[fitsbits] Bintable proposals

[Don Wells]

Dear Mark,

Mark Calabretta writes:
 > On Wed 2001/11/14 07:09:31 CDT, Bill Cotton wrote..
 > >   What you address is a fundamental FITS limitation rather than just
 > >a binary tables one.  Each header is required to say what the length
 > >of its data unit is..
 > 
 > Am I correct in saying that this arises from the fact that Modcomps
 > could not do file expansion..

No. 

This rule was a part of the Generalized Extensions Agreement of 1984.
It was specified at the same time that we were negotiating the basic
TABLEs Agreement, five years before the BINTABLE negotiations began.
The main purpose is to enable a reader to skip over FITS extensions
quickly.  A secondary motivation is to enable a reader to pre-allocate
space for data it has not yet encountered.

 > As I understand it the history goes something like this: the
 > genesis of FITS is linked to AIPS; AIPS was intended to run on
 > Modcomps; thus early versions of AIPS did not have file expansion;
 > therefore FITS didn't assume it.  This limitation has never been
 > relaxed.

Perhaps Eric Greisen may have had that motivation for the rule, but I
definitely didn't, and Preben Grosbol didn't either (Preben and I were
the negotiators on the Generalized Extensions Agreement of 1984).

Actually, the genesis of FITS was not linked to AIPS -- the real
history is that the design of AIPS was made to conform to FITS *after*
the Basic FITS negotiations were complete.  For my version of the
early history of FITS see:

    http://www.cv.nrao.edu/fits/documents/overviews/history.news

 > With regard to random groups data, the ATNF got around it from the
 > start (c1985) by inventing a FITS-like format called RPFITS..
 > ..RPFITS is not widely recognized and has sufficiently many other
 > idiosyncrasies (i.e. drawbacks) that we would like to move away
 > from it to something more portable.  Hence we decided it would be
 > better to bend the rules on binary tables hoping that the rest of
 > the world might catch up some day.  Part of the rationale is that a
 > BINTABLE reader which can't grok NAXIS2 = -1 would skip the file
 > and be no worse off contra a totally unrecognized format.

NAXIS2=-1 will cause an incorrect calculation of the length of the
BINTABLE extension.  Any reading application which tries to skip the
extension is likely to fail in some 'interesting' way, unless it has
been programmed to recognize this idiosyncrasy.  Of course, a reader
which applies a strict interpretation of the rules would declare a
negative NAXIS2 dimensionality to be in violation of the explicit rule
for NAXISn in Section 5.4.1.1 of the NOST Definition of FITS.

Regards,
Don
-- 
  Donald C. Wells      Scientist - GBT Project        dwells at nrao.edu
                    http://www.cv.nrao.edu/~dwells
  National Radio Astronomy Observatory                +1-434-296-0277
  520 Edgemont Road,   Charlottesville, Virginia       22903-2475 USA
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