[fitswcs] Status of WCS negotiations

[Don Wells]

Dear friends of FITS WCS,

Four questions seem to me to block the path toward agreement:

(1) SPLITTING-G&C: I perceive that there exists a substantial
consensus that the G&C paper needs to be split into two papers, the
first with a title something like "Generalized World Coordinate
Conventions for FITS" and the second with the same title as the
current G&C paper. The concept is that the first paper would contain
the meta-rules for FITS WCS, and would be designed to facilitate a
future agreement on spectroscopic interchange.  The first paper should
specify the lowest-level linear transformations and the pixel
correction function. It should specify the keyword name conventions,
and should address the issue of how to convey multiple WCS
descriptions in the same header. The second paper would contain those
parts of G&C which are concerned with celestial coordinates,
particularly the 25 projections.  I hope that there are now no
dissenters from this consensus; if such dissent exists I ask the
dissenters to Email me privately.

(2) MULTIPLE-WCS: It appears to me that there is a requirement to
support multiple WCS descriptions of the same data object. I recommend
that we do this by appending a character code [A-Z] to the WCS
keywords for secondary descriptions; the keywords without the
character would be the default operational ones. This notation would
require reducing the number of axes supported by the WCS keywords from
999 to 99. If any of you have alternative ideas for multiple-WCS,
please post them to 'fitswcs'; if you support my suggestion, post
about that. If the people on this mailing list could agree on this
notational convention, my bet is that the rest of the FITS community
would agree too.

(3) CD-MATRIX: I perceive that there exists a substantial consensus
that the lowest level linear transformation should be done with a
matrix of partial derivatives, of the sort called 'CD' in current
implementations. As far as I am aware, there is no longer a
science-driven requirement for the CDELTi/PCij notation which was
proposed in the G&C draft.  (I myself have always favored the latter,
out of feelings of nostalgia for CDELTi and of elegance regarding the
dimensionless PCij matrix, but I acknowledge that CDij works and has
an advantage of simplicity.)  I would very much like to see this old
debate brought to a consensus conclusion so that we could get on with
the process of revising G&C as discussed above. If there are any
dissenters from the consensus which I perceive, I ask them to Email me
privately.

(4) PIXEL-CORRECTION-FUNCTION: Many data acquisition systems produce
image matricies which are slightly distorted relative to canonical
projection formulae, due to misalignments and to the use of complex
corrector optics. There has been a school of thought which held that
producers of such data should be responsible for regridding it into
canonical geometries. I judge that this attitude really isn't
practical---(1) this class of problems is pervasive in modern systems
and (2) regridding corrupts the signals. I conclude (a) that we need
to agree on how to interchange a pixel correction function, and (b)
that we need to implement support for the function widely. I say
'pixel correction function' because I myself favor perturbing the
pixel coordinates before applying the linear transformation. I am
aware that other people prefer polynomial expansions in 'standard
coordinates' or other functional forms. I hope that we can agree that
these are mathematically equivalent notations. If any of you believe
that there is some notational form which is clearly best, please post
to 'fitswcs' on the subject so that I and others can learn about your
ideas. If any of you strongly object to some particular notational
form, please post your objection.  In the end we need to agree on a
canonical *interchange* notation (we could continue to use private
conventions in our software, of course).

I have been struggling to devise a 1-D index keyword notation for
N-dimensional polynomial coefficients. I have solved the 2-D case, and
am convinced that there is a solution for 3-D, but I have not yet
found that solution; I expect that the form of the 3-D case will imply
the n-D formulae. If any of you are interested in this notational
problem, please contact me privately.

-Don
-- 
  Donald C. Wells         Associate Scientist         dwells at nrao.edu
		    http://www.cv.nrao.edu/~dwells
  National Radio Astronomy Observatory                +1-804-296-0277
  520 Edgemont Road,   Charlottesville, Virginia       22903-2475 USA