[fitswcs] Draft#2 of FITS BoF report for review

[Doug Tody]

Hi Don,

I had a chance to read through your ADASS FITS BOF report this evening and
had the following comments.

    -	[minor] Define TG (Task Group) on the first occurrence, which is
	following "IAU Comm5 ...".

    -   [minor] In "multiple WCS support", "active set" (of WCS keywords)
	should be changed to "default set".  The active set is whatever
	the program using the WCS is using.  The default set is the default
	member of the WCS group defined by the WCS.

    -   [minor] I think "unrecognized comment keywords" should read
	"unrecognized keywords".  These are not comments, they are named
	keywords not recognized by naive FITS WCS readers.

    -   I was at first surprised to find WCS parameter values referred to
	as PVi_k where k is the axis, since there seemed to be general
	agreement in the meeting that giving the axis first and the
	axis-dependent card (iteration) number second made more sense.
	But I see you mentioned this point later on.  I infer that we
	don't want to make this kind of decision in a public discussion,
	but rather in followup TG discussions.

    -   The comment I made in the BOF about axis=0 possibly being useful
	for parameter values was not mentioned in the report (Eric responded
	that degenerate axis values were preferable).  I think that some
	aspects of PV need further thought.  There were at least 3 unresolved
	issues presented in our discussions, both during and after the
	BOF.  These were 1) nomenclature (PVi_k vs PVk_i), 2) possible
	usage of axis=0 for specifing general parameters that apply to
	the entire WCS of a multi-WCS group (but which may not fit Eric's
	degenerate axis model, e.g. the WCS system name, or a keyword
	explicitly associating several axes in a group), 3) whether or
	not the datatype of a PV keyword is restricted to numerical, or
	defined by the WCS function type to which the parameter is assigned
	(this case is unusual in that the PV keywords are not fully defined,
	rather their usage is defined by each WCS).  The discussion of
	whether parameters for coupled axes should be restricted to the
	"DEC-like" axis is obviously relevant only to projection class
	axes hence is not general (this is related to item 2).  I conclude
	that the whole issue of WCS-dependent parameter values, how to
	represent coupled axes for multidimension world functions, and
	annotation of multiple WCS, needs further thought.  We are close
	to solving all the problems now, and the major problems we had a
	year ago have been resolved, but there are still a few issues to
	think about.

    -   We may be at the stage where rewriting the draft of the G&C paper
	is warranted.  In private comments to Don I suggested a terse,
	formal interface summary (and perhaps some trial implementations
	or at least implementation studies) might be more appropriate so
	that we could iterate more rapidly and reach agreement on this in
	the TG before putting the work into rewriting the whole paper,
	but it could be done either way.  In either case, spelling things
	out in writing may be desirable to fully define the important
	details.  I suggest however that we should try to agree upon any
	drafts internally within the TG, before putting the next draft
	out for public comment, or we may be contributing to "WCS vintage
	pollution" as well as "namespace pollution".

The above may go into more detail than is needed for the BOF report, but
I include the comments anway for the other task group members who may have
missed some of this private discussion at the ADASS.

	- Doug