On Thu, 3 Aug 2006, Mark Taylor wrote:
> I imagine this will have to wait until Peter gets back, but if there's
> any other TST enthusiasts out there who want to chip in, feel free.
>
> Section 11.1 of SSN/75.1 says:
>
> \item By default, the first three columns in the table are: an identifier
> (that is, an object name), Right Ascension and Declination. These
> columns are usually called {\tt Id}, {\tt ra} and {\tt dec}
> respectively.
>
> \item If the first three columns of the table are not an identifier,
> Right Ascension and Declination then the TST should contain the
> following three parameters:
>
> \begin{quote}
> {\tt id\_col \\
> ra\_col \\
> dec\_col}
> \end{quote}
>
> The value of each parameter should be the number of the appropriate
> column or, if there is no such column, {\tt -1}. The column number is the
> sequence number of the column in the list of column names, starting
> counting at zero.
>
> However, if I write a TST table with the line
>
> id_col: -1
>
> then GAIA complains "Error: column index -1 out of range" and won't
> plot the data. This looks like a GAIA/SkyCat bug, but it may be an error
> in SSN/75. Either way, since it's presumably worked like that for
> a while, I presume that most TSTs in the field avoid writing id_col: -1,
> so for now I will try to do the same.
Hi Mark,
Clive's right and GAIA uses the Skycat parsing for TAB tables. In Skycat
only the ra_col/dec_col and x_col/y_col parameters can be set to -1, the
id_col value must either have a column set, or it will use the first
column, regardless. The reason for this is nothing to do with any
standards (where they exist), just the practicalities of the
implementation which requires a column of identifiers for controlling row
selection (these should be unique but don't need to be just to get the
plotting to work). When possible during an import of a table without an
obvious identifier column I just create a simple integer index column.
Cheers,
Peter.
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