/usr/share/doc/stilts/sun256/matchGroup.html is in stilts-doc 3.1.2-2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 | <html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<link rel="stylesheet" type="text/css" href="sun-style.css">
<title>Multi-Object Matches</title>
</head>
<body>
<hr>
<a href="plot2.html">Next</a> <a href="CombinedMatchEngine.html">Previous</a> <a href="match.html">Up</a> <a href="index.html">Contents</a> <br> <b>Next: </b><a href="plot2.html">Plotting</a><br>
<b>Up: </b><a href="match.html">Crossmatching</a><br>
<b>Previous: </b><a href="CombinedMatchEngine.html">Matcher Combinations</a><br>
<hr>
<h3><a name="matchGroup">7.2 Multi-Object Matches</a></h3>
<p>The generic matching in STILTS is determined by specified
match criteria, as described in <a href="MatchEngine.html">Section 7.1</a>.
These criteria give conditions for whether two items (table rows)
count as matched with each other. In the case of a pair match,
as provided by <a href="tmatch2.html"><code>tmatch2</code></a>,
it is clear how this is to be interpreted.
</p>
<p>However, some of the matching tasks
(<a href="tmatchn.html"><code>tmatchn</code></a> in group mode and
<a href="tmatch1.html"><code>tmatch1</code></a>)
search for match groups which may have more than two members.
This section explains precisely how STILTS applies the pair-wise
matching criteria it is given to identifying multi-object groups.
</p>
<p>In a multi-object match context, the matcher identifies a matched
group as the largest possible group of objects in which each is
linked by a pair match to <em>any</em> other object in the group -
it is a group of "friends of friends".
Formally, the set of matched groups is a set of disjoint graphs whose nodes
are input table rows and whose edges are successful pair matches,
where no successful pair match exists between nodes in
different elements of that set.
Thus the set has a minimal number of elements, and each of its elements
is a matched group of maximal size.
The important point to note
is that for any particular pair in a matched group,
there is no guarantee that the two objects match each other,
only that you can hop from one to the other via pairs which do match.
</p>
<p>So in the case of a multi-object sky match
on a field which is very crowded compared to the specified error radius,
it is quite possible for <em>all</em> the objects in the input table(s)
to end up as part of the same large matching group.
Results at or near this percolation threshold are
(a) probably not useful and
(b) likely to take a long time to run.
Some care should therefore be exercised when specifying match criteria
in multi-object match contexts.
</p>
<hr><a href="plot2.html">Next</a> <a href="CombinedMatchEngine.html">Previous</a> <a href="match.html">Up</a> <a href="index.html">Contents</a> <br> <b>Next: </b><a href="plot2.html">Plotting</a><br>
<b>Up: </b><a href="match.html">Crossmatching</a><br>
<b>Previous: </b><a href="CombinedMatchEngine.html">Matcher Combinations</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>
</html>
|