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.
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.
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,
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,
which does the same thing for sky geometry (and is probably a better
choice if you find yourself accumulating onto a longitude-latitude grid).