Marking up HEALPix index columns in VOTable

[Markus Demleitner]

Hi Apps,

On Thu, Aug 11, 2016 at 04:03:37PM +0200, Marco Molinaro wrote:
> if the order is what is needed,
> considering the MOC standardization to be in place,
> I vote for a solution that is a part of Rick's suggestion
> 
> N -> pos.healpix;meta.number
> 
> into a table level PARAM.
> Maybe mixed to a single addition to UCD words:
> 
> meta.order
> 
> if we want some more specific detail and a reusable word.
> (thus going pos.healpix;meta.order)
> 
> However I must say that I'm working on a tableset that,
> if it were to return such field, would have a non-homogeneous
> order for tessellation. But this case is not covered by the
> "horrible" solution either.

If I understand correctly what your problem is, I'd say it'd be
solved by what I think we need anyway to allow multiple healpix
columns (and hence potentially multiple orders) per table: some form
of grouping.

I'm still not a great fan of this since it involves referencing, and
getting referencing right always is a bit tricky -- but if people
really consider using VALUES as prohibitively ugly (while I'd still
say healpix ranges and hence orders are a perfectly legitimate use
case), there's probably no way around it.

Let's at least keep that as simple as we possibly can, then.  I think
the minimal thing we can say is:

  A FIELD containing a healpix index is marked up with a
  ucd="pos.healpix".  It must be referenced from a GROUP that will
  normally contain a PARAM with pos.healpix;meta.number that contains
  the healpix order (log_4(nsides)):

    <GROUP>
      <PARAM datatype="short" name="order_1"
        ucd="meta.number;pos.healpix" value="3"/>
      <FIELDref ref="hp_1"/>
    </GROUP>
   ...
   <FIELD id="hp_1" datatype="long" name="hp_1" ucd="pos.healpix"/>
  
  [meta.number is P, so it must be first, and I'd argue that's a good
  thing, too; I'd advocate having meta.order, too, though].
  In case a column contains healpixes of differing orders, the order 
  can also be stored in a separate column:

     <GROUP>
      <FIELDref ref="order_2"/>
      <FIELDref ref="hp_2"/>
    </GROUP>
    ...
    <FIELD id="order_2" datatype="short" name="order"
        ucd="meta.number;pos.healpix"/>
    <FIELD id="hp_2" datatype="long" name="hp_2" ucd="pos.healpix"/>

  From a consumer point of view, the rule is: If a FIELD has a ucd or
  pos.healpix, look for a GROUP that contains a FIELDref to that
  FIELD.  This GROUP must contain either a PARAM with ucd
  meta.number;pos.healpix or a FIELDref with this ucd.   Obtain the
  order number from there.  If no such GROUP, FIELDref, or PARAM can
  be found, ignore the FIELD or raise an error.

It may not be overly pretty, but it's not overly complex either and
neither requires prior nor likely complicates later DM work.

And I really can't convince you that simply going for VALUES (which,
yes, cannot cover the inhomogeneous case) would do the trick for now?

         -- Markus