refframe vocabulary

[Markus Demleitner]

Dear colleagues,

[apologies for the wide cross-post, but there's something in this for
everyone; I'd suggest followup to semantics]

For STC2, we will need an updated list of reference frames, and
current plans are to keep the list of reference frames in a
vocabulary, in particular to facilitate quick updates as they are
need (e.g., for solar system bodies).

With the resurrection of COOSYS, it further seems to me we should
keep VOTable's notion of refframes in sync with this effort, which in
turn would make it prudent to maintain existing VOTable terms as
well.

With these considerations, based the STC1 list, VOTable COOSYS, and
previous work by Mark CD, I've made a first draft vocabulary for the
reference frames at http://docs.g-vo.org/rdf/refframe/.

I'd be great if you could review the terms and their explanations.
Specific to-do points from me:

* I've removed the Earth reference systems for now; I suspect we
  might need them for observatory positions and such, but I'm sure we
  should only include here what we absolutely need (use cases!)

* There are also no planetary reference systems.  These probably
  should hang off of BODY when they come in.  I'd be really grateful
  if we could normative references for all of these (Galactic and
  ICRS show how I'd like this to look like).

* Speaking of references, once you start digging, it turns out
  ECLIPTIC becomes a tricky term.  If you can supply a reference for
  what this probably means where it's being used, I'd be really
  happy.  Oh, and if you have something ready for SUPER_GALACTIC, I'd
  like that as well.

* There are three VOTable terms I've not found sufficiently defined,
  and perhaps someone on Apps still remembers what they were supposed
  to be:

  (1) xy -- is this really UNKNOWN?  VOTable 1.1 speaks of "user
      defined" here, and it looks a bit... well, cartesian.  Hm.
  (2) barycentric -- this sounds a bit like ICRS BARYCENTER, where
      one thing would cover both the refpos and the refframe.  Is that 
      what it is?  If not, what else?
  (3) geo_app -- Ummm.  Do I appear extremely dumb if I admit I can't
      even speculate on this?

As usual, everyone is invited to do edits on volute:
https://volute.g-vo.org/svn/trunk/projects/semantics/vocabularies/refframe

The terms are stored in a simple CSV file, with just a bit of
hierarchy implied through the second column, so don't panic.  No RDF
fu required here.

Thanks,

            Markus