Marking up HEALPix index columns in VOTable

[Frederic V. Hessman]

Mark,

> On 8 Aug 2016, at 18:55, Mark Taylor <m.b.taylor at bristol.ac.uk> wrote:
> 
> Gregory Mantelet, Markus Demleitner and I have been having some
> discussions about exchange of HEALPix maps within the VO.
> ...
> So far so good; however, marking up a column like this provides
> insufficient information to make it useful, because to make sense
> of a HEALPix index you need to know what Order N of tiling it represents
> (HEALPix splits the sky into 12*4^N tiles, where N is an integer
> that for practical purposes falls in the range 0..29).
> Since UCD words cannot be parameterised (beyond composing them with
> other UCD words) there doesn't seem to be any way to encode this
> Order information in the UCD.
> 
> Markus's clever idea is to do something like this:
> 
>     <FIELD datatype="int" ucd="pos.healpix">
>       <VALUES type="legal">
>         <MAX>49151</MAX>
>       </VALUES>
>     </FIELD>
> 
> Since the legal maximum value for the column is 49151 (=12*(4^6)-1),
> it's possible to infer that N=6 for this healpix column.  Note that
> since type="legal" this does not say that the actual contents of
> the column hit that maximum (i.e. some pixels might be absent, but the
> declaration would not change).
> 
> This is kind of horrible, but at the same time the meaning is fairly
> unambiguous, and it requires no new standardisation activity.
> So, we are likely to go ahead with marking up HEALPix columns in
> this way (Markus's and Gregory's services will emit VOTables like
> this, and topcat will understand them) unless anybody persuades
> us different.

Clever but beyond horrible.  Why can’t you simply

>   - Some business with GROUP/FIELDref/PARAM elements at table level.
>     Note this also needs some convention to identify the healpix-order.

Yes, but the number of metadata elements needed is very small, say

	“pos.healpix.order"	=	pos.healpix;meta.number	=	N

	“pos.healpix.tiling"	=	pos.healpix;pos.wcs.ctype	=	“NESTED” or “RING"

The first works because the map needs some other number other than the number of tiles and the only other HEALPIX number is N.

I used pos.ctype in the 2nd in a slightly different manner from what we’re used to, but this UCD should be freed from its FITS fetters, since the meaning should be clear - the tiling scheme expresses a world coordinate system “type”.

Rick