[fitsbits] programming interfaces for WCS defaults

[Steve Allen]

With the FITS WCS papers I and II all but in print I will dare to raise
questions of interpretation regarding alternate coordinate descriptions.

WCS paper I permits 27 different coordinate systems to be defined for
a given situation.  The primary coordinate system has version
character ' ' (space), and the 26 alternate coordinate systems have
the version characters 'A' through 'Z'.

WCS paper I also defines default values for each of the WCS keywords
which are to be used if some of the keywords are not present.

This effectively means that all 27 coordinate systems are always
defined.  In the case of a PHDU or an IMAGE extension they default to
an un-named identity WCS which gives world coordinates that match the
Fortran-like FITS pixel array coordinates.

Note that it is not helpful to assert that at least one of the WCS
keywords for a particular version must exist in order to instantiate
that version.  The author of a FITS HDU may simply insert
WCSNAMEA= 'My silly instance of the default world coordinate system'
along with no other keywords for WCS version A, and by so doing the
author has instantiated WCS version A with a WCS that is not really
useful.

Here is the question in two variant forms:

How does the user interface of a FITS viewing application inform the
user whether or not there is a *useful* definition for any particular
version of WCS?

The same question can be generalized to the level of an application
programming interface -- how does the FITS toolkit library inform the
application programmer whether a WCS is defined, or whether it is only
partly defined and some values have reverted to default?

The answers will be important for the development of many applications,
and should probably end up in FITS guides to users and programmers.

--
Steve Allen          UCO/Lick Observatory       Santa Cruz, CA 95064
sla at ucolick.org      Voice: +1 831 459 3046     http://www.ucolick.org/~sla
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