[ImageDM] Mapping

[CresitelloDittmar, Mark]

Francois/Mireille.. I would appreciate your opinions on this matter.

Regarding this point on CoordSys and Characterisation..

I do not believe there is a restriction on the number of axes that can be
'characterized' under the Characterisation object.  If I have multiple
coordinate systems (CoordSys) in my data product, there can be a
CharacterisationAxis for each of the axes represented in all CoordSys.
There is a 'CoordSys' association within CharacterisationAxis to provide
the link.

So, for a data product (like Image) one should be able to define multiple
coordinate systems (CoordSys) and characterize each axis under
Characterisation.

For data discovery, it should be fine to require that a QueryResponse
contains only a single Coordinate system (CoordSys) and that all metadata
is provided in that coordinate system.  That would be defined in the
particular access protocol doc.

If the above assertion is true, then 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.

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.

Mark




On Thu, Nov 21, 2013 at 12:36 PM, Douglas Tody <dtody at nrao.edu> wrote:

> On Wed, 20 Nov 2013, CresitelloDittmar, Mark wrote:
>
>
>> On Tue, Nov 19, 2013 at 4:00 PM, Douglas Tody <dtody at nrao.edu> wrote:
>>       In the current ImageDM, CoordSys is mainly used to define the
>>       coordinate
>>       systems used globally for metadata describing the overall
>>       image dataset
>>       (Image, Observation, Char, etc.).
>>
>>
>> So, this defines a singular coordinate system which is to be used
>> throughout the metadata portions of the model?
>> pg 12: "Image also adds a CoordSys element defining a uniform set of
>> (default) coordinate frames and units for all Image metadata including
>> Observation and Characterisation."
>>
>> If my Data has WCS for the Spectral axis in both Wavelength and Energy,
>> (presumably via different Mapping-s), I CANNOT characterize my image in
>> both?
>>
>
> Correct; we want uniform coordinate frames to characterize datasets.
> The ultimate example of this is ObsTAP, where not only are all datasets
> characterized using the same frames and units, the spectral axis units
> are fixed as meters.  Characterization is mainly used in VO for
> discovery, for which uniform units are essential.  Mapping/WCS is not
> used for discovery, rather it is an astrometic calibration and is used
> for analysis, actual interaction with a specific dataset.
>
> That said, it is possible to have more advanced usage of
> Characterization, where additional metadata is added; this could be used
> to override the global coordinate system with something defined more
> locally.  However this is not typical usage.
>
>
>        It is highly desirable (from the client application
>>       perspective at
>>       least) to have a single set of coordinate frames used
>>       uniformly for all
>>       this higher level metadata.  It is pretty much required to do
>>       this in a
>>       use case such as a data discovery query response, where there
>>       are many
>>       table rows each describing a different dataset.  Otherwise we
>>       would have
>>       to add table columns to describe the frames used separately
>>       for each
>>       dataset/row, and the client would have a much harder time
>>       sorting out
>>       the information that comes back.
>>
>>
>> I can see that particular applications (like data discovery) may wish to
>> require a homogenized metadata set to simplify their use-case, but I'm
>> not sure that is a limitation on the generic model, but rather a
>> requirement of the application (SIAP).
>>
>
> Certainly SIAP or ObsTAP require homogenized metadata for discovery.
> For Image data we have two primary use cases or applications: discovery
> (SIAP or ObsTAP), and representing an actual Image dataset.  Probably
> one can allow more flexibility in terms of coordinate frames and units
> in a dataset instance: we already do for Mapping for example.  In
> practice though, I suspect data providers will only compute
> characterization metadata once for a dataset, so the primary use
> case (data discovery in the VO) will govern what is done for high
> level metadata.
>
> The main point of Characterization in the VO is to uniformly
> characterize datasets.  Here "uniform" means characterize in the same
> way for each measurement axis: location, bounds, support, etc. -
> obviously a simplified/approximate approach that cannot be precise, at
> least at the higher levels.  When we start to get into precise
> calibrations the models get more complex.  The Photometry model is a
> good example of that.  Mapping/WCS is still mostly a uniform model, but
> it is not describing the character of a dataset, it is precisely mapping
> sample coordinates to world coordinates and vice versa.
>
>         - Doug
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