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### B.32.2 Examples

Here are some examples of using `tmatch1`:

    stilts tmatch1 matcher=sky values="RA2000 DE2000" params=20 \
                   action=keep0 in=crowded.vot out=sparse.vot

  Copies an input catalogue "crowded.vot" to an output catalogue "sparse.vot", but omitting any objects (rows) which are within 20 arcsec of other objects. The output catalogue will contain no near neighbours.

    stilts tmatch1 matcher=skyerr values="RA2000 DE2000 RADIUS*4" params=40 \
                   action=keep0 in=crowded.vot out=sparse.vot

  This is similar to the previous example, but uses the `skyerr` matcher which determines the proximity threshold on a row-by-row basis from values in the table - in this case 4 times the value of the RADIUS column (this value must be in arc seconds). The `params=40` value does not affect the result, but it gives the algorithm an idea of the rough scale of object separation.

    stilts tmatch1 matcher=3d values="XPIX YPIX ZPIX" params=10 action=identify \
                   in=state.fit ocmd='select GroupSize>3' out=groups3+.fit

  Uses the "3d" matcher to identify groups of objects in terms of their proximity in a 3-dimensional Cartesian space, with positions given by the XPIX, YPIX and ZPIX columns in the input table. The `action=identify` parameter means that the input table is written out with the same rows, but with additional columns indicating which rows are associated with each other. One of these columns, "GroupSize" gives the number of objects in each group. The postprocessing filter `ocmd='select GroupSize>3'` selects only those rows which are part of groups of three objects or larger; singletons and pairs are discarded before writing the output file.

    stilts tmatch1 matcher=sky values="ra dec" params=3 action=wide2 \
                   ocmd='keepcols "id_1 ra_1 dec_1 id_2 ra_2 dec_2"'
                   in=galaxy.fits out=binaries.txt ofmt=ascii

  Identifies pairs of objects within 3 arcsec of each other from an input catalogue. Singles, and groups of three or more, will be discarded. The output table generated is a double-width version of the input table with pairs of objects next to each other on the same row. Here, the `ocmd` post-processing filter discards all of the columns except the identifiers and sky positions for each object. The output is to a text file.

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Next: tmatch2: Crossmatches 2 tables using flexible criteria
Up: tmatch1: Performs a crossmatch internal to a single table
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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