46,69 @@ def _get_crs(obj):
         data_array = obj
 
     sdims = spatial_dims(data_array, relaxed=True)
-    if sdims is None:
-        return None
-
-    crs_set = set(data_array[d].attrs.get('crs', None) for d in sdims)
-    crs = None
-    if len(crs_set) > 1:
-        raise ValueError('Spatial dimensions have different crs.')
-    elif len(crs_set) == 1:
-        crs = crs_set.pop()
+    if sdims is not None:
+        crs_set = set(data_array[d].attrs.get('crs', None) for d in sdims)
+        crs = None
+        if len(crs_set) > 1:
+            raise ValueError('Spatial dimensions have different crs.')
+        elif len(crs_set) == 1:
+            crs = crs_set.pop()
+    else:
+        crs = None
 
     if crs is None:
         # fall back option
         crs = data_array.attrs.get('crs', None) or obj.attrs.get('crs', None)
     return crs
 
 
+def _get_crs_from_coord(obj, mode='strict'):
+    """ Looks for dimensionless coordinate with `spatial_ref` attribute.
+
+        obj: Dataset | DataArray
+        mode: strict|any|all
+           strict -- raise Error if multiple candidates
+           any    -- return first one
+           all    -- return a list of all found CRSs
+
+       Returns
+       =======
+       None     - if none found
+       crs:str  - if found one
+       crs:str  - if found several but mode is any
+
+       (crs: str, crs: str) - if found several and mode=all
+    """
+    grid_mapping = obj.attrs.get('grid_mapping', None)
+
+    # First check CF convention "pointer"
+    if grid_mapping is not None and grid_mapping in obj.coords:
+        coord = obj.coords[grid_mapping]
+        spatial_ref = coord.attrs.get('spatial_ref', None)
+        if spatial_ref is not None:
+            return spatial_ref
+        else:
+            raise ValueError(f"Coordinate '{grid_mapping}' has no `spatial_ref` attribute")
+
+    # No explicit `grid_mapping` find some "CRS" coordinate
+    candidates = tuple(coord.attrs['spatial_ref'] for coord in obj.coords.values()
+                       if coord.ndim == 0 and 'spatial_ref' in coord.attrs)
+
+    if len(candidates) == 0:
+        return None
+    if len(candidates) == 1:
+        return candidates[0]
+
+    if mode == 'strict':
+        raise ValueError("Too many candidates when looking for CRS")
+    elif mode == 'all':
+        return candidates
+    elif mode == 'any':
+        return candidates[0]
+    else:
+        raise ValueError(f"Mode needs to be: strict|any|all got {mode}")
+
+
 def _xarray_affine(obj):
     sdims = spatial_dims(obj, relaxed=True)
     if sdims is None:
@@ -72,10 +126,26 @@ def _xarray_extent(obj):
 
 
 def _xarray_geobox(obj):
-    crs = _norm_crs(_get_crs(obj))
+    crs = None
+    try:
+        crs = _get_crs_from_coord(obj)
+    except ValueError:
+        pass
+
+    if crs is None:
+        try:
+            crs = _get_crs_from_attrs(obj)
+        except ValueError:
+            pass
+
     if crs is None:
         return None
 
+    try:
+        crs = _norm_crs(crs)
+    except ValueError:
+        return None
+
     dims = crs.dimensions
     return geometry.GeoBox(obj[dims[1]].size, obj[dims[0]].size, obj.affine, crs)
 

tests/test_xarray_extension.py

@@ -6,6 +6,8 @@
     _xarray_affine,
     _xarray_geobox,
     _xarray_extent,
+    _get_crs_from_attrs,
+    _get_crs_from_coord,
 )
 
 
@@ -49,6 +51,7 @@ def test_xr_geobox():
 
     assert ds.band[:, :2, :2].affine*(0, 0) == xy
     assert ds.band[:, :2, :2].affine*(1, 1) == tuple(a+b for a, b in zip(xy, rxy))
+    assert ds.band.isel(time=0, x=0).affine is None
 
     xx = ds.band + 1000
     assert xx.geobox is not None
@@ -59,3 +62,18 @@ def test_xr_geobox():
 
     assert mk_sample_xr_dataset(crs=None).geobox is None
     assert mk_sample_xr_dataset(crs=None).affine is not None
+
+
+def test_crs_from_coord():
+    xx_none = mk_sample_xr_dataset(crs=None)
+    xx_3857 = mk_sample_xr_dataset(crs=epsg3857)
+    xx_4326 = mk_sample_xr_dataset(crs=epsg4326)
+
+    assert _get_crs_from_coord(xx_none) is None
+    assert _get_crs_from_coord(xx_3857) == epsg3857
+    assert _get_crs_from_coord(xx_4326) == epsg4326
+
+    cc_4326 = xx_4326.geobox.xr_coords(with_crs='epsg_4326')['epsg_4326']
+    cc_3857 = xx_3857.geobox.xr_coords(with_crs='epsg_3857')['epsg_3857']
+    print(cc_4326)
+    print(cc_3857)