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## B.27 `tapskymatch`: Crossmatches table on sky position against TAP table

`tapskymatch` allows you to perform a positional crossmatch of a local table with one held in a remote TAP service, as long as that TAP supports upload queries. This task does three main jobs. First, it prepares the ADQL queries and TAP negotiations for you so that you don't need to remember the syntax for performing positional crossmatches against a TAP service. Second, it organises data transfer so that only those columns required (basically the positional ones) are transmitted to and from the service, to save on bandwidth. And third it divides the job up into chunks, so that the TAP service only has to perform a manageable-sized query at a time. If the job is large this chunking can be useful to monitor progress of the job, and it also allows you to perform a match which would otherwise hit the upload or output limits imposed by the service.

The positional match may be done in any spherical coordinate system, it's up to the user to ensure that the same coordinates are provided for the local and remote tables.

Note that `cdsskymatch` provides similar functionality by accessing a different external service, which is usually much faster; if the table you wish to match is part of the VizieR database, you may wish to use that command instead.

- B.27.1 Usage
- B.27.2 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