[ImageDM] Mapping

[Douglas Tody]

Hi Mark -

I got the impression earlier that you were suggesting doing this by
adding additional axes to Characterization; sure sounded like it.  In
any case, for ImageDM/Spectral CoordSys is a place we can put transforms
that don't fit into either Char or STC - FluxFrame/Photometry is an
existing example.  In principle Mapping could be be moved there, however
the complexity and size issue alone is sufficient to argue against this
(also encapsulation etc. as I noted earlier).  You yourself argued a
while back that Mapping was Data element specific and should be modeled
as part of the Data element.

In the case of STC, we did a comparison of STC to FITS WCS a while back.
STC does have some capability for this, but it is quite limited compared
to FITS WCS.  Hopefully we don't have to wade through all that again
(hah!).  Compatibility with FITS WCS and easy transformation to and from
our VO representations is mandatory if we wish to have any up-take by
the non-VO community since the community is heavily invested in WCS,
both in archive data and in client software.  STC could possibly be
supported as an optional representation, e.g., to specify an output
projection, and might work for simple projections so long as someone can
work out the transformation to/from WCS.

 	- Doug


On Mon, 25 Nov 2013, CresitelloDittmar, Mark wrote:

> Doug,
>
> The Image and Spectral models have CoordSys outside of Characterisation,
> and that object is described as basing from the STC astroCoordSys (?)
> element.
> So, I am not suggesting we replicate anything.. the STC recommendation
> already supports the definition of relations (transforms) between
> coordinate frames.
> I am suggesting that we use that.
>
> I'll do my best to get a diagram of what I have in mind out to the group
> today.. the Observation/Dataset separation took up my time this weekend.
>
> On Sun, Nov 24, 2013 at 6:07 PM, Douglas Tody <dtody at nrao.edu> wrote:
>
>> 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.
>