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<h3><a name="MatchEngine">7.1 Match Criteria</a></h3>
<p>Determining whether one row represents the same item as another
is done by comparing the values in certain of their columns to see
if they are the same or similar.
The most common astronomical case is to say that two rows match if
their celestial coordinates (right ascension and declination) are
within a given small radius of each other on the sky.
There are other possibilities; for instance the coordinates to compare
may be in a Cartesian space, or have a higher (or lower) dimensionality
than two, or the match may be exact rather than within an error radius....
</p>
<p>If you just need to match two tables according to sky position
with fixed errors you are recommended to use the simplified
<a href="tskymatch2.html"><code>tskymatch2</code></a> task.
For other cases, this section describes how to specify much more
flexible match criteria for use with
<code>tmatch1</code>, <code>tmatch2</code> or <code>tmatchn</code>
by setting the following parameters:
<dl>
<dt><strong><code>matcher</code></strong></dt>
<dd>Name of the match criteria type.
</dd>
<dt><strong><code>params</code></strong></dt>
<dd>Fixed value(s) giving the parameters of the match
(typically an error radius). If more than one value is required,
the values should be separated by spaces.
</dd>
<dt><strong><code>values*</code></strong></dt>
<dd>Expressions to be compared between rows. This will typically
contain the names of one or more columns, but each element may be
an algebraic expression (see <a href="jel.html">Section 10</a>) rather than just a
column name if required.
If more than one value is required, the values should be separated
by spaces.
There is one of these parameters for each table taking part in the
match, so for <code>tmatch2</code> you must specify both
<code>values1</code> and <code>values2</code>.
</dd>
<dt><strong><code>tuning</code></strong></dt>
<dd>Fixed value(s) supplying tuning parameters for the match
algorithm. If there is more than one value, they should be
separated by spaces.
This value will have a sensible default, so you do not
need to supply it, but providing adjusted values may make your match
run faster or require less memory (or the reverse).
Adjusting tuning parameters will not change the result of any match,
only the resources required to run it.
Looking at the progress output of a match will indicate what
tuning values have been used; adjusting the value a bit up or down
is a good way to experiment.
</dd>
</dl>
</p>
<p>For example, suppose we wish to locate objects in two tables which are
within 3 arcseconds of each other on the sky. One table has columns
RA and DEC which give coordinates in degrees, and the other has columns
RArad and DECrad which give coordinates in radians. These are the
arguments which would be used to tell <code>tmatch2</code> what the match
criteria are:
<pre>
matcher=sky
params=3
values1='RA DEC'
values2='radiansToDegrees(RArad) radiansToDegrees(DECrad)'
</pre>
It is clearly important that corresponding values are comparable
(in the same units) between the tables being matched,
and in geometrically sensitive cases such as matching
on the sky, it's important that they are the units expected by the
matcher as well. To determine what those units are, either consult
the roster below, or run the following command:
<pre>
stilts tmatch2 help=matcher
</pre>
which will tell you about all the known matchers and their associated
<code>params</code>, <code>values*</code> and <code>tuning</code> parameters.
</p>
<p>The following subsections list the basic <code>matcher</code> types and the
requirements of their associated <code>params</code>,
<code>values*</code> and <code>tuning</code> parameters.
The units of the required values are given where significant.
</p>
<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>
<hr><a href="SkyMatchEngine.html">Next</a> <a href="match.html">Previous</a> <a href="match.html">Up</a> <a href="index.html">Contents</a> <br> <b>Next: </b><a href="SkyMatchEngine.html">sky: Sky Matching</a><br>
<b>Up: </b><a href="match.html">Crossmatching</a><br>
<b>Previous: </b><a href="match.html">Crossmatching</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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