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[fitsbits] Repeated header keywords (was: Proposed Changes tothe FITS Standard)
Jonathan is quite correct that when duplicated keywords appear with different
values, it is not in general possible to know which value was intended. On Fri, 17 Aug 2007, Jonathan McDowell wrote: I agree with Thierry that there are many files which have repeated keywords, but I agree with another poster that there are existing implementations which assume it's the first instance, not the last instance, which prevails. So I think we should just strongly deprecate (not ban, and not impose an interpretation). But it is not that simple. How is a user or an application supposed to interpret the corrupted metadata? As Bill points out, depending upon which keyword is affected the problem may radically alter the scientific interpretation or, worse, render the object uninterpretable. I think we do not want to "deprecate" bad usage that was never explicitly permitted in the Standard, even if it was never forbidden either. The horse has long since bolted from the barn, and there is little to do but provide some guidance in the standard. Bill Pence wrote: Stepping back a little, I think the seriousness of this problem depends on what keyword is duplicated. If it is just some observatory-specific keyword that does not directly affect the scientific results, then it does not matter very much, and data providers need not worry about it. But if a critical WCS keyword, or exposure time keyword is duplicated in the file with different values, then surely the data providers need to take responsibility and fix the problem. One approach would be to say that headers _should not_ contain repeated keywords, and if a repeat does occur then the value is not defined (unless the values are identical). Ideally, this could have a few desired effects: it would encourage authors of FITS verifiers to flag instances of repeated keywords (though I suspect they do already), it would encourage FITS writers to pay attention to this problem, and it would encourage application developers to be refrain from silently adopting the first, last, or whatever instance of a keyword value without telling the user. |
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[fitsbits] Repeated header keywords
Dick Shaw wrote:
One approach would be to say that headers _should not_ contain repeated keywords, and if a repeat does occur then the value is not defined (unless the values are identical). Ideally, this could have a few desired effects: it would encourage authors of FITS verifiers to flag instances of repeated keywords (though I suspect they do already), it would encourage FITS writers to pay attention to this problem, and it would encourage application developers to be refrain from silently adopting the first, last, or whatever instance of a keyword value without telling the user. This looks to me like the best wording so far. I too have had many a weary battle with ``software engineers'' to get them to to conform to the existing standard; expecting a change which in so many cases just doesn't matter (the example of duplicated instrument temperatures) will just get zero priority. Would it be too complex to make the non-duplication of reserved words mandatory and of others just strongly recommended? In the real world, if crucial keywords like those in the WCS are repeated with different values, and applications randomly choose one or the other, that should soon be seen as a hard bug in the FITS writer and would get fixed - although of course it would be better if verification software caught such problems first. But if the verification program marks all my data files as non-conforming because of a duplicate temperature reading, I get into the regime of "oh, you can ignore those errors, they're not important" (er, I also support the less-able end of the software community). I really want to be able to say to the software authors "keep going till fitsverify passes". Pete. |
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