[ImageDM] Mapping

[CresitelloDittmar, Mark]

I wanted to make sure this topic gets some air time with the group.  Doug,
Francois and I have gone around a few times on it already for the earlier
draft.

This is in regards to the Mapping object in the ImageDM.

ImageDM doc
 Section 2, Paragraph 1,
  "The mapping of image axes to physical coordinates in measurement space
is described by a World Coordinate System (WCS) sub-model, referred to
herein as the Mapping model"

Section 2.2, pg 12
  "Mapping defines the world coordinate system (WCS) for a Data array"

Section 3.2.1, pg 18
  Naxes, Naxis etc on Image (for overall image), plus same on Data (for
subarray)

The model describes this object as defining a coordinate system.  The model
also includes a CoordSys class whose job is to define coordinate systems.
This object and the Frames it uses are derived from STC via the
Characterisation model.  STC also provides the ability to define a
PixelCoord system, and mechanism for defining a projection or mapping
between them.  (STC - pg 38 and others).

In my opinion, this model should be making as much use of this framework as
possible.  I don't think it is good to be defining coordinate systems in
multiple areas using different mechanisms.

Other questions:
  - I don't think the current document allows for > 1 Coordinate system.
So if I have a 2D image with "detector" and "sky" coordinate systems, I
don't think this works (but am not certain).
  - I think the Mapping definitions are for a Pixel -> World transform, but
it doesn't seem clear to me

Mark
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