[fitsbits] Proposed Changes to the FITS Standard

[William Pence]

Don Wells wrote:
> I have examined the 'Recommended Changes' document dated 07-18, and find 
> that the recommendations are acceptable.  My compliments to the chefs!

but,

> Item 21 recommends changes to section 7.3.3.2 that would "remove the 
> option to use the heap and the PCOUNT keyword in ways other than 
> described in the variable length array section 7.3.5".   This wording 
> makes me nervous because tricks with the heap and the gap area are an 
> obvious potential escape hatch that we left in the Binary Tables 
> Agreement in 1991.  I decided to examine the color-coded differences 
> document (very nice!).   I see changes marked that seem to be related to 
> this item, but I am unsure which changes accomplish the goal of the 
> item.  

The recommendation is to make a subtle change the wording of section 
7.3.3.2, which describes how the heap should be used, from this:

     "One use for this data area is described in section 7.3.5."

to this:

     "The use of this data area is described in section 7.3.5."

The current ambivalent wording (i.e. "One use" instead of "The use") is 
left over from when the variable length array convention was only 
described in an unofficial appendix.  Now that this convention has been 
officially approved as part of the FITS Standard (in 2005), it seems 
appropriate to state more definitely how the heap is intended to be 
used.  This is more consistent with all the other data structures (e.g. 
the primary array, or random groups) that are defined in the FITS 
Standard to be used in only a single specific way.

The technical panel also recommends deleting the blanket statement at 
the end of this section ("This does not preclude other uses for these 
bytes.") for a similar reason.  This boilerplate statement was routinely 
added at the end of all the conventions that were described in the 
unofficial appendices in the FITS standard.  Now that this convention 
has been officially moved into the body of this standard, this 
disclaimer is not necessary nor strictly appropriate.

As Don rightly points out, this change would eliminates a potential 
"escape hatch" in FITS that conceivably could serve a useful purpose in 
the future.  However, I would argue that it is better to eliminate this 
ambiguity for now;  if someone does come up with a clever alternate use 
for the heap in binary table, then the IAU FITS Working Group always has 
the power to modify the standard to allow this new use in the future, as 
long as it does not conflict with the use of the heap in existing FITS 
files.

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)