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#### 7.1.9 `1d_err`, `2d_err`, ...: Cartesian Matching with Per-Object Errors

    matcher=2d_err values*='<x> <y> <error>'
                   params='<scale>'
                   tuning='<bin-factor>'

> `values*`:
>   - `x`: Cartesian co-ordinate #1
>   - `y`: Cartesian co-ordinate #2
>   - `error`: Per-object error radius
> `params`:
>   - `scale`: Rough average of per-object error distance; just used for tuning in conjunction with bin factor
> `tuning`:
>   - `bin-factor`: Scaling factor to adjust bin size; larger values mean larger bins

The `1d_err`, `2d_err`, ... matchers compare positions in N-dimensional Cartesian space like the `1d`, `2d` matchers described in Section 7.1.6, except that the match radius can be different for each row. Rows are considered to match when the separation reckoned by Pythagoras between the `x`, `y`, ... positions is no larger than the sum of the two per-row `error` values. Matching in any number of Cartesian dimensions can be done by extending this syntax in the obvious way.

A variant form `1d_err_q`, `2d_err_q`, ... does the same thing but combines the two per-row `error` values in quadrature rather than by summation. In that case the separation beween the two positions must be no larger than sqrt(error12+error22).

The `scale` parameter must be supplied, and should be approximately the characteristic size of the per-object error values. In conjunction with the `bin-factor` tuning parameter its value affects the performance of the match, but not the result.

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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