SkyCoord to Dataset conversions

[calgray]

A problem with astropy SkyCoord is that it uses a structured dtype ndarray (i.e. sky_coord.data._values.dtype yields dtype([('lon', '<f8'), ('lat', '<f8')]) ).

While xarray could load this into a DataArray:

• There's no canonical way in xarray to access the fields other than using .values['field_name']
• loses the frame-specific mapping of the field name
• requires some considerable monkey patching to SkyCoord

From the general philosophy of xarray, it's more appropriate to unpack structured arrays and represent fields as separate variables in a Dataset when using xarray.

An API for this could look like following, which converts skycoords to xarray with astropy Quantities:

from astropy.coordinates import SkyCoord
from astropy_xarray.sky_coord import dataset_to_sky_coord, sky_coord_to_dataset

s = skycoord_to_dataset(
    SkyCoord(
        ra=[2, 6, 7, 4] * u.deg,
        dec=[4, 7, 4, 3] * u.deg,
        frame="icrs",
    ),
    coords=[
        ("field", [0, 1, 2, 3])
    ],
)
s

<xarray.Dataset> Size: 96B
Dimensions:  (field: 4)
Coordinates:
  * field    (field) int64 32B 0 1 2 3
Data variables:
    ra      (field) float64 32B [°] 2.0 6.0 7.0 4.0
    dec      (field) float64 32B [°] 4.0 7.0 4.0 3.0
Attributes:
    frame:    {'name': 'icrs', 'representation_type': 'spherical', 'different...

and back to SkyCoord:

dataset_to_skycoord(s)
# or s.astropy.to_sky_coord()

<SkyCoord (ICRS): (ra, dec) in deg
    [(2., 4.), (6., 7.), (7., 4.), (4., 3.)]>

[calgray]

an idea that can work alongside to this is a skycoord_to_dataarray(skycoord: SkyCoord, representation_axis: str) and likewise dataarray_to_skycoord(da: xr.DataArray, representation_coord: str).

can assume representation_coord is always the last axis. This may only work in frames where all units are the same.

[calgray]

From #13

• skycoord_to_dataset handles coordinates, skycoord_to_dataarray not added as its non-trivial extra skycoord component tensors (implemented using a _skycoord_to_dataarrays)
• to_sky_coord was added, but undocumented. Subject to be changed or removed in future.