UTYPEs in VOTABLE for data models

[Doug Tody]

Hi Jonathan -

On Wed, 31 Mar 2004, Jonathan McDowell wrote:
> > Doug Tody replied:
> > Short answer: given an element which has only a UTYPE tag, and nothing else,
> > you know exactly what it is, that is the interface to which it belongs
> > (protocol, data model, etc.), and the specific parameter or attribute of
> > that interface.
> 
> I disagree slightly.
> 
> That's not the same as 'know exactly what it is'. I think you mean
> 'know exactly how to process it'. If you mean this literally, UCD would
> be completely redundant. In fact, I argued (in an offlist email)
> that UCD and UTYPE are largely orthogonal. To amplify your comment:
> 
>   given an element which has only a UTYPE tag, and nothing else,
>   you know the interface to which it belongs
>   (protocol, data model, etc.), and the specific parameter or attribute of
>   that interface.
>   
>   BUT
> 
>   you do not necessarily know what physics it represents - that is the
>   job of the UCD. I believe there will be instances which
>   have identical UTYPE but different UCD. 

We are in agreement here.  The UTYPE tells us, for a data element,
precisely and directly "what it is" in some interface or data model.
>From UTYPE alone don't know anything about the physical meaning of an
element without understanding the role of the element in the associated
interface or data model.  UCD does convey some information about the
physical meaning of the element it is assigned to.  This is the essential
role of a UCD.  If the UCD does not do this (e.g., a UCD of "wcs.crval")
then probably we are misusing UCD and the UCD is just pretending to be
a UTYPE.

	- Doug