francois at vizir.u-strasbg.fr
Tue May 10 00:52:39 PDT 2005
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
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
More information about the ucd