Handling of CRS objects in coordinates

[David Hoese]

I started looking more at 2D coordinate arrays (especially when they are dask arrays) and using a pyproj CRS object as a coordinate and ran in to a weird situation. I'm hoping @snowman2 or someone else can help me decipher what is happening here.

from pyproj import CRS
import xarray as xr
import dask.array as da

crs1 = CRS.from_string('+proj=lcc +datum=WGS84 +lon_0=-95 +lat_0=25 +lat_1=25')
crs2 = CRS.from_string('+proj=lcc +datum=WGS84 +lon_0=-95 +lat_0=35 +lat_1=35')

a = xr.DataArray(da.zeros((5, 5), chunks=2), dims=('y', 'x'), coords={'yc': (('y', 'x'), da.zeros((5, 5), chunks=3)), 'xc': (('y', 'x'), da.zeros((5, 5), chunks=3)), 'crs': crs1})
b = xr.DataArray(da.zeros((5, 5), chunks=2), dims=('y', 'x'), coords={'yc': (('y', 'x'), da.zeros((5, 5), chunks=3)), 'xc': (('y', 'x'), da.zeros((5, 5), chunks=3)), 'crs': crs2})
c = xr.DataArray(da.zeros((5, 5), chunks=2), dims=('y', 'x'), coords={'yc': (('y', 'x'), da.zeros((5, 5), chunks=3)), 'xc': (('y', 'x'), da.zeros((5, 5), chunks=3)), 'crs': crs1})

# Adding DataArrays with different CRS coordinates
a + b

# Results in:
# <xarray.DataArray 'zeros-e5723e7f9121b7ac546f61c19dabe786' (y: 5, x: 5)>
# dask.array<shape=(5, 5), dtype=float64, chunksize=(2, 2)>
# Coordinates:
#     yc       (y, x) float64 dask.array<shape=(5, 5), chunksize=(3, 3)>
#     xc       (y, x) float64 dask.array<shape=(5, 5), chunksize=(3, 3)>
# Dimensions without coordinates: y, x

# Adding DataArrays with the same CRS coordinates
a + c

# Results in:
# <xarray.DataArray 'zeros-e5723e7f9121b7ac546f61c19dabe786' (y: 5, x: 5)>
# dask.array<shape=(5, 5), dtype=float64, chunksize=(2, 2)>
# Coordinates:
#     yc       (y, x) float64 dask.array<shape=(5, 5), chunksize=(3, 3)>
#     xc       (y, x) float64 dask.array<shape=(5, 5), chunksize=(3, 3)>
#     crs      object +proj=lcc +datum=WGS84 +lon_0=-95 +lat_0=25 +lat_1=25 +type=crs
# Dimensions without coordinates: y, x

I would have expected a + b to not be allowed since they differ and are not equal. Thoughts? Is this expected?

[Alan D. Snow]

I would have expected a + b to not be allowed since they differ and are not equal.

Interesting. It appears that the crs coordinate was lost in that operation. So, in that sense it failed. It must have dropped the parts that weren't equal and kept the parts that were equal.

Well, that is the extent of my insight on this one for now. Glad to see this moving forward 👍

[David Hoese]

New discovery tonight while working on something else, xarray makes the coordinates DataArrays no matter what and in the case of an object it becomes an object "scalar" array. So you have to do data_arr.coords['crs'].item() to get the original CRS object back. Maybe this is a problem and/or the reason the CRS gets dropped in the calculations described above.

<xarray.DataArray 'crs' ()>
array(<CRS: +proj=geos +a=6378137.0 +b=6356752.31414 +h=357860 ...>
Name: unknown
Ellipsoid:
- semi_major_metre: 6378137.00
- semi_minor_metre: nan
- inverse_flattening: 298.26
Area of Use:
- UNDEFINED
Prime Meridian:
- longitude: 0.0000
- unit_name: degree
- unit_conversion_factor: 0.01745329
Axis Info:
- Easting[E] (east) EPSG:9001 (metre)
- Northing[N] (north) EPSG:9001 (metre)
, dtype=object)
Coordinates:
    crs      object +proj=geos +a=6378137.0 +b=6356752.31414 +h=35786023.0 +lon_0=-89.5 +sweep=x +units=m +type=crs

[David Hoese]

Another data point on figuring this out:

In [27]: b = xr.DataArray(da.zeros((5, 5), chunks=2), dims=('y', 'x'), coords={'y': da.arange(1, 6, chunks=3), 'x': da.arange(2, 7, chunks=3), 'crs': crs2, 'test': 2, 'test2': 2})         

In [28]: a = xr.DataArray(da.zeros((5, 5), chunks=2), dims=('y', 'x'), coords={'y': da.arange(1, 6, chunks=3), 'x': da.arange(2, 7, chunks=3), 'crs': crs1, 'test': 1, 'test2': 2})         

In [29]: a + b                                                                                                                                                                              
Out[29]: 
<xarray.DataArray 'zeros-e5723e7f9121b7ac546f61c19dabe786' (y: 5, x: 5)>
dask.array<shape=(5, 5), dtype=float64, chunksize=(2, 2)>
Coordinates:
  * y        (y) int64 1 2 3 4 5
  * x        (x) int64 2 3 4 5 6
    test2    int64 2

Basically, xarray drops any non-dimensional coordinates that are not equal. So, as xarray sits right now, there is no easy way to force the CRS coordinate to be checked as far as I can tell.

[Ryan May]

This seems like a bug in xarray.

[David Hoese]

@dopplershift Do you know if you've gotten coordinates to behave like this in the past? I created this issue a couple days ago: pydata/xarray#2996

[Ryan May]

No, but I remember the coordinate approach being advocated because it shouldn't disappear like that.

[David Hoese]

@dopplershift This page http://xarray.pydata.org/en/stable/computation.html#coordinates seems to match what I'm seeing. Specifically non-dimensional coordinates (like my crs and test and test2 coordinates above) will not be preserved if there are conflicts.

[Ryan May]

😠

The rationale in the docs makes sense, but that doesn't make me happy about it for this case.

[David Hoese]

I'm going to close this. I merged #26 today and it copies the behavior of rioxarray regarding CRS handling. That is, .geo.crs will give you a pyproj CRS object, but this is dynamically determined in most cases. If the exact instance of the accessor is still alive then a private ._crs will contain the pyproj CRS object that was found before.

If you, the user, would like to persist this discovery you can use .geo.write_crs to store the CRS information in a CF-friendly way (a grid_mapping variable/coordinate with WKT information). I'm still working on the documentation to reflect the changes done in #26, but while I wait for CI on other PRs I thought I'd start cleaning up some of these issues.