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[fitsbits] Repeated header keywords (was: Proposed Changes tothe FITS Standard)



 
 
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  #1  
Old August 19th 07, 01:38 AM posted to sci.astro.fits
Dick Shaw
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Posts: 3
Default [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.
  #2  
Old August 20th 07, 08:46 AM posted to sci.astro.fits
Peter Bunclark
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Posts: 6
Default [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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