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[fitsbits] Abuse of EXTEND keyword
On Thursday 23 August 2007 03:25, Tim Pearson wrote:
My guess (it's a long time ago) is that there were FITS files out * there that had special records that did not conform to the rules for * standard extensions, so the EXTEND keyword was needed. Or perhaps * Grosbøl et al wanted to leave open the possibility of other uses for * the special records, which would be indicated by different header * keywords. At the time, special records were used (most noticeable by random groups). Thus, it was important to have a flag (EXTEND=T) to indicate that the file conformed to the new rules for FITS extensions. Due to the wide usage of random groups, special rules were introduced to allow coexistence but other less used types of special records existed. This flagging function (as used with GROUPS=T in random groups) indicates that all special records conform, as stated in the last part of the sentence. Thus, EXTEND=F could mean one of two things: 1) the writer knew about the new rules for extensions but did not care/conform, or 2) there are no extensions (which however is a subcase of EXTEND=T). As I said previously, it also visually showed the user, by listing the primary header, that information could be present in extensions. During the introduction of the extension rules, the lack of EXTEND=T showed clearly that the writer did not know about the new rules. Now almost 20 years later, this is not an issues. We may ask if anybody know of FITS files (possibly archival) which use their own 'special' special records. Even so I see no major problem in making the keyword optional butit's not a significant issue. I prefer such flags (GROUPS, BLOCKED, EXTEND) as they, by nature, just indicated an additional convention. If one introduces an explicit version, it may be too easy to make (too) strong changes. Preben PS: I do notice and appreciate the 'ø'. |
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[fitsbits] Abuse of EXTEND keyword
On Thu, 23 Aug 2007, Preben Grosbol wrote:
I prefer such flags (GROUPS, BLOCKED, EXTEND) as they, by nature, just indicated an additional convention. I share this preference, provided the convention is useful and actually used ... anyhow this does not imply the presence of the flag be mandatory. On Wed, 22 Aug 2007, Tim Pearson wrote: The Grosbøl et al. paper (1988 A&A) said: "Note that the presence of EXTEND=T in a primary FITS header ... indicates that the file _may_ have extensions records and that any special records will conform to the rules below." The important part of this statement is the last bit, "any special records will conform to the rules below" Thanks Tim for reminding us of the second part of the statement. However I wonder about the actual situation. I consider three cases for the EXTEND keyword, and four things it may signal (the possibility to have extensions, the presence of actual extensions, the presence of special records, their conformance to the standard). The answer to each thing can be YES NO or MAYBE. But what does "conformance of special records to the standard" mean ? Am I correct in assuming that in 1988 parliance "conforming extensions" were one possible kind of "generalized special records" while in 2007 parliance "conforming extensions" are no longer "special records" ... or at least the (deprecated) "special records" are all other possible kinds of "generalized special records" (mutually exclusive with "conforming extensions") ? I note that EXTEND=F was never *explicitly* defined I try to make this truth table ... may have ext's HAS ext's uses spec rec they conform --------------------------------------------------------------------- no EXTEND yes maybe maybe maybe EXTEND=T yes maybe maybe yes if present EXTEND=F ?? ?? maybe no if present I can't figure out if the "??" shall be NO or "maybe". But definitely the "maybe" in the first two lines are "maybe", so they are equivalent .... which seems to indicate that a mandatory EXTEND=T is not needed (which apparently is just the change in the wording of FITS 3.0 ... but no practical change in usage ?) Lucio Chiappetti -- ---------------------------------------------------------------------- is a newsreading account used by more persons to avoid unwanted spam. Any mail returning to this address will be rejected. Users can disclose their e-mail address in the article if they wish so. |
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