How precise should UCDs be?

[Bernard Debray]

Dear all,

Jonathan McDowell wrote:

>Francois Ochsenbein wrote:  
>
>>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  
>
>My own thoughts on the relation between UCD and utype/DM have evolved a
>little (although it comes to essentially the same answer as Francois). I
>believe that the UCD should have a precise and accurate definition, but
>that when you need metadata, or parameters,  i.e. you need to refer to 
>other UCDs, the utype/DM is needed. It's not a question of level of
>detail or "accurate", it's a question of "complete" - do you need
>more than one number of value to describe the idea?
>
>...
>But 
>  RA2000 and RA1950 are the same basic concept differing only by a parameter.
>  The concept 'RA' and the concept 'equinox' are the two different concepts
>  here:
>  pos.eq.ra      (value = 12:26:03.4) and
>  time.equinox   (value = 2000.0)
>
>and these are related to each other in the context of a coordinate frame
>(e.g. STC) data model. RA1950 and RA2000 are really the same physical
>concept simply with different metadata. pos.eq.ra is PRECISELY defined,
>it's just not COMPLETELY defined without tying it to the STC metadata.
>
>...
>
>Of course for pragmatic reasons we do break this paradigm for
>the "phot.*;em.*" UCDs where the  "em.IR" is really metadata for the
>"phot.*" concept. But in general I think this is a good rule of thumb
>for deciding whether you need a new UCD, or whether you need to
>relate two different UCDs via a data model.
>
>>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. 
>>    
>>
>The key phrase here is "requires a lot of extra parameters". In some
>other practical cases, a full description *is* required: so you need the
>parameters and a data model to relate them to each other. But if the
>data model is very generic you still need the UCDs to describe precisely
>what the individual parameters are.
>
>...
> - Jonathan
>
Yes, definitely.
If I go back to my particular initial concern about photometric 
bandpasses, I pointed that the definition of the UCDs for 
electromagnetic spectrum in the document NoteEMSpectrum-2004052-3.pdf 
which is still referred in the last version of the UCD1+ Working Draft 
Document (2005-04-29) is somewhat ambiguous for the optical and 
near-infrared parts. For example, the note in the table for em.opt.U 
says "B band" which, I think, for the average astronomer is Johnson B 
filter bandpass. On the other hand, there is no ucd to define a 
photometric system.  Therefore in this case,  the definition is not 
"precise" in the sense that it can be confusing.

On the other hand, I would tend to think that in order to be used in 
registries, UCDs should allow a more complete description than for being 
used in  statistical cross-matching (e.g. "I need a resource which has g 
magnitude in the SDSS photometric system for a given part of the sky"). 
 Does that mean this should be envisaged through data modelling as well  ?

 
Bernard Debray                       bernard.debray at obs-besancon.fr
Observatoire de Besançon             http://www.obs-besancon.fr/
BP 1615  25010 Besançon Cedex France