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## B.46 `tskymap`: Calculates sky density maps

`tskymap` calculates a weighted density map (or, to put it another way, a histogram) on the sky from columns of an input table. The sky is divided up into some discrete set of tiles according to a specified tessellation scheme (currently HEALPix or HTM are supported), and the required quantities are aggregated into bins corresponding to these tiles. The output table has a column giving the pixel index identifying each tile, plus one or more columns each representing an aggregation of a quantity from the input table.

By default the number of rows from the input table falling within each tile is included as the first column in the output table. But by specifying the `cols` and `combine` parameters you can add more columns giving the sum, mean, median or other statistics of input table columns or expressions as well.

The output table can then, for instance, be plotted using `plot2sky`'s `healpix` layer type (though an alternative is to do that plot directly using a `skydensity` layer).

In the case of HEALPix tiling, the result can also be output in a FITS file suitable for use by external applications that understand the semi-standard FITS-Healpix convention. Note in this case, for maximum compatibility, the `fits-healpix` output format should in general be used.

See also `tgridmap`, which does the same thing for N-dimensional grid geometry.

- B.46.1 Usage
- B.46.2 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