[fitsbits] Primary & Alternate WCS Keyword Order

[Arnold Rots]

I suppose another consideration (which I think was mentioned a few
days ago) for ordering certain keywords is to make it easier on humans
reading the headers.

Another question on your earlier post:
You stated that it does not make sense for WCSAXES to be less than the
number of non-degenerate axes in the image.
However, would it not make sense, if one had a RA-Dec-Doppler cube to
define a coordinate system that only provides an alternate for the
spatial axes, in which case WCSAXESA would be 2?

  - Arnold

Mark Calabretta wrote:
> 
> On Tue 2012/06/26 16:00:41 -0400, William Pence wrote
> 
> >Regarding the ordering of FITS header keywords, it's interesting to note 
> >that other than the 9 mandatory structural FITS keywords that must 
> >appear in a specific order (SIMPLE, XTENSION, BITPIX, NAXIS, NAXISn, 
> >PCOUNT, GCOUNT, TFIELDS, and END), WCSAXES is the only other keyword 
> >defined in the FITS Standard that has any restrictions on the position 
> >that it can have in the header.   Maybe this requirement was useful back 
> >in the days of having to read FITS files sequentially off of magnetic 
> >tape, but now I think it is hard to justification why this one keyword 
> >should have this special positional requirement?
> 
> Indeed, it is clear why some of these need a specific order
> (SIMPLE, XTENSION, END).
> 
> However, it is less clear why others do.  Specifically, why
> would I need to know the data format (BITPIX, NAXISj, PCOUNT,
> GCOUNT) until I have finished reading the header?
> 
> Others seem intended to facilitate one-pass sequential scanning
> (NAXIS, TFIELDS, WCSAXESa) as these define the number of
> parameterised keywords to expect.
> 
> Regards,
> Mark Calabretta
> 
> 
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Arnold H. Rots                                Chandra X-ray Science Center
Smithsonian Astrophysical Observatory                tel:  +1 617 496 7701
60 Garden Street, MS 67                              fax:  +1 617 495 7356
Cambridge, MA 02138                             arots at head.cfa.harvard.edu
USA                                     http://hea-www.harvard.edu/~arots/
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