Re: Format of concatenated UCD's

Rob,

On 2005 Jun 9 , at 22.19, Rob Seaman wrote:

> Is there some technical reason that Rick's example can't become
> "con:event.burst ucd:em.opt"? Let's try to avoid getting lost in the
> semantic underbrush. Please tell me why we can't have separate
> namespaces?

Myself, I don't think there's anything wrong with con:event.burst;ivoa:em.opt. The strong nervousness which others have about the use of UCD namespaces, as expressed for example in the UCD1+ document, I understand but do not fully share.

There seem to be two issues.

1. (semantic) If you have a namespaced UCD atom, then not all software will understand it. This is true, but it's also rather in the nature of namespaces that they are an acknowledgement that the namespaced thing is special or non-standard, so that it will only be useful to certain processors. They also avoid the atom being confused with other atoms which happen to have the same name (this is of course The Point), but _usually_ give some indication of where a fuller explanation may be found. That is, they're a complication, but avoid a worse problem.
2. (syntactic) How do you associate a namespace prefix with a URI (for that is the only sane way of pinning down a namespace)? In the XML context, this is straightforward, since you can simply use the same namespace resolution algorithm as exists in XML (I believe it would be bad to specify a different one). That has the problems that it makes namespaced UCDs somewhat context-dependent, and that the namespace algorithm has some confusing edge cases. It represents more complication, but is perfectly manageable, I think.

How do you do the association in non-XML contexts, such as FITS headers, or in a database? That's more complicated, and is probably inevitably messy. One possibility, I suppose, would be to follow the unstandardised but common XML habit of writing {namespace-uri}namespaced-thing, as in
'{http://www.ivoa.net/ucd}em.opt'. Not pretty, and it could easily get very long-winded.

All this is a long way of saying that there are technical worries about using namespaces, but that they don't seem to me to be killing ones.

The advantage of using namespaces is that they allow sets of `ucds' like the VOConcepts to be defined, which don't clutter the generic UCD vocabulary, but which do inherit the precision in the UCD spec, and any updates follow it. ...it seems to me.

All the best,

Norman

-- 
----------------------------------------------------------------------
Norman Gray  :  Physics & Astronomy, Glasgow University, UK
http://www.astro.gla.ac.uk/users/norman/  :  www.starlink.ac.uk