The Gaia DR2 includes a pre-computed crossmatch with optical/near infra-red photometric and spectroscopic surveys.
A subset of the matched external catalogues were also used by Gaia CU9 Validation and were of help in validating Gaia results.
The external catalogues matched with Gaia DR2 are all obtained in the optical/near-IR wavelength region (with the exception of allWISE (Cutri et al.2013) which
extends in the medium-IR domain), are general surveys not restricted to a specific class of objects and have an angular resolution lower than Gaia.
The external catalogues are not homogeneous enough among themselves to use exactly the same
algorithm for all of them. We thus broadly separated the external catalogues into two different groups: large dense surveys and sparse catalogues,
and we defined two slightly different algorithms for the two groups.
External catalogues are defined as dense surveys when it is possible to define a precise (i.e. based on a reasonable number of objects) and accurate
(i.e. local) density around the majority of their objects.
The algorithms we define are both not symmetric: for dense surveys we use Gaia DR2 as the leading catalogue,
while for sparse catalogues we use Gaia DR2 as the second catalogue. This means that for dense surveys for each Gaia DR2 source possible counterparts are searched for in the external catalogue, while for sparse catalogues for each external catalogue source possible counterparts are searched for in Gaia DR2.
Both cross-match algorithms are positional only and use the Gaia DR2 positions, their errors, their correlation and, when available, parallax, proper motions and
the corresponding errors and correlations.
Details on the crossmatch algorithms are given in Marrese et al. 2018 (in preparation) and in Marrese et al. (2017).