[fitswcs] forwarded message from Steve Allen

[Eric Greisen]

I am forwarding a portion of Steve's reply to Mark and me.  This
portion needs wider consideration.

Some of Steve's other suggestions have been reflected in the documents
at

           http://www.aoc.nrao.edu/~egreisen

Eric Greisen
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From: Steve Allen <sla at ucolick.org>
To: Eric Greisen <egreisen at cv3.cv.nrao.edu>,
        Mark Calabretta <Mark.Calabretta at atnf.csiro.au>
Cc: Steve Allen <sla at ucolick.org>
Subject: Re: [fitswcs] new WCS papers
Date: Thu, 13 Dec 2001 16:58:03 -0800

########

Paper I, Section 2.4

"If an alternate coordinate description is specified, all coordinate
keywords for that version must be given even if they do not differ..."

Here I have a problem that I'd like to try to solve.  In the examples
for papers II and III there are several instances where we see a bunch
of alternate coordinate descriptions.  You are setting up people to
expect to do such things.  (This is not to assure that they'll follow
the example.  They certainly didn't follow Wells' degenerate axis
example.  But in this case they might...)  The examples in
papers II and III are relatively simple, and have no DCF.

Now consider the case of an optical image with a precise celestial WCS
that requires a DCF with hundreds of parameters.  In the (admittedly
somewhat unlikely case) where the FITS file writer wants to express
alternate coordinate systems (galactic, ecliptic) then this
requires that *all* of the hundreds of parameter values be repeated.

This repetition is tedious, wasteful, and prone to error.  Is there a
chance you would consider a means of letting a WCS inherit values from
another?

As fodder for discussion I suggest a keyword WCSBASEs which would have
a string value consisting of a single character.  The meaning of
WCSBASEA = 'B' would be that the WCS parameters for alternative WCS
A would default to those of WCS B unless explicitly overridden by
keywords for WCS A.

- --
Steve Allen          UCO/Lick Observatory       Santa Cruz, CA 95064
sla at ucolick.org      Voice: +1 831 459 3046     http://www.ucolick.org/~sla
PGP: 1024/E46978C5   F6 78 D1 10 62 94 8F 2E    49 89 0E FE 26 B4 14 93
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