# join_skycoord¶

astropy.table.join_skycoord(distance, distance_func='search_around_sky')[source]

Helper function to join on SkyCoord columns using distance matching.

This function is intended for use in table.join() to allow performing a table join where the key columns are both SkyCoord objects, matched by computing the distance between points and accepting values below distance.

The distance cross-matching is done using either search_around_sky or search_around_3d, depending on the value of distance_func. The default is 'search_around_sky'.

One can also provide a function object for distance_func, in which case it must be a function that follows the same input and output API as search_around_sky. In this case the function will be called with (skycoord1, skycoord2, distance) as arguments.

Parameters
distanceQuantity (angle or length)

Maximum distance between points to be considered a join match

distance_funcstr or function

Specifies the function for performing the cross-match based on distance. If supplied as a string this specifies the name of a function in astropy.coordinates. If supplied as a function then that function is called directly.

Returns
join_funcfunction

Function that accepts two SkyCoord columns (col1, col2) and returns the tuple (ids1, ids2) of pair-matched unique identifiers.

Examples

This example shows an inner join of two SkyCoord columns, taking any sources within 0.2 deg to be a match. Note the new sc_id column which is added and provides a unique source identifier for the matches.

>>> from astropy.coordinates import SkyCoord
>>> import astropy.units as u
>>> from astropy.table import Table, join_skycoord
>>> from astropy import table

>>> sc1 = SkyCoord([0, 1, 1.1, 2], [0, 0, 0, 0], unit='deg')
>>> sc2 = SkyCoord([0.5, 1.05, 2.1], [0, 0, 0], unit='deg')

>>> join_func = join_skycoord(0.2 * u.deg)
>>> join_func(sc1, sc2)  # Associate each coordinate with unique source ID
(array([3, 1, 1, 2]), array([4, 1, 2]))

>>> t1 = Table([sc1], names=['sc'])
>>> t2 = Table([sc2], names=['sc'])
>>> t12 = table.join(t1, t2, join_funcs={'sc': join_skycoord(0.2 * u.deg)})
>>> print(t12)  # Note new sc_id column with the IDs from join_func()
sc_id   sc_1    sc_2
deg,deg deg,deg
----- ------- --------
1 1.0,0.0 1.05,0.0
1 1.1,0.0 1.05,0.0
2 2.0,0.0  2.1,0.0