Next Previous Up Contents
Next: healpix
Up: Layer Types
Previous: skyellipse

8.3.24 skydensity

Plots a density map on the sky. The grid on which the values are drawn uses the HEALPix tesselation, with a configurable resolution. You can optionally use a weighting for the points, and you can configure how the points are combined to produce the output pixel values.

The way that data values are mapped to colours is usually controlled by options at the level of the plot itself, rather than by per-layer configuration.

Usage Overview:

   layerN=skydensity levelN=<-rel-level|+abs-level>
                     transparencyN=0..1 lonN=<deg-expr> latN=<deg-expr>
                     weightN=<num-expr> inN=<table> ifmtN=<in-format>
                     istreamN=true|false icmdN=<cmds>

All the parameters listed here affect only the relevant layer, identified by the suffix N.


   stilts plot2sky in=tgas_source.fits lon=l lat=b
                   layer1=skydensity weight1=astrometric_excess_noise combine1=mean level1=4
                   projection=aitoff auxmap=cubehelix auxfunc=log
                   xpix=580 ypix=250

combineN = sum|mean|median|min|max|stdev|count|hit       (Combiner)
Defines how values contributing to the same density map bin are combined together to produce the value assigned to that bin (and hence its colour).

For unweighted values (a pure density map), it usually makes sense to use count. However, if the input is weighted by an additional data coordinate, one of the other values such as mean may be more revealing.

The available options are:

[Default: sum]

icmdN = <cmds>       (ProcessingStep[])
Specifies processing to be performed on the layer N input table as specified by parameter inN. The value of this parameter is one or more of the filter commands described in Section 6.1. If more than one is given, they must be separated by semicolon characters (";"). This parameter can be repeated multiple times on the same command line to build up a list of processing steps. The sequence of commands given in this way defines the processing pipeline which is performed on the table.

Commands may alteratively be supplied in an external file, by using the indirection character '@'. Thus a value of "@filename" causes the file filename to be read for a list of filter commands to execute. The commands in the file may be separated by newline characters and/or semicolons.

ifmtN = <in-format>       (String)
Specifies the format of the input table as specified by parameter inN. The known formats are listed in Section 5.2.1. This flag can be used if you know what format your table is in. If it has the special value (auto) (the default), then an attempt will be made to detect the format of the table automatically. This cannot always be done correctly however, in which case the program will exit with an error explaining which formats were attempted.

[Default: (auto)]

inN = <table>       (StarTable)
The location of the input table. This may take one of the following forms: In any case, compressed data in one of the supported compression formats (gzip, Unix compress or bzip2) will be decompressed transparently.
istreamN = true|false       (Boolean)
If set true, the input table specified by the inN parameter will be read as a stream. It is necessary to give the ifmtN parameter in this case. Depending on the required operations and processing mode, this may cause the read to fail (sometimes it is necessary to read the table more than once). It is not normally necessary to set this flag; in most cases the data will be streamed automatically if that is the best thing to do. However it can sometimes result in less resource usage when processing large files in certain formats (such as VOTable).

[Default: false]

latN = <deg-expr>       (String)
Latitude in decimal degrees.

The value is a numeric algebraic expression based on column names as described in Section 10.

levelN = <-rel-level|+abs-level>       (Integer)
Determines the HEALPix level of pixels which are averaged over to calculate density.

If the supplied value is a non-negative integer, it gives the absolute level to use; at level 0 there are 12 pixels on the sky, and the count multiplies by 4 for each increment.

If the value is negative, it represents a relative level; it is approximately the (negative) number of screen pixels along one side of a HEALPix sky pixel. In this case the actual HEALPix level will depend on the current zoom.

[Default: -3]

lonN = <deg-expr>       (String)
Longitude in decimal degrees.

The value is a numeric algebraic expression based on column names as described in Section 10.

transparencyN = 0..1       (Double)
Transparency with which components are plotted, in the range 0 (opaque) to 1 (invisible). The value is 1-alpha.

[Default: 0]

weightN = <num-expr>       (String)
Weighting of data points. If supplied, each point contributes a value to the histogram equal to the data value multiplied by this coordinate. If not supplied, the effect is the same as supplying a fixed value of one.

The value is a numeric algebraic expression based on column names as described in Section 10.

Next Previous Up Contents
Next: healpix
Up: Layer Types
Previous: skyellipse

STILTS - Starlink Tables Infrastructure Library Tool Set
Starlink User Note256
STILTS web page:
Author email:
Mailing list: