[fitsbits] INHERIT and Hierarchical Grouping

[Doug Tody]

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                       pence at milkyway.gsfc.nasa.gov
> NASA/GSFC Code 662       HEASARC        +1-301-286-4599 (voice)
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>
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