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[fitsbits] Start of the 'INHERIT' Public Comment Period
Bob says:
I have to express some concern about registering the INHERIT convention. The documentation notes a number of potential problems that can occur when FITS software that is unaware of the convention is used to read, interpret, and write new copies of files that use INHERIT. 1) Isn't the notion of the convention registry to record what is already being used around the FITS community? NOAO relies on INHERIT for our MEF data from the MOSAIC DHS and will rely on it for the NEWFIRM DHS. The NOAO Pipeline understands INHERIT. The convention has served us well. 2) NOT documenting INHERIT certainly won't help the community. 3) IRAF has understood INHERIT for three-score-and-ten dog years. It would be cleaner, I think, to define more clearly the rules for how primary headers pertain to extension headers (e.g., the concept of inheritance applies by default, or whatever). 4) Yes, but that boat has sailed. The community has been on a course to deal with inheritance since this note from the image extension paper: "Although allowed, it is recommended that the primary header does not set the keyword NAXIS=0, since it would not make sense to extend a non-existing image with another image." FITS is either going to tie the contents of separate HDUs together semantically or not. The community eagerly - and widely - adopted the notion of the primacy of the primary HDU - likely before the words above were published. Implicit here is that the primary header of an empty HDU is often used for information that applies to the entire file. 5) If not INHERIT, then what? 6) And we'd still be left with gazillions of files that rely on this convention as an organizing semantic principle. Clearly the first step in revisiting the fundamental semantics of a FITS file (of which keyword inheritance is only a small part) would be to protect our investment in previous data products by documenting the current de facto standards. 6a) In any event, the first step in deprecating any convention would be to recognize its existence. This is something we might recommend to the recently formed FITS review panel to discuss. 7) By all means, but only in an advisory capacity. I presume we're not thinking of changing the fundamental FITS standards process? It has served us well for many years. 8) Really - isn't documenting the current usage the simplest thing to do? All of these conventions are conventional, rather than standard, precisely because they reflect issues that were thorny to deal with the first time around. Few will fall into the same category as the checksum keywords - i.e., pre-existing legal FITS usage demanding no clarification of the standard. Rob |
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