[ImageDM] Mapping

[CresitelloDittmar, Mark]

All,

I've been wanting to get this out for discussion before we get too close to
the holidays.

In the hopes of settling the debate about whether or not STC provides the
capabilities encapsulated in the Mapping object, I have gone through the
trouble of making a step-by-step transition of Mapping elements to an STC
based model.

You can find the document in Volute.  I have also posted the link on the
twiki.

https://volute.googlecode.com/svn/trunk/projects/dm/ImageDM/doc/Mapping-STC.pdf

I'm not sure if this will help or hinder my point in the end, Mapping takes
a LOT of information about Frames, Axis linkages, Transforms, etc.. and
flattens it to an encapsulation of the FITS WCS serialization (for the most
part).  Pulling that out gets somewhat complicated, but in my opinion,
anything not in the diagram would need to be explained in the text.

ImageDM (and other earlier models) simplified STC elements (Frames),
cutting off the complicated inter-system relations that are in the STC
objects.  I hope this shows pretty definitively that by NOT simplifying
them we get the desired capability, but properly modeled and more
flexible.  I think this scenario provides some important benefits:
  - uses established IVOA recommendation, so ImageDM can spend its space
with use-case diagrams rather than re-defining objects
  - enables the 'intermediate' axis set to be realized as actual axes.
  - enables scaling of 'Observable' data values
  - reduces redundant info.  The Mapping object provides partial WCS Frame
information, presumably the full definitions would have to be stored
somewhere else.
  - reduces irrelevant attributes.
  - provides a consistent framework from which we can generalize for
general hypercube data and specialize/reuse in the Spectral model to enable
'virtual' columns (which are currently not possible).

Please have a look.. hopefully we can have some good discussion over the
next couple weeks.
Francois and I had some discussion about this approach back in July.. I'll
need to dig up those comments and see how they relate to this effort.

Mark
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