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[fitsbits] INHERIT and Hierarchical Grouping
Just glancing through the proposal, I noticed that it continues to
misuse EXTNAME, although understandable given an omission from the generalized extensions paper. (Sorry Bill if I sound like a broken record on this point.) GROUPING is a type; it describes the function or semantics of the HDU or component in a data structure, it is not a named instance. Don added these EXT* keywords specifically with Starlink hierarchical data in mind. It's my fault for not picking the omission of an EXTTYPE keyword from the extension paper before publication. It only came blindly obvious once I tried to convert a Starlink NDF dataset into FITS and back to the hierarchical format. Once I'd added an EXTTYPE to record the component data type, to supplement the path within the hierarchy stored in EXTNAME, it was possible to convert to FITS and back to NDF recovering the original structure. "Generalized extensions and blocking factors for FITS", Section 9 does suggest using the component paths "to establish easy to understand relationships between different extensions and even between different extensions in different FITS files." The overall concept looks good and goes beyond the single file. However, I should re-read it more carefully. I do prefer the VOTable way of grouping as explained by Doug, through familiarity with that perspective in our own packages. Malcolm Currie Rutherford Appleton Laboratory |
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