[fitswcs] inherit vs. concatenate

[Don Wells]

Steve Allen writes:
 > .. I think.. that concatenations of WCSs should be considered as
 > relatively common operations.  Indeed, they are so common that
 > perhaps they should be included as part of the WCS standard.
 > 
 > Example code implementing concatenations of FITS WCS has already been
 > written by Doug Mink at Harvard/SAO.  It is part of the WCSTools
 > version 3 library described at
 > http://tdc-www.harvard.edu/software/wcstools/
 > 
 > The particular feature of concatenation is included in Doug's
 > "dependent coordinate systems" and as seen in
 > ftp://cfa-ftp.harvard.edu/pub/gsc/WCSTools/Versions
 > that is a feature that has been present since version 2.9.0 in February
 > of 2001.
 > 
 > It appears that Doug Mink has not documented the notion of WCS
 > concatenation as implemented in his code.  If I recall correctly it
 > adds a keyword WCSDEPs for which the semantics are that
 > WCSDEPB = 'A'
 > means that WCS version B uses as inputs not the pixel coordinates
 > but rather the outputs of WCS version A..

I recommend that Doug and/or Steve produce a description of Doug's WCS
concatenation convention as an 'unofficial' appendix for WCS-I.  I.e.,
I am recommending that this convention be treated in the same way that
we treated the three advanced topics which accompanied the BINTABLE
standard ten years ago, but which were not considered to be an
official part of the standard.  If we add this appendix, WCS-I can be
modified to reserve the keyword, but the committees will not actually
approve the appendix itself.  Of course, we know the history: those
three BINTABLE appendicies have been regarded as almost de facto
standards during the past decade, and probably they now have enough
support to get official approval in the committees.  My bet is that
the same process of slow consensus-building will occur again.

-Don Wells

PS: I infer that a FITS reader which does not recognize/implement
     WCSDEPB will not crash, but will simply produce 'incorrect' WCS
     calculations. In the practical concatenation cases which have
     motivated this proposal, what errors occur if WCSDEPB is ignored?
-- 
  Donald C. Wells      Scientist - GBT Project        dwells at nrao.edu
                    http://www.cv.nrao.edu/~dwells
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