  The NASA/IPAC Extragalactic Database (NED) has deployed a new rule-based
cross-matching algorithm called Match Expert (MatchEx), capable of
cross-matching very large catalogs (VLCs) with >10 million objects. MatchEx
goes beyond traditional position-based cross-matching algorithms by using other
available data together with expert logic to determine which candidate match is
the best. Furthermore, the local background density of sources is used to
determine and minimize the false-positive match rate and to estimate match
completeness. The logical outcome and statistical probability of each match
decision is stored in the database, and may be used to tune the algorithm and
adjust match parameter thresholds. For our first production run, we
cross-matched the GALEX All Sky Survey Catalog (GASC), containing nearly 40
million NUV-detected sources, against a directory of 180 million objects in
NED. Candidate matches were identified for each GASC source within a 7.5
arcsecond radius. These candidates were filtered on position-based matching
probability, and on other criteria including object type and object name. We
estimate a match completeness of 97.6% and a match accuracy of 99.75%. MatchEx
is being used to cross-match over 2 billion catalog sources to NED, including
the Spitzer Source List, the 2MASS Point-Source Catalog, AllWISE, and SDSS DR
10. It will also speed up routine cross-matching of sources as part of the NED
literature pipeline.
