Subject: Re: Proposed TAXISnnn/TDESCnnn/TNDIMnnn conventions

From: tody at noao.edu (Doug Tody)
Date: 1 Oct 91 16:24:19 GMT

Bill, I suppose any number of conventions can be proposed and tried.
Since conventions are not part of the FITS standard one is free to try
anything in this regard.  For the sake of argument, I will offer some
comments on your proposals.

TAXISnnn
	The one-dimensional array-of-records format upon which binary tables
	is based was not chosen arbitrarily, it is what is used in a
	relational database.  There is a wealth of experience with this
	model, not only in commercial databases, but in the existing "table"
	implementations found in various astronomical software systems.
	Your N-dimensional array of structures, on the other hand, is in my
	opinion a fairly arbitrary generalization of the simple table, and
	does not justify departing from the relational model.  If we want to
	abandon the relational model and propose a database with more complex
	structures then there are no end of ways this can be done - and
	currently, no standards (this is a hot area of research currently).

TDESCnnn
	You are starting to reinvent world coordinate systems.  See the
	paper by Hanisch et.al. describing a proposed convention for
	representing world coordinate systems in FITS.  In the general case
	this is a very difficult problem.

TNDIMnnn
	This is a good idea (allowing the array dimension to be specified
	separately for each record).  A generalization of the TDIMnnn
	convention along these lines is probably needed.

This discussion illustrates further why Appendix A is a convention and not
part of the core proposal.

These proposed conventions would all be layered upon the basic binary tables
format.  Ultimately many such conventions are likely to be developed to
represent different types of data.  Discussion of the basic binary tables
format is also welcome.
-- 
Doug Tody, National Optical Astronomy Observatories, Tucson AZ, 602-325-9217
UUCP: {arizona,decvax,ncar}!noao!tody  or  uunet!noao.edu!tody 
Internet: tody at noao.edu             SPAN/HEPNET: NOAO::TODY (NOAO=5355)