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[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) _______________________________________________ fitsbits mailing list http://listmgr.cv.nrao.edu/mailman/listinfo/fitsbits |
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