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#### 8.3.32 `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>
                         combineN=sum|sum-per-unit|count|...
                         perunitN=steradian|degree2|arcmin2|arcsec2|mas2|uas2
                         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`.

Example:

       stilts plot2sky in=tgas_source.fits lon=l lat=b
                       layer1=skydensity weight1=parallax combine1=mean level1=4
                       projection=aitoff auxmap=PuRd auxfunc=histogram
                       xpix=540 ypix=250

`combineN = sum|sum-per-unit|count|...` (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. The combined values are the weights, but if the `weight` is left blank, a weighting of unity is assumed.

  For density-like values (`count-per-unit`, `sum-per-unit`) the scaling is additionally influenced by the `perunit` parameter.

  The available options are:

  - `sum`: the sum of all the combined values per bin
  - `sum-per-unit`: the sum of all the combined values per unit of bin size
  - `count`: the number of non-blank values per bin (weight is ignored)
  - `count-per-unit`: the number of non-blank values per unit of bin size (weight is ignored)
  - `mean`: the mean of the combined values
  - `median`: the median of the combined values (may be slow)
  - `min`: the minimum of all the combined values
  - `max`: the maximum of all the combined values
  - `stdev`: the sample standard deviation of the combined values
  - `hit`: 1 if any values present, NaN otherwise (weight is ignored)

  [Default: `sum-per-unit`]

`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, and lines which are blank or which start with a '`#`' character are ignored.

`ifmtN = <in-format>` (String)
  Specifies the format of the input table as specified by parameter `inN`. The known formats are listed in Section 5.1.1. 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. This parameter is ignored for scheme-specified tables.

  [Default: `(auto)`]

`inN = <table>` (StarTable)
  The location of the input table. This may take one of the following forms:
  - A filename.
  - A URL.
  - The special value "`-`", meaning standard input. In this case the input format must be given explicitly using the `ifmtN` parameter. Note that not all formats can be streamed in this way.
  - A scheme specification of the form `:<scheme-name>:<scheme-args>`.
  - A system command line with either a "`<`" character at the start, or a "`|`" character at the end ("`<syscmd`" or "`syscmd|`"). This executes the given pipeline and reads from its standard output. This will probably only work on unix-like systems.
  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). This parameter is ignored for scheme-specified tables.

  [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.

`perunitN = steradian|degree2|arcmin2|arcsec2|mas2|uas2` (SolidAngleUnit)
  Defines the unit of sky area used for scaling density-like combinations (e.g. `combine`=`count-per-unit` or `sum-per-unit`). If the Combination mode is calculating values per unit area, this configures the area scale in question. For non-density-like combination modes (e.g. `combine`=`sum` or `mean`) it has no effect.

  The available options are:

  - `steradian`: (180/Pi)^2 deg^2
  - `degree2`: square degrees
  - `arcmin2`: square arcminute, (1/60)^2 deg^2
  - `arcsec2`: square arcsecond, (1/3600)^2 deg^2
  - `mas2`: square milliarcsecond, (.001/3600)^2 deg^2
  - `uas2`: square microarcsecond, (1e-6/3600)^2 deg^2

  [Default: `degree2`]

`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.

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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