[fitsbits] Proposed Changes to the FITS Standard

[Thierry Forveille]

Jonathan McDowell a écrit :
> 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.
All cases I have seen were instances of "updating" the information by
appending a second copy of the keyword an the (then) end of the
header. Some of these might trace back to real time writing on
9-track tapes, when rewinding to update would have been a significant
cost and this was done as an optimisation/compromise (better have
slightly ambiguous information that plainly incorrect one), while
others certainly are just plain laziness/carelessness. It is easier
to write at the end of the header, so I'd expect the latest version
of the keyword to almost always be last in the header. Does anyone on
the list have examples of FITS files where the prefered version (when 
that can be determined...) of a duplicated keypword is not the last
occurence?

> So I think we should just strongly deprecate (not ban,
Agreed.

> and not impose an interpretation).
Depends on what the situation is in the field: if we can conclude
that in >90% of the affected files the prefered version is always
the last occurence (or always the first), I think that recommanding
that choice would be useful. If both endianesses occur with similar
probabilities (or if we cannot conclude), then of course let's
document that behaviour is undefined.


PS: The duplicated keywords I can remember were all for relatively
secondary information (things like DATE-WRITTEN, for instance), which
probably explains why the issue could be ignored for so long.