refframe vocabulary

[Francois Ochsenbein]

Hi Markus, and friends,

Having recently looked in details in the various coordinate frames, it
turns out that the definition of the Galactic frame is somewhat fuzzy:
it was originally defined in the FK4, but Hipparcos and Gaia introduced
a slightly modified version by fixing the galactic pole and the node
with angles rounded to 10⁻⁵° directly in the ICRS, and therefore
differs from the original at the level of ~50mas. And since the
supergalactic system is defined relative to the galactic, the same
uncertainty exists for that one. Anyway, the list of the frames will
likely have to evolve in the near future...

Cheers, François Ochsenbein

==> Le lundi 2019-04-08 à 16:41+0200,
    Markus Demleitner <msdemlei at ari.uni-heidelberg.de> a écrit:

>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


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