1. UCDs Notes on discussion 2003 October 16 14:00 – 16:00

2. Roy Williams and Sébastien Derriere Introduction Need for revision of current UCD scheme Presented UCD working draft V1.9.9 General form: : ; Examples: –arith.radio;phot.flux;em.radio –Ivoa:meta.id;src

3. Wide-ranging discussion How to support covariance, e.g. proper motion and declination? Where is boundary between UCDs and Data Models? Ideally need normalised vocabulary, but IAU Thesaurus is 10 years out-of-date. What happens to UCD if columns are combined, e.g. arith.sum ?

4. Patricio Ortiz Presented analysis of most popular UCDs showing the complexity of the problem. This was followed by a further inconclusive discussion. Some topics raised: –How/whether to support grouping – generally felt to be responsibility of data model: UCD should be applied to a column as an entity. –Is equinox needed as attributed to positional coordinates? –Can UCDs be applied to parameters as well as columns?

5. Tom McGlynn Proposed somewhat simplified scheme for UCD2 –Fewer top-level items, mainly physical concepts such as position or flux. –To be followed by one or more attributes/qualifiers Waveband, measurement type, etc. Value (this one alone may be omitted as the default) Error, max, min, etc.

6. Straw vote taken Do we need new UCDs, or keep old ones –In favour of existing UCDs: ~4 –In favour of some form of new UCDs: ~25 –There were also a few abstentions.

7. Demonstration of Aladin by Pierre Fernique Showed Aladin using UCDs to limit the number of stars plotted on an image –Select [PHOT*] < 16 Can also be done in UCD-2, perhaps even more effectively.

8. Conclusions A move to some form of UCD-2 was generally supported. The simplification and rationalisation proposed by Tom McGlynn attracted relatively few criticisms and several favourable comments. Coordination is desirable on the appendix to UCD-1.9.9 which has a table of waveband names, e.g. em.UV.200-300nm –These should be aligned with the waveband names or classifications proposed by the Registry and Data Model groups.