Comments on Image Data Model

[Omar Laurino]

Hi DMers,

Those of you who were in the last DM session (and weren't doing your email)
know that this point was already raised. Just to reassure Arnold and Markus
that their feedback got there just in time ;)

I think we have currently spotted three (maybe four) different options that
*do not* require rewriting utypes for a standard that we already have:

1. Map WCS to STC
2. Use a single CLOB, with utype WCS.header (or whatever the DM dictates)
and keep the FITS keywords as they are
2b. Use a single PARAM/FIELD with utype WCS.header (or whatever the DM
dictates) and an STC-S representation of the WCS "coordinates". (see
discussion started by Arnold in the utypes list).
3. Use the WCS keywords as utypes.

I would vote for 2b.: it uses an IVOA standard, it is more compact than any
other solution and is very "portable" and more easily parseable than a
complicated XML/VOTABLE structure.

I also like 1. but it is less compact and (I suspect) more troublesome in
actual implementations.

3. is OK, but it defeats the idea of having meaningful utypes which encode
structure and can be parsed by the human mind (which is a requirement I
don't understand/like but that has been fought hard for by somebody).

I am sure I am overlooking something (e.g. are we sure we can map WCS to
STC and express WCS information with STC-S strings?)

Omar.

On Mon, Oct 29, 2012 at 7:29 AM, Markus Demleitner <
msdemlei at ari.uni-heidelberg.de> wrote:

> Dear DMers,
>
> On Fri, Oct 26, 2012 at 09:50:03AM -0400, Arnold Rots wrote:
> > I just noticed Doug Tody's presentation in the last DM session (which
> > ran in parallel with DCP) on an image DM.
> >
> > He tries to relate FITS WCS concepts to IVOA standard concepts, but I
> > find the table that he presents totally unacceptable.
> > The IVOA has a direct equivalent of FITS WCS: STC. Therefore, this
> > list of associations needs to be cast in terms of STC concepts.
> >
> > I'll be happy to provide that list - if anyone cares.
>
> I've also not been in that session (which clashed with DALI), and I'm
> afraid I have to agree to some extent with Arnold.  One may deplore
> that WCS and our STC data model don't always easily map to one
> another, but adhoccing utypes from WCS and ignoring STC doesn't seem
> like a good way either, in particular not if we blindly carry over
> historical baggage of WCS (both CDMatrix and CDelt have utypes?).
>
> I'd *much* prefer a streamlined version of this (things being what
> they are, the basis should really be the approved IVOA RECs where
> applicable, which in this case means large parts fall under STC) with
> recommendations on how to map historical and current WCS to it
> (remeber CROTa?).  I realize that's a bit of an effort, but I guess
> it's effort well spent.
>
> If we're not willing to spend it, then I'd have the only slightly
> tongue-in-cheek solution of defining
>
> WCS.header
>
> that's then a CLOB containing all the WCS-relevant FITS cards, simply
> referring to the WCS papers for the actual definition.  Works at
> least as well as what's proposed now, doesn't need larget software
> adaption, and is, as far as I can see, about as "VO" as the what's on
> page 5 of the PDF at
> http://wiki.ivoa.net/internal/IVOA/InterOpOct2012DM/dt-imagedm.pdf
>
> Cheers,
>
>           Markus
>



-- 
Omar Laurino
Smithsonian Astrophysical Observatory
Harvard-Smithsonian Center for Astrophysics
100 Acorn Park Dr. R-376 MS-81
02140 Cambridge, MA
(617) 495-7227
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