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### 5.7 Matching against a Remote Table

TOPCAT provides a number of facilities for positional crossmatches against tables which are exposed via Virtual Observatory web services (Cone Search, Simple Image Access and Simple Spectral Access) and general SQL-like matching (TAP). These work rather differently from the other functions described in this section, which operate on local tables, though conceptually the result is similar. See Section 6 for more details.

It also provides access to the X-Match service provided by the CDS, as described in Appendix A.10.1, which allows fast matching against any VizieR table or SIMBAD.

In many cases the CDS Upload X-Match window is the easiest, fastest and best way to match against remote tables. TAP is more flexible, but has a somewhat steeper learning curve, and may not be as fast or scalable for large local tables. The multi-SIA and multi-SSA windows may be useful for performing per-row searches of modest size against remote image or spectral archives. The multi-cone window is (since version 4.2) present mostly for historical reasons, and is not usually a good choice, since it is much less efficient than TAP or CDS X-Match.

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TOPCAT - Tool for OPerations on Catalogues And Tables
Starlink User Note253
TOPCAT web page: http://www.starlink.ac.uk/topcat/
Author email: m.b.taylor@bristol.ac.uk
Mailing list: topcat-user@bristol.ac.uk