[fitsbits] Primary & Alternate WCS Keyword Order

[William Pence]

Alternatively, why not just eliminate this requirement entirely, by 
deleting this sentence: "This keyword, if present, must precede all WCS 
keywords except NAXIS in the HDU."  What is the purpose and benefit of 
this requirement?

-Bill


On 06/25/2012 12:41 PM, Dick Shaw wrote:
> Randy's question uncovers an ambiguity in the FITS v3.0 standard that really
> ought to be fixed. I think the need for WCSAXESa remains well justified, but
> how much does the keyword order matter? Sect. 8.2 technically requires that
> this keyword precede _all_ WCS keywords except NAXIS, but for headers with
> more than one WCS description this requirement as worded cannot be met.
> (WCSAXESA must precede all WCS keywords... but what about WCSAXES and
> WCSAXESB, etc.?)
>
> I propose that the description of WCSAXES in Sect. 8.2 be amended as follows:
>
> WCSAXES -- [integer; default: NAXIS or largest of WCS indexes i or j]
> Number of axes in the named* WCS description. This keyword, if present, _must_
> precede all WCS keywords for its named WCS description except NAXIS in the
> HDU. The value of WCSAXES _may_ exceed the number of pixel axes for the HDU.
> The default value mentioned above applies to any named WCS "a" if WCSAXESa is
> absent, but does not apply to a named WCS otherwise.
>
> [Footnote *]: The "a" refers to the name of the WCS represented by all WCS
> keywords with the "a" suffix. Its primary function is to provide a means by
> which to specify a particular WCS if multiple versions are defined in the HDU.
> See Sect. 8.2.1.
>
> The intent of the above wording (in case I flubbed it up)  is to allow all of
> the cases mentioned in Bill Pence's earlier e-mail to be valid. As far as I
> can tell, this change would not invalidate any existing FITS file that was
> valid before. A reasonable alternative, in my view, would be to change the
> word "_must_" to "_should_" if no compelling reason can be found for
> specifying the order of these keywords.
>
> -Dick
>
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-- 
____________________________________________________________________
Dr. William Pence                       William.Pence at nasa.gov
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