[UCD] Question about representation of HEALPix pixel IDs

[Laurent Michel]

Hi Gregory,


Using UCDs to retrieve the Healpix frame supposes that the client will pick here and there values from which it could 
reconstruct that frame.
In a model approach, the healpix frame is defined by the model, so that the client just has to build a model instance to get 
that frame. This second approach is more consistent, a bit more heavy though, since the way data are tied is not inferred by the 
client.

As Markus said, I experimented this model oriented approach in my Tessellation project.

    https://volute.g-vo.org/svn/trunk/projects/dm/Tesselation/

The idea was to extend a little bit STC with an Healpix frame and coords.
This is a relevant use case for both playing with the VO-DML model extension and fulfilling a real requirement.

Bye
Laurent


Le 19/04/2018 à 09:09, Markus Demleitner a écrit :
> Hi Gregory,
> 
> On Tue, Apr 17, 2018 at 11:49:14AM -0700, Gregory Dubois-Felsmann wrote:
>> 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
> 
> Yes, that has been the intention, and so far, it doesn't go beyond
> that.
> 
>> 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
> 
> Do you actually see many RING and NUNIQ healpixes in datasets
> intended for the wider public and plausibly showing up in VOTables?
> I personally so far had hoped we might get by with NESTED in the VO.
> 
>> 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'?
> 
> That would be my preferred way forward if non-nested healpix schemes
> needed annotation, yes.
> 
>> 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.
> 
> Much appreciated!
> 
>> 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.
> 
> That one definitely a topic for Apps, and I'd say this should be
> discussed once we understand what client receiving such a message
> should expect.
> 
>> 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.
> 
> Yeah -- unfortunately, this wasn't followed up.  As to annotating
> healpix kind and order, I think if you took it into your hands,
> prepared a proposal and then just went with it, chances are good it'll
> be taken up (I suppose it would eventually be standardised in DALI,
> so the DAL list would be a good place to discuss that).
> 
> Also note Laurent Michel's effort on having a data model-based
> annotation of sky pixelisation schemes at
> https://volute.g-vo.org/svn/trunk/projects/dm/Tesselation/ -- I've
> not really have a good look at it, and so I'll leave it to Laurent
> (in cc:) to comment.
> 
> As to the reference frame (and position) used, I'd be strictly
> against improvising something.  Until the STC data model and a usable
> serialisation for it come around, just use COOSYS, probably somewhat
> like this:
> 
>    <COOSYS ID="main_system" system="ICRS"/>
> 
>    <COLUMN name="healpix" ... ref="main_system"/>
> 
> I don't think any healpix consumers at this point evaluate it, but
> it's a straightforward extension of current practices, so I'd be
> surprised if that were to cause any pain.
> 
>         -- Markus
> 

-- 
jesuischarlie/Tunis/Paris/Bruxelles/Berlin

Laurent Michel
SSC XMM-Newton
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Fax : +33 (0)3 )3 68 85 24 32
Université de Strasbourg <http://www.unistra.fr>
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