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#### A.9.1.3 Multiple Positional Search Panel

Multiple Positional Search Panel

The Multiple Positional Search Panel appears in VO-based multiple query windows which specify a procedure in which one query is made for each row of an input table. Each of the input rows defines a (probably small) region on the sky. The purpose is to find data records (about catalogue entries, images or spectra) in a remote catalogue corresponding to each row of a local catalogue. As such, it is used to define a kind of join, of a local Apparent Table with a remote one, where the remote one is exposed via a VO data service.

To use it you must fill in the URL field to select the remote service, and fields defining how each row of the input table can be turned into a positional query. You also need to define what form the result will be returned in. These parts are described below.

The URL field specifies the data service which is to be queried:

URL
  This must contain the service URL for the data service that you are querying. Usually, this will be filled in by selecting one of the services obtained by the registry query. However, you can fill it in manually with the URL of a service you know about if you prefer. If you know what you're doing, it's also possible to doctor a service URL filled in by the registry selection, for instance by adding `&name=value` parameters to it.

The following fields define what queries are to be sent to the service:

Input Table
  Select one of the tables loaded into TOPCAT whose row positions you want to search around.
RA and Dec columns
  Fill these in with the columns in the input table which contain J2000 Right Ascension and Declination (you can perform coordinate conversions using the Sky Coordinates Window if required). Select the units as appropriate. If the input table has appropriate metadata, the correct values may be filled in by default, but it may occasionally guess wrong, so it's wise to check that they are filled in correctly.
Search Radius column
  Fill this in with a value that represents the size of search that you want to make around each row's position. You can either use a constant value, or a column name, or any numeric algebraic expression based on the table (see Section 7). Select units as appropriate. This field has slightly different appearance and meaning for the different data service types; hover over it with the mouse to see a tooltip with the details. For SIA and SSA, but not for Cone Search, it is permissible to leave it blank (though certain services don't seem to like that). Select the units as appropriate.

The Output Mode selector indicates what will be done with the result.

Output Mode
  When the queries have been performed, there are different ways that the results can be returned. Since the operation here is basically a crossmatch between a local table and a remote one, this effectively describes what sort of join is to be done. The options are as follows:
  New joined table with best matches
    A new table will be generated, containing one row for each row in the input table which returned at least one search result from the remote table. The best (closest) match will be included; any others will be discarded. The new table will have all the columns of the input table and all the columns returned by the data service.
  New joined table with all matches
    A new table will be generated, containing one row for each search result returned. Rows from the input table will be duplicated where more than one search result was returned corresponding to that row. The new table will have all the columns of the input table and all the columns returned by the data service.
  New joined table, one row per input row
    A new table will be generated containing the same number of rows as the input table. If any search results were returned for each row, the best (closest) one is included. Any others will be discarded, and cells will be empty for rows with no search results. The new table will have all the columns of the input table and all the columns returned by the data service.
  Add subset for matched rows
    No new table will be generated, but a new subset will be added to the table which includes only those rows which returned at least one search result.

The final controls adjust the details of how the individual queries are submitted to the remote service.

Parallelism
  Controls how many cone search queries are running at once. By making several queries to the service concurrently, the time it takes to fill in the whole table can be much quicker than making the query for the first row, waiting for the result, making the query for the second row, etc. The value here is approximately how many queries will be run at the same time. Increasing the value might make your multiple query run faster; or it might overload the server and make you unpopular with the service administrator, or result in your query taking longer or failing altogether, or both. The default value of 3 is probably reasonable, but experiment with adjusting it if you want.
Error Handling
  Determines what happens if one of the queries fails with an error. The options are:
  abort
    terminate the operation; the whole multiple search fails
  ignore
    treat a failed query the same as one which returns zero rows
  retry N times
    try N times (with increasing delays) to get a non-fail result
  retry indefinitely
    keep trying (with increasing delays) for a non-fail result
  The best setting for this depends on the way the service is set up; the default is abort, but for unreliable or poorly implemented services it may be better to continue operation in the face of a few failures.

When all of the fields are filled in (defaults can be used for many of them), hit the Go button, and the search will commence. It may be quite slow; a progress bar is shown at the bottom of the window. When it completes, a popup window summarising the result will be shown. If you get bored waiting, you can hit the Cancel button, and perhaps adjust the parameters and try again.

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TOPCAT - Tool for OPerations on Catalogues And Tables
Starlink User Note253
TOPCAT web page: http://www.starlink.ac.uk/topcat/
Author email: m.b.taylor@bristol.ac.uk
Mailing list: topcat-user@jiscmail.ac.uk