[fitsbits] FITS vis a vis XML

[Steve Allen]

On Wed 1999-12-22T14:11:39 +0100, Francois Ochsenbein hath writ:
> => FITS has 2 formats for tabular data; the binary tables (which seem to
>    be preferred ?) have either to be encoded in some way, or to be loaded
>    by a second connection; iven with the "ascii tables" format, the data
>    can't just be displayed with the standard tools that are available
>    on a PC or a Unix station.

Hogwash.  I'm doing it with dd, sed, and awk.  I could switch to perl alone.

> => there is no facility (yet?) in FITS for managing links -- definitions
>    which specify how to get the related data or documentation, which is
>    a requirement in a distributed environment. Just the limitation to 70
>    characters in the headers prevents from writing (not so long) URLs !

There is the FITS Grouping Proposal
<http://fits.gsfc.nasa.gov/group.html> which does not solve the
problem, but gives good ideas on how to start.  We are toying with
alternative representations that use some of the ideas, but we're not
done.

> => mixing XML and FITS which both provide metada describing the data
>    contents and structures in the same document would inevitably generate
>    conflicts --- any modification in the XML dscription would have to be
>    reflected in the FITS headers and vice-versa; the convertibility
>    paradigm looks more appropriate, and safer too.

Indeed, as I contemplate XML I've come to realize that for semantic
purposes we've already been using it.  The database of FITS keywords,
interrelationships and structures that we created to hasten our
instrument development projects could be recast as a DTD.

We constrained our usage of FITS to conform to the rules of the
database.  Our FITS table HDUs plus the database schema are equivalent
to XML.  The part that we are missing is a way to put the database
schema (effectively the DTD) explicitly into the FITS files.

This could result in a new type of FITS extension, or maybe just a new
set of constraints for the use of existing facilities, which would
make the interconversion between FITS tables and XML completely
explicit.  We'd like to develop this, but we are limited to producing
only what is necessary for an operational spectrograph.

--
Steve Allen          UCO/Lick Observatory       Santa Cruz, CA 95064
sla at ucolick.org      Voice: +1 831 459 3046     http://www.ucolick.org/~sla
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