[fitswcs] Status of WCS negotiations

[Don Wells]

Mark Calabretta writes:
 > 
 > On Wed 1998/07/15 14:36:27 -0400, Don Wells wrote
 > >(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..
 >
 > ..  The single PC matrix has a number of virtues: ..
 >    2) Dimensionless elements make it conceptually cleaner.

I agree with you on this, Mark, but it appears to me that the
consensus of the community is that this fact is unimportant.

 >    3) CDELTn allows for easy change of units/scale.

Yes, but changing scale for CDij is no big deal

 >    4) Old FITS files look like new ones with a default PC matrix.
 >    5) New FITS files without a PC matrix look just like old ones.

This is true for files that follow the Classic-AIPS notation, but
isn't true for those that follow the IRAF/SDAS notation. By now, in
1998, there are vast quantities of data in astronomy archives with
both WCS notations.  Therefore, as Chair of IAU-FWG, I expect to have
difficulty in getting the whole FITS community to accept this argument.

Furthermore, the water has been muddied by the argument (primarily
advanced by Pat Wallace I think) that the CDELTi are defined in a
subtly different manner in G&C from the definition used in older WCS
implementations, and that therefore the keyword names should be
changed. Whether or not this argument is technically valid, it also
makes it difficult for me to form a consensus of the FITS community.

 >    2) CD matrix elements have implied units.  Off-diagonal elements may have
 >       physically meaningless units.

I too am unsure what to say about "Stokes" axes in CDij matrix
notation, but it appears to me that the long-slit spectrogram case is
represented properly in CDij notation. The units issue needs further
review. My bet is that, in the end, we will find a way around it.

 >    3) Readers and processing software have to recognize and provide for many
 >       matrices of size up to 999*999 (4 Mbyte).

I recommend that we agree to limit axes requirements for WCS to 99.
Real implementations are certainly entitled to limit themselves to
much smaller dimensionality, like 9 (Eric Greisen told me that the
worst case he has thought of has NAXIS=7).

 >    6) Rescaling one axis requires matrix row multiplication.

I expect that IRAF/SDAS implementors have had to address the need for
transformation of their CDij matricies when they subimage, decimate or
interpolate their imagery; I would like to hear their comments on your
criticism.

 > It boils down to a cost/benefit analysis, not introducing complexity where
 > it's not needed.  ..

There are several existing astronomy data processing packages (IRAF,
SDAS) which have used CDij notation in production for many years now;
they write CDij into their headers currently and could easily support
alternative keyword names for a compatible CDij concept. So, looking
at the situation from their viewpoint, adopting CDELTi+PCij would
introduce a complexity where it's not needed.

 > .. if someone can show me
 > just one legitimate example of where it's necessary to maintain multiple
 > linear transformations then I will reconsider.

As I noted in another posting, an image pixel represents both a point
on the sphere (celestial coordinates) and a point in an instrumental
coordinate system (linear coordinates). This statement is true in a
wide variety of detector systems.

-Don