[fitswcs] update of Paper III

[Steve Allen]

Hi Eric,

On Fri 2005-05-20T09:15:38 -0600, Eric Greisen hath writ:
> The answer is no - no more parameters are need.  Instead the full WCS
> nomenclature of Papers I and II should always be used.  Since this was
> not obvious to David I decided to add a paragraph to the paper - I do
> not know if the wording is good or where exactly it should go.  At
> present it is on page 22 "real-world complications".

I see the paragraph.  This may be the best that can be done within
the largely syntactic world of FITS.  The relationship between
spectral coordinate meaning and celestial coordinate meaning seems
pretty well expressed there.

Beyond prose, the semantic requirements for relations between the
spectral and celestial quantities are more rigorously expressable in
the realm of UCDs and XML schemas for data modeling.  The
consideration of the need for this extra paragraph strongly resembles
the current activities of the Virtual Observatory working groups.

The VO stuff is fluid and evolving rapidly as understanding of its use
increases.  The FITS paper needs to offer reasonable syntax for
implementing this without requiring anything that might later (soon)
prove to be problematic.

The only thing that strikes me immediately is that it always seems
good to remind people that in the context of tables the Greenbank
convention can be used with header keywords specifying the content of
some "virtual columns" whose values are constant.  E.g., RADESYSs
is likely to have the same value for every object in a table.

--
Steve Allen                 <sla at ucolick.org>                   WGS-84 (GPS)
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