[fitsbits] FITS Binary Table Proposals

[William Pence]

In anticipation that the FITS committees will soon be asked to vote on the 
proposals to incorporate the TDIMn and variable length array conventions 
into the FITS Standard, I have updated the draft proposal documents 
available at

http://fits.gsfc.nasa.gov/bintable_proposals.html

to explicitly list every single editorial change that will need to be made 
to the text of the FITS Standard document.   The main change will be to move 
the text that is currently in the Appendices B.1 and B.2 into section 8.3 
that defines the binary table format, but in addition, a number of other 
small editorial changes will be needed, for example to update all the 
existing references to Appendix B.

The only new modification of any technical substance that has been made to 
this latest draft is to clarify that the minimum allowed value for the THEAP 
keyword (which gives the byte offset to the start of the heap data area) 
shall be the product of the NAXIS1 and NAXIS2 keyword values.  This forces 
the heap to follow the main table data and prevents any overlap of the two. 
  As Tom McGlynn previously pointed out, this requirement is not clearly 
stated by the current wording.

Without this requirement someone might design a FITS table in which some of 
the variable length array descriptors point to a location in the fixed 
length part of the table.  Even more extremely, if negative THEAP values are 
allowed, then the descriptor could point to a previous header keyword 
record, or conceivably to data in a previous HDU in the file.  While it is 
possible that this feature might be put to good use by a clever FITS file 
designer, I think it is clear that this usage was never intended, and that 
allowing this could cause complications for software that supports variable 
length arrays.

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