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<html>
   
   <head>
      <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
      <link rel="stylesheet" type="text/css" href="sun-style.css">
      <title>skydensity</title>
   </head>
   
   <body>
      <hr>
      <a href="layer-healpix.html">Next</a> <a href="layer-skycorr.html">Previous</a> <a href="LayerType.html">Up</a> <a href="index.html">Contents</a> <br> <b>Next: </b><a href="layer-healpix.html">healpix</a><br>
       <b>Up: </b><a href="LayerType.html">Layer Types</a><br>
       <b>Previous: </b><a href="layer-skycorr.html">skycorr</a><br>
      
      <hr>
      <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=&lt;-rel-level|+abs-level&gt;
                     combineN=sum|mean|median|min|max|stdev|count|hit
                     transparencyN=0..1 lonN=&lt;deg-expr&gt; latN=&lt;deg-expr&gt;
                     weightN=&lt;num-expr&gt; inN=&lt;table&gt; ifmtN=&lt;in-format&gt;
                     istreamN=true|false icmdN=&lt;cmds&gt;
</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> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<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 = &lt;cmds&gt;</code> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<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 = &lt;in-format&gt;</code> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<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 = &lt;table&gt;</code> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<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>&lt;</code>" character at the start,
                     or a "<code>|</code>" character at the end
                     ("<code>&lt;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> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<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 = &lt;deg-expr&gt;</code> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<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 = &lt;-rel-level|+abs-level&gt;</code> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<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 = &lt;deg-expr&gt;</code> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<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> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<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 = &lt;num-expr&gt;</code> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<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>
      <hr><a href="layer-healpix.html">Next</a> <a href="layer-skycorr.html">Previous</a> <a href="LayerType.html">Up</a> <a href="index.html">Contents</a> <br> <b>Next: </b><a href="layer-healpix.html">healpix</a><br>
       <b>Up: </b><a href="LayerType.html">Layer Types</a><br>
       <b>Previous: </b><a href="layer-skycorr.html">skycorr</a><br>
      
      <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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