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## B.2 `cdsskymatch`: Crossmatches table on sky position against VizieR/SIMBAD table

`cdsskymatch` uses the CDS X-Match service to join a local table to one of the tables hosted by the Centre de Données astronomiques de Strasbourg. This includes all of the VizieR tables and the SIMBAD database. The service is very fast, and in most cases it is the best way to match a local table against a large external table hosted by a service. It is almost certainly much better than using `coneskymatch`, though it is less flexible than TAP (see the `tapquery` task for flexible access to TAP services, and `tapskymatch` for positional matches).

The local table is uploaded to the X-Match service in chunks, and the matches for each chunk are retrieved in turn and eventually stitched together to form the final result. The tool only uploads sky position and an identifier for each row of the input table, but all columns of the input table are reinstated in the result for reference.

For a better understanding of the details of how this service operates, including exactly what coordinates are matched against the uploaded positions (roughly: integrated to J2000 using proper motions if available) and what columns are included in the output (roughly: a subset of the most commonly used columns), please consult the service documentation.

Acknowledgement: CDS note that if the use of the X-Match service is useful to your research, they would appreciate the following acknowledgement:

> "This research made use of the cross-match service provided by CDS, Strasbourg."

- B.2.1 Usage
- B.2.2 Examples

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STILTS - Starlink Tables Infrastructure Library Tool Set
Starlink User Note256
STILTS web page: http://www.starlink.ac.uk/stilts/
Author email: m.b.taylor@bristol.ac.uk
Mailing list: topcat-user@jiscmail.ac.uk