Re: Expressing 2- and 3-D coordinates

About the question of arrays in cells -- as far as VOTable is concerned the question is not the _possibility_ of storing the position as a vector (it's quite possible, Alberto !) but the _requirement_ of grouping the components of the position, their error, etc in dedicated structures that cannot always be compatible with the VOTable definition.

There are two main problems:

• it happens that only 1 component of the position is present (e.g. meridian catalogs measure the RA, not the Dec).
• an array is not just a set of numbers -- in VOTable it's required that all elements are expressed with the same data type (e.g. double) and expressed with the same unit (this makes physical sense, too). It's obviously a problem for the spherical 3D coordinates (2 angles and a distance); errors ellipses are also problematic

One way of resolving this dilemna in VOTable was proposed at the Spanish IVOA meeting in last October, with a convention on the contents of the "utype" attribute (the attribute which aims at describing exactly what the column represents, as opposed to the broad classification provided by the "ucd" attribute): use conventions similar to the XPath/XQuery syntax to refer to the exact parameter or quantity described in a data model. With such a convention it becomes possible to use in VOTable either the vector representation, or the individual components:

• <FIELD name="pos" type="double" arraysize="2" ucd="pos.eq" unit="rad" utype="stc:AstroCoords[coord_system_id='J2000-OPTICAL-ET']/Position2D" > implies that the position is stored as a 2D vector e.g. <TD>187.2779 2.0524</TD>
• <FIELD name="RA" type="double" ucd="pos.eq.ra" unit="rad" utype="stc:AstroCoords[coord_system_id='J2000-OPTICAL-ET']/Position2D/Value2(1)"
  >
  implies that the column contains the first component of the position e.g. <TD>187.2779</TD>

The contents of the utype becomes somewhat indigest and not intuitive -- but on the other hand it has the decisive advantages of:

• non-ambiguity: "RA" becomes "the first component of the 2-D value of the astrononomical coordinate system defined in the STC model under the designation 'J2000-OPTICAL-ET'") and a program can easily gather the various components of a datamodel refered in a VOTable and take the appropriate action (even ask a robotic telescope to point successively to the positions given in the table rows...)
• transformability: the relevant columns of each row can be converted into the schema defined by the data model (e.g. the STC model)
• verifiability: if necessary, check that all the required parameters of the data model are present, with the proper data types, etc.

This proposal was not accepted at the last IVOA meeting -- but I feel it's really worth to consider this possibility of bridging the existing data models with large data sets expressed as VOTables. Any comments ?

Cheers, francois

Francois Ochsenbein       ------       Observatoire Astronomique de Strasbourg
   11, rue de l'Universite F-67000 STRASBOURG       Phone: +33-(0)390 24 24 29
Email: francois-at-astro.u-strasbg.fr   (France)         Fax: +33-(0)390 24 24 32
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