patricio UCD/Tony

[Francois Ochsenbein]

Following Tony's post on "My understanding of UCDs", I would like
to thank first Tony for his action -- it helps to clarify the discussions --
and I support Patricio's details. I would like to add a few more details:

1. UCDs as Data Types:
   yes, UCDs help to define what is a "legal" operation, but they do not
   replace the units.

2. UCDs as Keywords:
   Querying from UCDs to retrieve which resources do match a set of UCDs
   is one of the possible applications of the UCDs. For example, there is
   a version of VizieR which retrieves the catalogues containing a set
   of UCD at       http://vizier.u-strasbg.fr/viz-bin/VizieR?-ucd
   
   Note that this URL does more than retrieving resources: it can 
   retrieve directly the results by specifying e.g. PHOT_STR* in one
   of the UCD input specification, and a position on the sky, 
   the search find those objects close to the position and which have
   a Stroemgren photometry.

   The UCDs allow more than just retrieving catalogues having this parameter:
   it allow to specify CONSTRAINTS on that UCD (Patricio's Scenario 2)
   something like
   Select ID, POS_EQ_RA_MAIN, POS_EQ_DEC_MAIN, REDSHIFT_HC
     From [all catalogues]
    Where REDSHIFT_HC > 3

   PROVIDING that a rule is defined to specify what to do in case of 
   multiple columns matching a UCD. The default rule could be something
   like for, say, the REDSHIFT_HC parameter:
   -> if a REDSHIFT_HC has a qualifier MAIN , use this one;
   -> otherwise, use the first non-null value of the possible REDSHIFT_HCs.

   The real advantage in using UCDs for actual queries is to be able to
   use GENERIC queries without having first to ask for the details of
   the catalogues (at the registry level or from the resource server).
   And when thousands of catalogues are involved, it makes a difference !
   Notice also that the rule of the "first non-null" value is not
   that easy to write as an SQL statement...
   
3. UCDs as Pointers into Data Model
   I'm not sure to understand the problem -- why could not a DM point
   refer to a set of UCDs ?

--Francois
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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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