UTYPEs and UCDs

[Jonathan McDowell]

About UTYPEs and UCDs:

> If I get you correctly, Frames holds all the necessary info.
> Are you saying that we do NOT need UTYPES at all?
> Alberto

I think UTYPEs are needed only for VOTable. They are a way to describe
the mapping of the VOTable back into the schema. You wouldn't need a
UTYPE if you serialized the model as `pure' XML.

UCDs are another story. I see the role of UCDs in the data model as
distinguishing different physics which is represented by the same
computational structure. I will trot out here the same example I've used
in the discussions we've had about SIAP/SSAP: consider a spectrum. The
same data model (and hence, if in VOTable, the same UTYPEs) could be
used to describe a spectrum of surface brightness versus wavelength, and
of flux density versus frequency; we would use UCDs to describe the
physical nature of the observable and the spectral coordinate. I think
UCDs should also be used wherever the VALUE of a data model attribute is
one of an enumerated set (e.g. in the STC the possibilities of
heliocentric, geocentric, etc.).

Now Gerard and Pat (and maybe even Brian?) would say, I imagine, that
the right way to do this in a data model is not to use UCDs, but to have
separate classes for each physical concept. What that loses, for me, is
the fact that for a broad group of applications the two different
observables `flux density' and `surface brightness' and the two
different coordinates `wavelength' and `frequency' will be handled by
software in the same way; all we care about for many purposes is that
it's a one-dimensional array, not what the axes are. Using a model in
which the commonality is explicit and the differences are handled by
having the UCD attribute with different values instead of by having
different classes is a compromise between
old-style-everything-hard-coded and extreme OO approach; we simplify the
problem by having relatively few software classes and handing the hard
work over to the UCD definition list.

I haven't assimilated the 50 mails that happened while I was at lunch;
more this evening.

 - Jonathan