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[Francois Ochsenbein]

Hi Patricio, et al.,

This question of accurate/inaccurate description was discussed
many times --- and one of the results was the definition of 2 
different entities :
-- the UCDs which have the role of broad classification
-- the "utype" was introduced to fulfil the role of the accurate
   description -- the "utype" being in principle connected to a 
   data model which gives the necessary level of detail.

In the photometric domain, each of the two entities has its role:
-- from a ucd: 2 quantities showing the same UCD can be compared 
   at least in a statistical way; 
-- from a utype: the accurate specification of each filter must be
   accessible; quantities can then be combined. 
   In practice however the required level of details is frequently 
   not fully known. 

There is exactly the same problem in the astrometric domain:
while UCDs can be used to define the RA and Dec, an accurate
definition of these values requires a lot of extra parameters
-- have a look at Arnold's Space-Time model and be sure you
have specified everything if you want to be accurate...
In many practical cases -- e.g. for statistical cross-matching --
a full description is not required, and then the UCDs have their
place. 

--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 17
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