Sexagesimal metadata

[Anita M S Richards]

Dear all,

Apologies for butting in as a lurking user, but if the many service providers can be persuaded to be more consistent in the sexagesimal format required for queries this would save users many hours!

CDS at least provides examples and documentation.  But there are so many variants out there (including ALMA using 12.12.12.12 for +12:12:12.12 Dec) that it is usually impossible to take a list from one data provider and use it to query another without extensive use of SED or whatever...  If standardisation is not possible is there any chance of developing and providing translation code to service providers so that if I entered e.g. 12h12m12.12s +12d12m12.1s or any likely variant, this would be translated into whatever their service needed?


many thanks
Anita

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Dr. A.M.S. Richards
Senior visiting fellow, JBCA, University of Manchester, M13 9PL
a.m.s.richards at manchester.ac.uk (please do not use old  @jb.man.ac.uk email)
I work the hours which suit me. I do not expect instant replies, I respect whatever hours you work.

________________________________
From: dal-bounces at ivoa.net <dal-bounces at ivoa.net> on behalf of Mark Taylor <m.b.taylor at bristol.ac.uk>
Sent: 10 March 2022 17:17
To: semantics at ivoa.net <semantics at ivoa.net>
Cc: Gilles Landais <gilles.landais at astro.unistra.fr>; dal at ivoa.net <dal at ivoa.net>
Subject: Sexagesimal metadata

Dear Semantics and DAL,

following the report I gave on VOUnit usage in the VO at the last interop
(https://wiki.ivoa.net/internal/IVOA/InterOpNov2021Ops/uuc.pdf)
Gilles Landais at CDS has done a good job of tidying up the units
reported by VizieR to conform to the VOUnit standard (the results are
not yet visible in the public VizieR service).

But one issue that remains is how to flag up sexagesimal quantities,
which are currently marked up like this:

   <FIELD name="RAJ2000" ucd="pos.eq.ra;meta.main" ref="J2000"
          datatype="char" arraysize="12" unit="'h:m:s'">
   <FIELD name="DEJ2000" ucd="pos.eq.dec;meta.main" ref="J2000"
          datatype="char" arraysize="13" unit="'d:m:s'">

The unit, by long VizieR tradition, is quoted as 'h:m:s' or 'd:m:s'
(including the single quotes), which is probably recognised as an
ad hoc indication by quite a bit of client code out there.
However this is not legal VOUnit syntax, and they're not really units
anyway since the columns they apply to are not numeric, but it's not
obvious how to indicate sexagesimal content of columns in VOTable metadata.
You can infer it by noticing ucd="pos.eq.*" and datatype="char",
but it seems like there ought to be a more explicit way to do this,
especially considering how common this case is in the tables we deal
with.

So: what would be the VO-compliant way for CDS to proceed here?
(note I'm not claiming that they are going to follow any such advice,
that's for Gilles to decide).  Do we just leave consumers to make a
good guess based on UCD and datatype?  Could it be a job for DALI/xtype?
Something else?

Mark

--
Mark Taylor  Astronomical Programmer  Physics, Bristol University, UK
m.b.taylor at bristol.ac.uk          http://www.star.bristol.ac.uk/~mbt/
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