[fitswcs] Status of WCS negotiations

[Mark Calabretta]

On Tue 1998/07/21 16:21:46 -0400, Don Wells wrote
in a message to: Mark Calabretta <Mark.Calabretta at atnf.csiro.au>

> > .. extra complexity.. should be quarantined as far as possible from
> > those who don't need it..
>
>I want to agree with you on this philosophical point, Mark, but I
>can't.  The problem is that radio and X-ray users now need fully
>functional optical astrometry technology to support their
>research. This was the conclusion of my initial posting on the
>scientific requirements for WCS agreement.  Our whole community
>(optical, radio, X-ray, infrared, ground-based and space-based) must
>now implement WCS functionality for the worst cases. 

I am saying that we should support the complexity required but in such a way
that those who don't need it don't see it (if that is possible).

> As a practical
>matter, this means that we need at least one software package which
>implements the functionality and which can be imported and used by
>astronomy datasystems everywhere.  Your wcslib package fills this
>need. If you program the extra complexity into wcslib and describe it
>in the published paper so that others can produce equivalent
>implementations now or in the future as needed, the science
>requirement will be satisfied.

WCSLIB won't need to know about multiple representations, it deals with them
one at a time - i.e. you give it a set of keywords and it translates them into
internal data structures that it understands and returns to the caller to use
as required (what I refer to as a "context").  Multiple representations are
the same as multiple contexts.  Maintenance of the contexts occurs at a higher
level, e.g. one of the WCSLIB test programs maintains six simultaneously.  So
if, rather than my test program, you were writing a general purpose package
which reads FITS files, you'd have to make an assumption about how many
contexts you were likely to have to handle, and you'd have to disentangle the
FITS keywords yourself and feed them in.

> > .. extending the CDij in this way [is]
> > .. virtually ruled out on practical grounds by the
> > requirement for inverse coordinate transformations.
>
>Higher-ordered distortion terms are invertible by iteration.

True, but I said impractical, not impossible.  Think about how you would code
it.


I could go on (and on) but like you I have other work to catch up on.  It
would be useful to have a formal statement of what the optical people actually
want, a formal counter proposal or change proposal.

Cheers, Mark