VODML base types : request for enhancement of the IVOA.1.0 template model

[Markus Demleitner]

Dear DM,

On Mon, Feb 16, 2026 at 09:39:55AM +0000, Paul Harrison via dm wrote:
> I think that if a particular model element have a constant UCD then
> it should conceptually by modelled with the SemanticConcept - but
> it seems that the type of “topConcept” means that it cannot be used
> to express the UCD string and there is possibly no conventional
> value for the vocabularyURI for UCDs. (see
> https://github.com/ivoa/vo-dml/issues/19)

First off, VO-DML predates the current vocabulary spec, and therefore
limits itself to SKOS.  That limitation should be lifted
independently of the UCD business.  I'd be happy to review the
language on vocabulary usage for VO-DML 1.1, but I'd need some
poking.  Perhaps we should even have an RDF serialisation for the
instances?

For the UCD question, however, all that iss not terribly helpful.
The reason is that UCDs have syntax.  Modelling the UCD atoms as a
proper vocabulary is probably possible, and there's even a draft PR
for that <https://github.com/ivoa-std/Vocabularies/pull/31>, but
you'd want ("compound") UCD words here, and RDF can't (really) do
that.

Me, I'm still skeptical that VO-DML should say a lot about UCDs
beyond mentioning that Quantity-s can be adorned with them in
sufficiently capable serialisations.  But *if* there is a strong
reason to say "this must be a valid UCD", that'll need extra logic
beyond RDF.

Thanks,

            Markus