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[fitsbits] CDELTn
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[fitsbits] CDELTn
On Thu, 20 Sep 2007, Mark Calabretta wrote:
It's perfectly illegal, Sect. 2.1.2 of WCS Paper I leaves no room for doubt: "The PCi_j matrix must not be singular; it must have an inverse. Furthermore, all CDELTi must be non-zero. In other words, To repeat what I said in a previous post, WCS papers will be "incorporated by reference" in the draft FITS standard 3.0, so there will be no doubt they are applicable. But maybe it is worth adding ranges of legal values to table 8.2 of thereabout (Bill are you keeping track of this for when you return from the ADASS ?) The only other point is that CDELTn were around before WCS, and we have in 3.0 the rule that a change of a standard cannot make invalid what done before ... 3) If CDELTia isn't meaningful, as for a degenerate axis, then simply omit it. It defaults to 1.0. Looks a sensible suggestion. Not using degenerate axes is perhaps even more sensibile :-) Lucio Chiappetti -- ---------------------------------------------------------------------- is a newsreading account used by more persons to avoid unwanted spam. Any mail returning to this address will be rejected. Users can disclose their e-mail address in the article if they wish so. |
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[fitsbits] CDELTn
The only other point is that CDELTn were around before WCS,
Around before the WCS paper yes, but not around before WCS: a minor source of confusion in this discussion has been that WCS existed from the very begining of FITS (the WCS phrase may have appeared later, not really sure, but the concept was there from the begining), in their basic linear and orthogonal form. What the WCS paper did was add support for distortion, projections, but basic WCS is much older. 3) If CDELTia isn't meaningful, as for a degenerate axis, then simply omit it. It defaults to 1.0. Looks a sensible suggestion. Not using degenerate axes is perhaps even more sensibile :-) Not at all :-) Degenerate axes are actually a most natural (if perhaps initially slightly counterintuitive, for some) way of conveying position information along the missing axes. One classical example is using two degenerate axes to convey RA & Dec (or lII & bII, or whatever) information for a set of individual spectra obtained on a grid, where you can set CDELTn to the grid spacing, CRVALn to RA at grid center, and CRREFn to the pixel position of the grid center relative to current datum (i.e. minus the pixel position of current datum in the grid). Very convenient, and commonly used for single-dish radioastronomical spectra. |
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[fitsbits] CDELTn
On Thu, 20 Sep 2007, Thierry Forveille wrote:
Looks a sensible suggestion. Not using degenerate axes is perhaps even more sensibile :-) Not at all :-) Degenerate axes are actually a most natural (if perhaps initially slightly counterintuitive, for some) way of conveying position information along the missing axes. This has long been a point of argument. In one view a dataset can be viewed as N-dimensional, where N is large and there can be many "axes", any of which are potentially degenerate (single valued). In the other view, one has an N-dimensional sampled dataset and any number of other dataset attributes which are constant for the entire dataset but which do not qualify as sampled axes. The degenerate axis view might make sense for a very few things which are often sampled (polarization, frequency/velocity, possibly time), where a common model can be used whether or not a given physical "axis" is sampled (Characterization in VO is similar). But in the general case, applied to any "image" attribute, this is a poor model. - Doug |
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[fitsbits] CDELTn
On Thu, 20 Sep 2007, Thierry Forveille wrote:
The only other point is that CDELTn were around before WCS, Around before the WCS paper yes, but not around before WCS: [...] but basic WCS is much older. I was of course referring to the WCS papers. The "basics" (CRVAL CRPIX CDELT and CROTA) were probably there since the very beginning (but CROTA was later deprecated). I am not able now to verify if any of the earliest (pre-WCS-paper) standards explicitly specified constraints for CDELT ... and hence to tell whether Francois's CDELT=0 files were once legal. Looks a sensible suggestion. Not using degenerate axes is perhaps even more sensibile :-) Not at all :-) Degenerate axes are actually a most natural (if perhaps initially slightly counterintuitive, for some) way of conveying position information along the missing axes. One classical example is using two degenerate axes to convey RA & Dec (or lII & bII, or whatever) information for a set of individual spectra obtained on a grid, I was thinking of much sillier cases. People writing a 1-d spectrum as a FITS image (and this instead of a bintable is something I accept) as NAXIS=2 NAXIS1=nbins NAXIS2=1 -- ---------------------------------------------------------------------- is a newsreading account used by more persons to avoid unwanted spam. Any mail returning to this address will be rejected. Users can disclose their e-mail address in the article if they wish so. |
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