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[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. -- Steve Allen WGS-84 (GPS) UCO/Lick Observatory Natural Sciences II, Room 165 Lat +36.99855 University of California Voice: +1 831 459 3046 Lng -122.06015 Santa Cruz, CA 95064 http://www.ucolick.org/~sla/ Hgt +250 m |
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