[fitsbits] Primary & Alternate WCS Keyword Order

[Steve Allen]

On Fri 2012-06-22T13:42:28 -0400, William Pence hath writ:
> and I think the following case DOES NOT conform to the standard:
> Case 2:

agreed

> The question is, do either of the following cases conform to the standard?:

> Case 3b:  (This is the case Randy cited)

If we are not already using this, we might be.
In my opinion 3b must be legal.
However it raises the question about what value to assume for WCSAXESP
in the absence of that explicit keyword; i.e., would the value for
nonexistent WCSAXESP be presumed inherited from WCSAXES, or from NAXIS ?
The notion that WCSAXES is supposed to be very early in the header
seems to allow that it could be inherited to alternate version WCS.

> Case 3a:
>
> WCSAXESP=                    2 / number of WCS physical axes
> CTYPE1P = 'RAWX    '           / physical WCS axis 1 type CCD col
> CRPIX1P =                    1 / reference CCD column
> CRVAL1P =                  482 / value at reference CCD column
> WCSAXES =                    2 / number of WCS axes
> CTYPE1  = 'RA---TAN'           / right ascension coordinate type
> CRPIX1  =    3.629693431287876 / [pixel] reference pixel along image axis 1
> CRVAL1  =          284.3632905 / [deg] right ascension at reference pixel

In this I expect no question that an absent WCSAXES would not inherit
the value from WCSAXESP; i.e., the value of a missing WCSAXES would
inherit from NAXIS.

My impression is that no inheritance of the value of WCSAXES was
intended to be allowed; i.e., that in the absence of WCSAXES or
WCSAXESa the value of any of them would be inherited from NAXIS.

If we concur on that notion then I think it is sufficient for
WCSAXES and WCSAXESa simply to occur prior to any other WCS keywords
for that primary or alternate WCS.

--
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