Next Previous Up Contents
Next: Match Criteria
Up: Top
Previous: tosql


6 Crossmatching

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 join.

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 Appendix A.

Matching can in general be a computationally intensive process. The algorithm used by STILTS, except in pathological cases, scales as O(N log(N)) or thereabouts, where N 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 105-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 (-Xmx200M).

In the current release of STILTS the only crossmatching task is tmatch2 which finds matches between pairs of tables. In future versions however facilities for finding matches within the same table, and in more than two tables, will be introduced.


Next Previous Up Contents
Next: Match Criteria
Up: Top
Previous: tosql

STILTS - Starlink Tables Infrastructure Library Tool Set
Starlink User Note256
STILTS web page: http://www.starlink.ac.uk/stilts/
Author email: m.b.taylor@bristol.ac.uk