[TRANSFORMS] - What is the scope of this work package?

[Arnold Rots]

David,

Thank you for this very nice summary

1. We will need these conversions, whether or not there are going to
be Quantities, though I suspect that we need them beyond Quantities,
unless those things are going to be all-encompassing.  but it does
make sense to wait and see for a while what's going to happen with the
Quantities.

2. I think you summed it up pretty well.
At this point I would be inclined to go with the FITS appraoch, for
the reasons you list and because it means that people do not have to
learn yet another language for expressing mappings.  However, I am
happy to change my mind if someone can convince me that we need
additional capabilities - I do like the AST rationalized approach.

  - Arnold

David Berry wrote:
> I've uploaded the "Toolkit approach to WCS" presentation which I gave at
> Cambridge to the DM wiki.
> 
> As I see it, the two main areas where transformations could be of use
> within the data model are:
> 
> 1) Transforming Quantities: given a Quantity, how do you transform it into
> some other system. This includes simple transformation between Units (e.g.
> Hz -> GHz), but also more complex transformations such as
> (frequency->wavelength), or even (topocentric frequency -> heliocentric
> radio velocity). How to structure these transformations depends very much
> on the eventual definition of a Quantity, so maybe we should defer
> discussion until the [QUANTITY] work package is developed further.
> 
> 2) WCS: Actually, I would drop the "W" since in my world-view "all
> coordinate systems are born equal" - including pixel coordinates, etc.
> In fact, I think WCS is also closely related to [QUANTITY] since
> each coordinate axis in a WCS corresponds in concept to a Quantity.
> 
> What is the place of WCS within the data model? A FITS file containing
> data will usually have a set of FITS-WCS headers. So do we need to
> consider WCS further? As I said in my Cambridge presentation, my opinion
> is that a toolkit approach to WCS would be much more scalable and easy to
> extend than the prescriptive approach embodied within the (excellent and
> extremely carefully though-out) recipes of the FITS-WCS papers. But the
> reality is that FITS-WCS is the currently accepted system for storing WCS
> by the majority (not all) of the astronomical community - so should the
> IVOA just adopt FITS-WCS for use through-out its data model?
> 
> Of course, it would be possible to convert a FITS-WCS representation into
> some form of a toolkit representation (the AST library does just that),
> but there would seem to be little point in doing this unless you then take
> advantage of the extra flexibility offered by the toolkit. But it would
> then be very easy to end up in a situation in which the resulting WCS
> uses features which are not representable within the recipes prescribed
> by FITS-WCS, making it hard, if not impossible, to go back to a FITS file.
> 
> I suppose if the IVOA were to produce a definition of a pluggable
> component-based WCS system, there may be some data providers who would
> choose to use it (those who found it hard to fit within the
> prescribed FITS-WCS recipes). The problem with this is that client
> analysis software would then need to understand the IVOA-WCS system
> as well as the FITS-WCS. This may not be a problem for new software, but
> would be for legacy software.
> 
> Any comments?
> 
> David
> 
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Arnold H. Rots                                Chandra X-ray Science Center
Smithsonian Astrophysical Observatory                tel:  +1 617 496 7701
60 Garden Street, MS 67                              fax:  +1 617 495 7356
Cambridge, MA 02138                             arots at head-cfa.harvard.edu
USA                                     http://hea-www.harvard.edu/~arots/
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