Axis identifier in the coords model

[CresitelloDittmar, Mark]

Laurent/All,

I'm really glad to see you working through these questions!

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The reasoning for the current representation..

The previous version of the Coords model had explicit references from the
Coordinate to the associated Axis as well as the CoordFrame.
This revision combined the CoordSpace(containing Axis-s) + CoordFrame into
CoordSys, and Coordinate now references CoordSys.

So, there is a path from Coordinate to the associated Axes via CoordSys, as
you noted.

Rather than having 2 paths from Coordinate to Axis, which may be out of
sync, the direct relation between them was removed, and the Point.axis#
values MUST map to the CoordSpace axes, in order.

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>From here, I think the discussion may branches a bit into 'how are the
models to be used?'.
Where do things need to be 'modeled'?, and what are serialization issues?
For example:
  would it be sufficient for the model to be as-is, but allow the
serialization to help bridge the gap to assist applications?
     A VOTable serialization could use the UCD to tag each Point.axis*
value with "pos.eq.ra", "pos.eq.dec"  which applications could queue on to
*shortcut* tracing the model elements through the
CoordSys.SpaceFrame.refFrame and CoordSys.SphericalCoordsSpace.Axis[] .  If
absent, they'd have to trace the path manually.
  OR, if we concede that this is a very common action for applications, is
the model REQUIRED to provide the information at that point?

The model has iterated between these two points of view.

I don't know the answer to this, but it seems to me that this is the sort
of question that we are currently trying to resolve, with the Mapping
Syntax discussion, the Transform implementation project, and your CAB-MSD
work.

Mark
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