Subject: hierarchical keywords

From: Michael Olberg <[email-masked]>
Date: Tue, 28 Nov 89 09:45:13 -0100

28-Nov-89

I'd like to comment on Bob Hanish's summary of the meeting
at ST ScI where he reports on the critical attitude of the
attendees towards a hierarchical keyword scheme in FITS which
would hide all but the globally agreed upon keywords from a
'classical' FITS reader program.

At the Greenbank meeting I was myself in favour of this scheme
compared to the Bob's BEGIN/END proposal which would leave all
low hierarchy keywords accessible to the present FITS parsers,
although they may not be able to deduce the full meaning of a 
keyword without the help of a human interpreter.

The implementation of a keyword hierarchy by simple BEGIN/END
grouping can in my opinion in fact be implemented in FITS 
without changing anything, i.e. without the BEGIN/ENDing 
of keywords. It might be a quick solution to  the  hierarchy
problem  while we are waiting for the gurus to come up with some-
thing more final and  sophisticated. The  whole  idea  was
triggered by a comment from Preben at the Greenbank meeting, where
he pointed out that there is in general  no   meaning  attached
to  the sequence in which FITS keywords appear on tape (except
for a very  few).  I  propose  that  we agree  on such a meaning,
namely that certain keywords define the domain of lower hierarchy
keywords that come  afterwards(!).  The  keyword   TELESCOP   for
instance   would  have  to  appear  before  all  keywords that we
thought should be site  specific.  Equally,  the keyword INSTRUME
could  trigger  some  extra meaning for the instrument specific
keywords following after. It is conceivable that we find  on  the
same  tape  from  a  given site two scans which were taken with
two  different  spectrometers,  where  the  spectrometer  charac-
teristics  are  given with the same set of keywords, although the
interpretation of them should  be  slightly  different,   because
they were preceeded by different INSTRUME cards. Of course the scope
of keywords like TELESCOP and INSTRUME would always be until the 
apearance of another keyword like that or the END card. If one 
introduced a keyword like DISCIPLN or BRANCH or whatever you like,
you would already have a scheme which was at least as powerful as
the Morgan/Stobie idea of SNGLDISH GENERAL and SNGLDISH NRAO-12M etc.

					  Michael Olberg
					  Onsala Space Observatory