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<h4><a name="layer-skydensity">8.3.26 <code>skydensity</code></a></h4>
<p>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.
</p>
<p>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.
</p>
<p>
<strong>Usage Overview:</strong>
<pre>
layerN=skydensity levelN=<-rel-level|+abs-level>
combineN=sum|mean|median|min|max|stdev|count|hit
transparencyN=0..1 lonN=<deg-expr> latN=<deg-expr>
weightN=<num-expr> inN=<table> ifmtN=<in-format>
istreamN=true|false icmdN=<cmds>
</pre>
</p>
<p>All the parameters listed here
affect only the relevant layer,
identified by the suffix
<code>N</code>.
</p>
<p>
<strong>Example:</strong>
</p>
<div align="center"><img src="plot2-layer-skydensity.png" alt="" align="middle"></div>
<p><pre> stilts plot2sky <strong>in=tgas_source.fits</strong> <strong>lon=l</strong> <strong>lat=b</strong>
<strong>layer1=skydensity</strong> <strong>weight1=astrometric_excess_noise</strong> <strong>combine1=mean</strong> <strong>level1=4</strong>
projection=aitoff auxmap=cubehelix auxfunc=log
xpix=580 ypix=250</pre></p>
<p>
<dl>
<dt><strong><code>combineN = sum|mean|median|min|max|stdev|count|hit</code> <em>(<a href="http://andromeda.star.bris.ac.uk/starjavadocs/uk/ac/starlink/ttools/plot2/layer/Combiner.html">Combiner</a>)</em></strong></dt>
<dd>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).
<p>For unweighted values (a pure density map),
it usually makes sense to use
<code>count</code>.
However, if the input is weighted by an additional
data coordinate, one of the other values such as
<code>mean</code>
may be more revealing.
</p>
<p>The available options are:
<ul>
<li><code>sum</code>: the sum of all the combined values
</li>
<li><code>mean</code>: the mean of the combined values
</li>
<li><code>median</code>: the median of the combined values (may be slow)
</li>
<li><code>min</code>: the minimum of all the combined values
</li>
<li><code>max</code>: the maximum of all the combined values
</li>
<li><code>stdev</code>: the sample standard deviation of the combined values
</li>
<li><code>count</code>: the number of non-blank values (weight is ignored)
</li>
<li><code>hit</code>: 1 if any values present, NaN otherwise (weight is ignored)
</li>
</ul>
</p>
<p>[Default: <code>sum</code>]
</p>
</dd>
<dt><strong><code>icmdN = <cmds></code> <em>(<a href="http://andromeda.star.bris.ac.uk/starjavadocs/uk/ac/starlink/ttools/filter/ProcessingStep.html">ProcessingStep[]</a>)</em></strong></dt>
<dd>Specifies processing to be performed on
the layer N input table as specified by parameter <code>inN</code>.
The value of this parameter is one or more of the filter
commands described in <a href="filterSteps.html">Section 6.1</a>.
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.
<p>Commands may alteratively be supplied in an external file,
by using the indirection character '@'.
Thus a value of "<code>@filename</code>"
causes the file <code>filename</code> 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.
</p>
</dd>
<dt><strong><code>ifmtN = <in-format></code> <em>(String)</em></strong></dt>
<dd>Specifies the format of the input table as specified by parameter <code>inN</code>.
The known formats are listed in <a href="inFormats.html">Section 5.2.1</a>.
This flag can be used if you know what format your
table is in.
If it has the special value
<code>(auto)</code> (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.
<p>[Default: <code>(auto)</code>]
</p>
</dd>
<dt><strong><code>inN = <table></code> <em>(<a href="http://www.starlink.ac.uk/stil/javadocs/uk/ac/starlink/table/StarTable.html">StarTable</a>)</em></strong></dt>
<dd>The location of the input table.
This may take one of the following forms:
<ul>
<li>A filename.</li>
<li>A URL.</li>
<li>The special value "<code>-</code>",
meaning standard input.
In this case the input format must be given explicitly
using the <code>ifmtN</code>
parameter.
Note that not all formats can be streamed in this way.
</li>
<li>A system command line with
either a "<code><</code>" character at the start,
or a "<code>|</code>" character at the end
("<code><syscmd</code>" or
"<code>syscmd|</code>").
This executes the given pipeline and reads from its
standard output.
This will probably only work on unix-like systems.
</li>
</ul>
In any case, compressed data in one of the supported compression
formats (gzip, Unix compress or bzip2) will be decompressed
transparently.
</dd>
<dt><strong><code>istreamN = true|false</code> <em>(Boolean)</em></strong></dt>
<dd>If set true, the input table
specified by the <code>inN</code> parameter
will be read as a stream.
It is necessary to give the
<code>ifmtN</code> 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).
<p>[Default: <code>false</code>]
</p>
</dd>
<dt><strong><code>latN = <deg-expr></code> <em>(String)</em></strong></dt>
<dd>Latitude in decimal degrees.
<p>The value is a numeric algebraic expression based on column names
as described in <a href="jel.html">Section 10</a>.
</p>
</dd>
<dt><strong><code>levelN = <-rel-level|+abs-level></code> <em>(Integer)</em></strong></dt>
<dd>Determines the HEALPix level of pixels which are averaged
over to calculate density.
<p>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.
</p>
<p>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.
</p>
<p>[Default: <code>-3</code>]
</p>
</dd>
<dt><strong><code>lonN = <deg-expr></code> <em>(String)</em></strong></dt>
<dd>Longitude in decimal degrees.
<p>The value is a numeric algebraic expression based on column names
as described in <a href="jel.html">Section 10</a>.
</p>
</dd>
<dt><strong><code>transparencyN = 0..1</code> <em>(Double)</em></strong></dt>
<dd>Transparency with which components are plotted,
in the range 0 (opaque) to 1 (invisible).
The value is 1-alpha.
<p>[Default: <code>0</code>]
</p>
</dd>
<dt><strong><code>weightN = <num-expr></code> <em>(String)</em></strong></dt>
<dd>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.
<p>The value is a numeric algebraic expression based on column names
as described in <a href="jel.html">Section 10</a>.
</p>
</dd>
</dl>
</p>
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<hr><i>STILTS - Starlink Tables Infrastructure Library Tool Set<br>Starlink User Note256<br>STILTS web page:
<a href="http://www.starlink.ac.uk/stilts/">http://www.starlink.ac.uk/stilts/</a><br>Author email:
<a href="mailto:m.b.taylor@bristol.ac.uk">m.b.taylor@bristol.ac.uk</a><br>Mailing list:
<a href="mailto:topcat-user@jiscmail.ac.uk">topcat-user@jiscmail.ac.uk</a><br></i></body>
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