UCD versioning?

[Rob Seaman]

Hi,


Could somebody comment on the relationship between http:// 
www.ivoa.net/internal/IVOA/IvoaUCD/UCD-1.9.9b.pdf and http:// 
www.ivoa.net/Documents/latest/UCD.html?  Is the former obsolete?  It  
hasn't been updated in two years.

In any event, I don't see any discussion of a versioning strategy.   
Unfortunately, I also can't seem to get a search to work on the list  
archive (http://www.ivoa.net/forum/ucd).  Surely this topic has been  
discussed (likely in gory detail)?  Or is there some notion of "once  
UCD, always UCD"?

The recent discussion about UCD maintenance seems to me to be  
orthogonal to the question of explicit versioning.  Are new UCDs  
simply added to the end of a list that preserves no historical  
memory?  Or are new UCDs appended to only the latest version of the  
list?  Are UCDs ever removed from the list?  Are their names or  
locations in the hierarchy never to be changed?

And is the interpretation of a document that uses UCDs going to vary  
as the master list changes over time?  The process for adopting UCDs  
is intended to be very conservative, but even so, one imagines that  
some UCDs will be adopted that will later be rethought.  Note that  
even if the UCDs associated with a particular document (or image or  
catalog) don't change, that the nuances of the interpretation of  
these items might change as new - unselected - UCD options are added  
to the master list.  An assertion versus three options may turn into  
an assertion versus six options.

I gather there is some thought of occupying UCD-space only until we  
enter the promised land of the ontology-sphere.  One might be  
forgiven, perhaps, for imagining that this may well take a sizable  
fraction of the Biblical 40 years of wandering, however.  That being  
the case, what provisions are in place to allow UCDs to thrive in the  
ontology-free wilderness?

Could someone summarize how UCD versioning is - or is not - supposed  
to work?  Alternately, feel free to point me toward pertinent  
discussions.

Thanks!

Rob Seaman
NOAO