WCS questions

[Mark Calabretta]

On Tue 1996/12/17 19:47:32 GMT, Stephen Walton wrote
in a message to: fitsbits at fits.cv.nrao.edu

>But if I read the WCS spec correctly, the values of CRVAL1 and CRVAL2
>would simply be the values of latitude and longitude at the pixels given
>by CRPIX1 and CRPIX2.  However, this raises another problem:  for the
>linear WCS I propose, CRVAL1 and CRVAL2 would both be zero.  It may still
>be that two WCS's in the same header would be needed.

If you want to measure distances in metres, rather than solar radii as
originally proposed, then use alternate axis descriptions (Section 2.3 of WCS
paper).

>well.  To use this would, though, require coding in the CD matrix
>formalism, which I'd assume would be back-supported by any software
>developed once the present WCS draft is made final.

The current draft recognises the CD matrix for backward compatibility but
states that it must not be written.  In particular, use of a CD matrix would
imply that the CmDELTn secondary descriptor cards should be ignored
(presumably, but in fact the behaviour is undefined) which is certainly not
what you want in light of the above.

Mark Calabretta
ATNF