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#### 7.1.2 `skyerr`: Sky Matching with Per-Object Errors

    matcher=skyerr values*='<ra/deg> <dec/deg> <error/arcsec>'
                   params='<scale/arcsec>'
                   tuning='<healpix-k>'

> `values*`:
>   - `ra/deg`: Right Ascension
>   - `dec/deg`: Declination
>   - `error/arcsec`: Per-object error radius along a great circle
> `params`:
>   - `scale/arcsec`: Rough average of per-object error distance; just used for tuning to set default pixel size
> `tuning`:
>   - `healpix-k`: Controls sky pixel size. Legal range 0 - 29. 0 is 60deg, 20 is 0.2".

The `skyerr` matcher compares positions on the celestial sphere using error radii which can be different for each row. Rows are considered to match when the separation between the two `ra`, `dec` positions is no larger than the sum of the two per-row `error` values.

The `scale` parameter should be a rough average value of the error distances. It is used only to set a sensible default for `healpix-k` tuning parameter, and its value does not affect the result. If you set `healpix-k` directly, its value is ignored.

As with `sky` matching, other longitude/latitude coordinate pairs may be used in place of right ascension and declination.

A variant form `skyerr_q` does the same thing but combines the two per-row `error` values in quadrature rather than by summation. In that case the separation between the two positions must be no larger than sqrt(error12+error22).

Note: the semantics of this matcher have changed slightly at version 2.4 of STILTS. In earlier versions the single parameter was named `max-error` and provided an additional constraint on the maximum accepted separation between matched objects. For most uses, the old and new behaviours are expected to give the same results, but in cases of difference, the new behaviour is more likely what you want.

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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