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#### 7.1.4 `sky3d`: Spherical Polar Matching

    matcher=sky3d values*='<ra/degrees> <dec/degrees> <distance>'
                  params='<error/units of distance>'
                  tuning='<bin-factor>'

> `values*`:
>   - `ra/degrees`: Right Ascension
>   - `dec/degrees`: Declination
>   - `distance`: Distance from origin
> `params`:
>   - `error/units of distance`: Maximum Cartesian separation for match
> `tuning`:
>   - `bin-factor`: Scaling factor to adjust bin size; larger values mean larger bins

The `sky3d` matcher compares positions in the volume of the sky taking account of distance from the observer. The position in three-dimensional space is calculated for each row using the `ra`, `dec` and `distance` as spherical polar coordinates, where `distance` is the distance from the observer along the line of sight. Rows are considered to match when their positions in this space are within `error` units of each other. The units of `error` are the same as those of `distance`.

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

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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