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<h2><a name="match">7 Crossmatching</a></h2>
<p>STILTS offers flexible and efficient facilities for crossmatching tables.
Crossmatching is identifying different rows, which may be in the
same or different tables, that refer to the same item.
In an astronomical context such an item is usually,
though not necessarily, an astronomical source or object.
This operation corresponds to what in database terminology
is called a <em>join</em>.
</p>
<p>There are various complexities to specifying such a match.
In the first place you have to define what is the condition that
must be satisfied for two rows to be considered matching.
In the second place you must decide what happens if, for a given row,
more than one match can be found. Finally, you have to decide what
to do having worked out what the matched rows are; the result will
generally be presented as a new output table, but there are various
choices about what columns and rows it will consist of.
Some of these issues are discussed in this section, and others
in the reference sections on the tools themselves in <a href="cmdUsage.html">Appendix B</a>.
</p>
<p>Matching can in general be a computationally intensive process.
The algorithm used by the <code>tmatch*</code> tasks in STILTS,
except in pathological cases, scales as <i>O(N log(N))</i> or thereabouts,
where <i>N</i> is the total number of rows in all the tables being matched.
No preparation (such as sorting) is required on the tables prior to
invoking the matching operation.
It is reasonably fast; for instance an RA, Dec positional match
of two 10<sup>5</sup>-row catalogues takes of the order of 60 seconds
on current (2005 laptop) hardware. Attempting matches with large tables can
lead to running out of memory; the calculation just mentioned required a
java heap size of around 200Mb (<code>-Xmx200M</code>).
</p>
<p>In the current release of STILTS the following tasks are provided
for crossmatching between local tables:
<dl>
<dt><strong><a href="tmatch2.html"><code>tmatch2</code></a></strong></dt>
<dd>Generic crossmatching between two tables.
</dd>
<dt><strong><a href="tskymatch2.html"><code>tskymatch2</code></a></strong></dt>
<dd>Crossmatching between two tables where the matching criterion is
a fixed separation on the sky. This is simply a stripped-down
version of <code>tmatch2</code> provided for convenience when the
full generality is not required.
</dd>
<dt><strong><a href="tmatch1.html"><code>tmatch1</code></a></strong></dt>
<dd>Generic crossmatching internal to a single table.
The basic task this performs is to identify groups of rows within a
single table which match each other.
</dd>
<dt><strong><a href="tmatchn.html"><code>tmatchn</code></a></strong></dt>
<dd>Generic crossmatching between multiple (>2) tables.
</dd>
<dt><strong><a href="tjoin.html"><code>tjoin</code></a></strong></dt>
<dd>Trivial join operation between multiple tables in which no row
re-ordering is required. This barely warrants the term "crossmatch"
and the concepts explained in the rest of this section are not
relevant to it.
</dd>
</dl>
</p>
<ul>
<li><a href="MatchEngine.html">7.1 Match Criteria</a></li>
<ul>
<li><a href="SkyMatchEngine.html">7.1.1 <code>sky</code>: Sky Matching</a></li>
<li><a href="SkyMatchEngine-err.html">7.1.2 <code>skyerr</code>:
Sky Matching with Per-Object Errors</a></li>
<li><a href="EllipseSkyMatchEngine.html">7.1.3 <code>skyellipse</code>:
Sky Matching of Elliptical Regions</a></li>
<li><a href="SphericalPolarMatchEngine.html">7.1.4 <code>sky3d</code>: Spherical Polar Matching</a></li>
<li><a href="EqualsMatchEngine.html">7.1.5 <code>exact</code>: Exact Matching</a></li>
<li><a href="IsotropicCartesianMatchEngine.html">7.1.6 <code>1d</code>, <code>2d</code>, ...:
Isotropic Cartesian Matching</a></li>
<li><a href="AnisotropicCartesianMatchEngine.html">7.1.7 <code>2d_anisotropic</code>, ...:
Anisotropic Cartesian Matching</a></li>
<li><a href="CuboidCartesianMatchEngine.html">7.1.8 <code>2d_cuboid</code>, ...:
Cuboid Cartesian Matching</a></li>
<li><a href="ErrorCartesianMatchEngine.html">7.1.9 <code>1d_err</code>, <code>2d_err</code>, ...:
Cartesian Matching with Per-Object Errors</a></li>
<li><a href="EllipseCartesianMatchEngine.html">7.1.10 <code>2d_ellipse</code>:
Cartesian Matching of Elliptical Regions</a></li>
<li><a href="sec7.1.11.html">7.1.11 Custom Matchers</a></li>
<li><a href="CombinedMatchEngine.html">7.1.12 Matcher Combinations</a></li>
</ul>
<li><a href="matchGroup.html">7.2 Multi-Object Matches</a></li>
</ul>
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<b>Previous: </b><a href="mode-tosql.html">tosql</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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