[fitswcs] WCS documents

[Bob Hanisch]

Despite my silence, I have been thinking about the FITS WCS issues a great
deal.  The main sticking point seems to me to concern the possibly differing
dimensionality of the data array, as described by NAXIS, and the WCS.  One
argument - a logically and mathematically correct one - is to use NAXIS to
describe both the data array and WCS dimensionality, and use degenerate axes
(NAXISn = 1) as needed to insure consistency.  A counter-argument is that
this approach is neither natural to many users, nor consistent with common
practice in many applications programs, and such an approach will cause many
programs to fail.  Having recently helped a colleague to deal with serious
interoperability problems between radio astronomy and optical astronomy
coordinate systems, I am keenly aware that we cannot simply prescribe a
solution without regard for issues of common use AND common understanding.
We should not, however, endorse a solution in which the attributes of the
WCS are implicit rather than explicit, and where the existence of WCS
keywords with indices higher than the value of NAXIS might, or might not, be
present.

I think we need one additional keyword, a keyword that explicitly describes
the dimensionality of the WCS, leaving NAXIS to describe the dimensionality
of the data array.  WCAXIS or something like that.  This by itself will not
deal with different usage conventions in radio astronomy and elsewhere, but
it will avoid ambiguity and help us avoid the need for private keyword
spaces in which the subtleties of a WCS are hidden away.  Such a keyword is
not strictly necessary in the "mathematically pure" approach, though I would
argue that even in this case we will have increased clarity for programmers
and end-users.

Bob

----- Original Message -----
From: "Eric Greisen" <egreisen at NRAO.EDU>
To: <fitswcs at NRAO.EDU>
Sent: Monday, September 17, 2001 11:45 AM
Subject: [fitswcs] WCS documents


> The silence on this exploder has been remarkable.  Mark and I have
> made some progress and I have added references to that progress and
> the NOAO proposal to my web site.  Revised **drafts** of Papers I and
> III are available there:
>
>      http://www.aoc.nrao.edu/~egreisen
>
> The FITS BoF will probably mention these Monday Oct 1, 2001 in
> Victoria.
>
> Eric Greisen
>