Comments on Image Data Model

[Markus Demleitner]

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