Sexagesimal metadata

[Mark Taylor]

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/