[fitsbits] Proposed Changes to the FITS Standard

[William Pence]

Rob Seaman wrote:
> William Pence wrote:
> 
>> One of the proposed changes, however,  is to require that the  
>> PCOUNT and GCOUNT keywords must  immediately follow the last NAXISn  
>> keyword in all conforming extensions,
> 
> It seems to me that if this or any future revision to the standard is  
> going to attempt to require newly defined usage that this MUST be  
> paired with a version mechanism.  Otherwise "once FITS always FITS"  
> is violated big time.  Requiring a feature (a demand) is not the same  
> as deprecating its converse (a suggestion).  Rather, a feature that  
> isn't already required can only in the future be required against  
> some specially labeled subset of FITS files.

The "once FITS always FITS" philosophy captures the spirit of FITS, but 
in practice each new version of the FITS Standard has imposed new 
requirements that in principle could invalidate existing FITS files. 
For example, version 2.0 of the FITS Standard introduced a new 
requirement that the value and comment fields in a keyword MUST be 
separated by a slash character.  I think the FITS community in general 
has taken a somewhat pragmatic view that it is OK to add a new 
requirement to FITS that *might* invalidate older FITS files, as long as 
the benefit of the new requirement is perceived to out weigh the 
possible negative affects.

Of course, if the FITS community thinks a new requirement would cause 
too much dislocation to existing data or software, then an alternative 
would be to just "strongly recommend" instead of "require" the new 
feature.  It's also possible to specify that a new requirement will come 
into effect at some point in the future to allow time for software 
systems to adapt, as was done with the Y2000 change to the DATE keyword 
format.

In the new draft of the FITS Standard, the technical panel listed 22 
specific changes to the FITS requirements in the "Summary of Recommended 
Changes" document (at http://fits.gsfc.nasa.gov/fits_draft.html), but 
most of these either remove requirements, or only add a recommendation. 
  There are only 3 proposed new absolute requirements in this list:

  1. Keywords that have a value shall not be repeated in a header.

  2. PCOUNT and GCOUNT must immediately follow the last NAXISn
     keyword in all conforming extensions (as is already required
     in IMAGE, TABLE, and BINTABLE extensions).

  3. Embedded space characters are now forbidden within numeric
     values in an ASCII Table (e.g.  "1 23 4.5"  is no longer
     allowed to represent the decimal value 1234.5)

The public comment period on these, as well as all the other recommended 
changes, remains open here on this email list/newsgroup until at least 
the end of September...

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)