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[fitsbits] CDELTn



 
 
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Old September 18th 07, 06:14 PM posted to sci.astro.fits
Steve Allen
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Default [fitsbits] CDELTn

On Tue 2007-09-18T12:26:21 +0100, David Berry hath writ:
Section 2.1.2 of FITS-WCS paper 1 says "Furthermore, all CDELTi must
be non-zero." Even if NAXISn is 1, that one pixel should still have
some width. If any CDELT is zero then the transformation from pixel to
WCS cannot be inverted.


In the lingo of RFC, I think this is a SHOULD and not a MUST.

More explicitly, CDELTi SHOULD be non-zero, and FITS intepreters
SHOULD NOT fail if CDELTi is zero.

The subject of non-invertible transformations does arise in
FITS images. The simplest and most historic example would be
a photographic plate exposed with an objective prism.
The draft of the WCS Distortions paper already says more on this
subject.

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Steve Allen WGS-84 (GPS)
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