[UCD] Question about representation of HEALPix pixel IDs

[Mark Taylor]

Gregory,

On Tue, 17 Apr 2018, Gregory Dubois-Felsmann wrote:

> I am in the position of defining a compact representation that can be used to transmit a HEALPix ID (or a list of them) from one application to another, representing, for instance, the selection of a pixel to query or operate on.  In my specific application I could just specify “out of band” that the IDs must be in NUNIQ form, but I thought I’d try to be a good citizen and make the data packet generic enough that it could support the other representations.

I applaud your thoughtfulness about it, but I'll mention that to some
extent in the VO we try to foster interoperability by *avoiding*
that kind of flexibility rather than catering for it; for instance
the MOC format mandates (out of band) NUNIQ, NESTED and ICRS,
and Simple Cone/Image/Spectrum access protocols mandate ICRS.
The general idea is to avoid causing extra implementation work
for clients by reducing the number of ways to encode essentially
the same information.  Whether that's an appropriate consideration
for you depends on the details of your application, but it may
be worth thinking about.

> In this regard, I also don’t see an MType for the transmission of a HEALPix ID or list of IDs in SAMP, which is something I’d also like to support.

I don't know of a HEALPix MType either.
If you have two clients that want to exchange HEALPix IDs, then
I suggest you draft an MType of your own with reference to the
existing http://wiki.ivoa.net/twiki/bin/view/IVOA/SampMTypes list
and  try it out - if it seems to be useful, then report it to
the apps mailing list and you or somebody else can add it to
list on the wiki.  If you want to discuss the details on the apps
mailing list before implementing it, that's fine too.

However, note that there is an MType for exchanging MOCs -
coverage.load.moc.fits.  See http://www.ivoa.net/documents/MOC/
for a description of the MOC concept and format if you're not
already familiar with it.
If what you want to exchange is coverage information (the encoding
of a sky region) rather than HEALPix pixel IDs or ID lists as such,
that might be a better option for the SAMP communication.
Use of MOCs rather than HEALPix pixel lists might also be worth
considering for your wider problem, since it could address your
semantics issues as well as reducing the amount of data you
need to exchange, and avoid you having to come up with a new
format.

Mark

--
Mark Taylor   Astronomical Programmer   Physics, Bristol University, UK
m.b.taylor at bris.ac.uk +44-117-9288776  http://www.star.bris.ac.uk/~mbt/