UCDs and XML

[Ray Plante]

Hi Roy,

I think I missed an important point of your message regarding the
relationship between UCDs and XML.  Are you commenting on whether an
XPath-based UCD can be used in the ways we are currently using UCDs.  I
believe they can, and I don't see any barrier to doing any of the things
you described.  Simply put, instead of using a UCD like "BH_mass", we
would say "BlackHole/mass" (which still fits nicely in a ucd= attribute).  
I think I missed something, though, so please set me straight.

> There is a lot that can be done with UCD, and the real question is how to
> allow this semantic flexibility to blossom, but without having everyone make
> their own UCD and losing all interoperability!

The attraction of an XML-based data model (from which UCDs can flow) is 
that it provides a framework around which we can build interoperable, 
standard metadata.  As a community (through the IVOA), we can adopt 
standard schemas; the standard is represented by a highly structured
document (a schema).  The XML mechanism of namespaces and intermixing of 
schemas allow one to straight forwardly define and use new schemas to 
cover new concepts (e.g. black holes).  If you want wide interoperability 
for those new concepts, it requires adoption by the whole community. 

cheers,
Ray