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

[David Berry]

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