[fitsbits] Proposed Changes to the FITS Standard

[William Pence]

Rob Seaman wrote:
> Speaking of which, it is the duplicate keyword requirement that seems  
> most onerous.  To implement this efficiently for all keywords, one  
> would have to build a hash table or some such for each header.  Then  
> one is left with the question of what to do upon detecting a  
> duplicate.  The sense of a requirement is to simply throw an error  
> and exit.  How helpful is that? 

This is missing the main point of this new requirement.  No current 
software system that I am aware of (except for the FITS verifier code) 
checks for duplicated keywords, so users have no idea which of the 
duplicated keywords is being used by a particular program.  The software 
might be using the first, the 'next', or the last instance of the 
keyword.  This could easily cause the user to derive incorrect 
scientific results.  What is the best way to prevent this from 
happening?   Seems to me we should focus on the root of the problem and 
(formally at least) disallow duplicated keywords in a conforming FITS 
file.  This doesn't mean software should automatically throw out a file 
that inadvertently has a duplicated keyword.

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.

Bill Pence
-- 
____________________________________________________________________
Dr. William Pence                       pence at milkyway.gsfc.nasa.gov
NASA/GSFC Code 662       HEASARC        +1-301-286-4599 (voice)
Greenbelt MD 20771                      +1-301-286-1684 (fax)