[UCD] Question about representation of HEALPix pixel IDs

[Gregory Dubois-Felsmann]

Dear Semantics WG,

I note that there is a type-‘Q’ entry in the UCD vocabulary “pos.healpix” in the Position and Coordinates category.  I assume that this is intended to describe a data item as an integer ID of a pixel in the HEALPix system.  Since there are three numbering conventions for such pixels - NESTED and RING (which require the additional specification of a HEALPix order or the quantity usually known as “Nside”), and NUNIQ (which encodes both the order and the pixel ID) - some additional information is required in order to interpret the values described by this UCD.  Beyond the order and numbering convention, a coordinate system reference is also required (equatorial, galactic, planetary, etc.), and this is not a theoretical issue given the variety of HEALPix datasets (e.g., HiPS data) which exist in the community.

Is there any existing practice for how to provide additional semantic metadata that allows the recipient of IVOA-style data to determine which numbering system a given dataset described with “pos.healpix” is actually following?

Has a further specialization of the UCD vocabulary been considered, e.g., including something like “pos.healpix.nuniq”?  

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.

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’ve seen the discussion in the archives for this list and the Apps list in August/September 2016.  It didn’t seem to run to a conclusion, and I don’t see a followup discussion in the October 2016 Interop meeting agenda.

This feels like a more “semantics” than “apps” issue, though in practice the result would be of real interest to application developers.  If there’s a sense that the discussion should move to the “apps” list, that’s fine.

Thanks,

Gregory

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Gregory P. Dubois-Felsmann, Ph.D.        Caltech/IPAC LSST science lead
LSST Science Platform Scientist and Data Management Interface Scientist
gpdf at ipac.caltech.edu                              tel: +1-626-395-6693