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## B.37 `tgridmap`: Calculates N-dimensional density maps

`tgridmap` scans an input table to create one or more N-dimensional density maps, or equivalently N-dimensional histograms, of the values in an input table, and outputs the result as an, optionally sparse, table containing a row for each grid cell. The maps/histograms can optionally be weighted by some quantity from the input table, and various options such as summing, averaging and counting are available for aggregation of inputs into the output bins.

The supplied `coords` parameter defines which N numeric columns of the input table form the coordinates of the bin grid, and the `cols` parameter defines which quantities are aggregated into each bin. Either the `binsizes` or `nbins` parameter must be supplied to define the extents of the bins on each axis. The output table contains a row for each bin, with columns giving the central (and upper/lower bound) values of each grid coordinate, and a column for each aggregated value. The rows are output in first-coordinate-slowest sequence, and the `sparse` parameter determines whether a row is written for every cell in the hypercube defined by the grid dimensions, or only for those cells with non-blank data.

The tabular form of the output may not be the most appropriate or compact way to write a density map, especially for multi-dimensional grids, but it means the output can be manipulated later by other STILTS commands or by TOPCAT. To do a similar job with more compact output, see `tcube`. See also `tskymap`, which does the same thing for sky geometry (and is probably a better choice if you find yourself accumulating onto a longitude-latitude grid).

- B.37.1 Usage
- B.37.2 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