[fitsbits] INHERIT and Hierarchical Grouping

Hi Bill -

I just took a quick look at this. A word of warning: when I first
heard of this convention some weeks ago, I thought from the name that
what was being referred to was the HIERARCH (hierarchical keyword)
convention from ESO. I see now that this is something completely
different.

I only had time to skim the document, but it does appear to offer a
more general alternative to INHERIT, and does much more. It reminds
me somewhat of the GROUP construct in VOTable, which might be worth
looking at as a comparision (the VOTable GROUP refers to fields rather
than tables or extensions, but otherwise is similar in providing a
way to describe hierarchical relationships). One difference is that
in the FITS HGC a group tells what other groups it is a member of,
as well as lists member extensions, whereas in VOTable GROUP, a GROUP
only lists its member elements, which can be either other GROUPs or
simpler member elements.

Both of these constructs provide generic ways to specify a logical
structure which applies to an otherwise unstructured collection of
objects; since the structure is not explicit, the data can be viewed
either way. An alternative approach is a data model, for example
the entity-relationship model often used in relational databases.
This can describe more complex relationships and does not require
use of an explict grouping construct, but is less direct.

- Doug



On Tue, 17 Apr 2007, William Pence wrote:

Doug Tody wrote (06-04-2007) regarding INHERIT:
I agree with Steve that this is a simple example of a broader problem
of associating relational entities. FITS is in essence a relational
system; every FITS object (even an image) is actually a table.
INHERIT is a simple means for specifying the relationship between two
or more tables composed as an MEF. A FITS MEF is a simple container
with one level of structure. ...
The real problem with INHERIT is that it is a simplistic solution to
what is a more general problem.

Since no one else has commented on this yet, I'll just point out that
the Hierarchical Grouping convention (now also open for public comment)
provides a more general mechanism for specifying the relationship
between multiple HDUs that may be in different files or even on
different computer systems. Information about both the INHERIT and
Hierarchical Grouping conventions is available at
http://fits.gsfc.nasa.gov/fits_registry.html.

Does anyone have any comments about the Hierarchical Grouping convention
itself? Are there any deficiencies or limitations in this convention
that have not been considered? Are there alternative ways of
accomplishing the same thing that might be simpler or offer more
features than this convention?

Bill Pence
--
__________________________________________________ __________________
Dr. William Pence
NASA/GSFC Code 662 HEASARC +1-301-286-4599 (voice)
Greenbelt MD 20771 +1-301-286-1684 (fax)


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