[ImageDM] Mapping

[Douglas Tody]

On Sun, 24 Nov 2013, CresitelloDittmar, Mark wrote:

> ... I reiterate that the Mapping
> information, which defines coordinate systems, should be contained within
> the CoordSys umbrella, using existing VO standards as much as possible.

The existing WCS formalism (as captured in Mapping) does a lot more than
just define the coordinate systems used in the Mapping.  The current WCS
model is comparable in size to Characterization.  Are we suggesting
trying to replicate all of this within the Characterization model (e.g.,
the CD matrix, tabular coordinate value / index arrays, etc.).  Note
also, that in defining a WCS we (or a Photometric calibration and the
like) are no longer merely defining the characteristics of the dataset.

> If the access protocols have a need for an object which encapsulates all of
> the Mapping information under one object, then I suppose that could also be
> provided.  It would not be part of the data product, but would hold
> information extracted from a data product.

It is not merely a matter of providing the Data element metadata in an
object (although that is important), but a matter of complexity,
information hiding, and re-use.  Data models become increasingly baroque
and awkward the larger they become.  We are better served by having data
models with a well-defined scope and purpose, and associating multiple
such data models to model complex use cases.  Re-use is important not
just for the VO data models but for existing data models in wide use
within the astronomical community, FITS WCS being a prime example.

Re adding additional axes to Characterization - yes that can be useful.
Adding an axis to characterize visibility data could be a good example.

 	- Doug