Re: How precise should UCDs be?

On Tue, 10 May 2005, Alberto Micol wrote:
> On May 10, 2005, at 14:06, Jonathan McDowell wrote:
> > 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.
> >
> > This is of course fuzzy, since sometimes - as in parts of STC - the
> > utype implies what the UCD must be. But sometimes the utype describes
> > only the precise relationship between two concepts, and the UCDs
> > describe the precise identity of the concepts - both precise, but
> > doing different jobs. Thus you might use utypes to
> > link a velocity and its standard of rest, but you still need to
> > use UCDs to explain that this is the escape velocity from the
> > star (src.veloc.escape) and not the expansion velocity of the
> > wind (src.veloc.expansion) that you're talking about.
> > [...]
> > Group 2
> > - the velocity of what? (distinguishing different physics concepts):
> > src.veloc.dispersion
> > src.veloc.escape
> > src.veloc.expansion
> > src.veloc.microTurb
> > src.veloc.pulsat
> > src.veloc.rotat
> >
>
> Devil's advocate point of view
>
> It looks to me as if you are trying to model a generic astronomical
> source
> by using UCDs. Wouldn't that be the role of a "Generic Astronomical
> Source Data Model"
> instead?
> That is, the UCD could be src.veloc leaving to the utype of the GAS-DM
> to tell which part of the source is being described.
>
> Drawing a line between UCDs and UTYPEs is something we have not
> achieved yet.
>
> The fact is that we have never really tried. I have never seen a VOTABLE
> using both UCDs and UTYPEs; until we see one and we experiment with it
> I fear we could philosophically talk about it for ages.
> We need real examples (self-criticism: hence we need some DM :-) ).
>
> In my view, precise UCDs means: a pragmatic, though incomplete,
> approach;
> an approach that is useful to fill a gap: the absence of a DM.
>
> To counter-balance such sentence I can certainly add that
> on the other hand, we will never have all the DMs we would need.
>
> Help!
> :-)
>
> Alberto

Alberto,

that's exactly my problem! I've never seen any VOT with both ucds and utype that would allow me to think how to identify quantities which are "kind of equivalent" or "equivalent enough" so the user has a short list to pick what is of his/her interest.

Whether that line exists, or how it will be defined is quite important. We have already a number of people (astronomers and non-astronomers) wondering why we bother with UCDs (let alone utype!). I second your position that we need practical examples, not just philosophical discussions on how we should write this or that. In my case, the clash with reality comes when consulting a registry (or the META tables in vizier) trying to retrieve resources of a particular type. Personally if I want to get catalogues with HST blue data, I'd like to get back a list with as little overhead as possible, not five times larger, poluted by all other observations in the blue bands which fall into the same box... again, I haven't seen utype widely used yet. If I could narrow down the search using utype + ucd or utype alone, fine, we can always write a piece of software to perform this task.

My complaints last night had to do with the fact that up to this point in time, using only ucd1+ our searches would bring back too much overhead.

Cheers,

Patricio

---
Patricio F. Ortiz			pfo-at-star.le.ac.uk
Department of Physics & Astronomy	Phone: +44 (0)116 252 2015
University of Leicester			
Leicester, LE1 7RH, UK