UCDs status and perspectives

[Ray Plante]

Hi,

> However, is there a means of transition from the present use of UCDs, e.g.
> by associating columns
> FREQ  FLUX_DENSITY FLUX_DENSITY_ERROR
> GHz      mJy           mJy
> 1.4     5              1
> 4.8     10             1
> 22.0    20             3
> 
> or whatever?

The need to associate columns and attach a collective meaning to them is
an issue that has come up numerous times.  The obvious case is that of RA
& Dec values that are stored in separate columns; however, as the above
example implies, the problem is more general.

In the NVO project, we have discussed ways in which VOTable might be 
extended to include descriptions of so-called "frames" (as in reference 
frame) that encapsulates these associations.  This would be made easier if 
it were possible to use exented metadata (e.g. Arnold's space-time 
coordinates schema).  We've discussed XML mechanisms to allow this; I 
think the next step, if we want to pursue this, is for someone to make a 
proposal.

> Secondly, however, how much can be done 'seamlessly' based on the
> existing, working CDS UCD system?  It is important to keep that going, not
> just because astronoemrs depend on it and want to see it do these things
> they hear about in reports from VO meetings, but because we need to test
> developments on the community as much as possible, and we will find
> subjects much more easily if we can give people prototypes which look as
> familiar as possible (as with the AVO demo).  Also many new functions e.g.
> the SED tool and VOPlot are being developed based on the existing UCDs.

I completely agree.  It will take some time to shake out a refactored 
model that covers everything already in the current UCDs.  Therefore, we 
will need a mechanism for continuing to support UCDs as they are now.

It's important to note that the current UCDs represent the most 
practical use-case as they were drawn directly from real catalogs.  As we 
build up more complicated models, e.g. photometry measurements, even 
images, we will need to make sure that the relevent UCD concepts are 
covered.  In fact, that could be made a formal of the definition process.

cheers,
Ray