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[fitsbits] Proposed Changes to the FITS Standard
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 NASA/GSFC Code 662 HEASARC +1-301-286-4599 (voice) Greenbelt MD 20771 +1-301-286-1684 (fax) |
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