[fitsbits] Fwd: Re: multiple keyword occurance in header

[Arnold Rots]

The way we solved fixing the order of HISTORY keywords in headers of
Chandra FITS files was to reserve the last 8 characters for a sequence
label (3 chars) + sequence number (5 digits).
This would result in something like this:

HISTORY  TOOL  :mytool  12.03.09                                        ASC00001
HISTORY  PARM  :infile=something.img                                    ASC00002
HISTORY  PARM  :outfile=/local/data/xebec/arots/mydata/axaf/b1509/resultASC00003
HISTORY  CONT  :s/fileWithALongName                                     ASC00004


  - Arnold

Walter Jaffe wrote:
> Two (smallish) problems with 
> HISTORY text 1
> HISTORY text 2 ...
> 
> 1. If the original texts+comments fill the full 80 byte card
>    image, the last 8 characters, usually comments, will be lost.
> 
> 2. HISTORY records are, after the initial 8 bytes, free format.
>    If we don't use the BLOCK HISTORY construction (I think this
>    is a terrible suggestion of mine for keywords, 
>    but that can be fixed later), then we will have mixed series
>    of "real" HISTORY records (thus free format) and "BLOCK"
>    history records, which are strictly formatted according to
>    FITS standards (e.g. must be KEYWORD= or COMMENT or HISTORY after
>    the initial HISTORY keyword).  If an interpreter wants to unpack
>    the BLOCK records (maybe LITERAL, or NAMESPACE are  better words, 
>    see below) without having the special BEGIN/END as clues, it
>    will run into unparseable standard HISTORY records.
> 
> 
> >Well, it's a good thought that we might want to understand why issues  
> >like duplicate keywords occur before making changes to the standard.   
> >But I'm not sure I see the advantage of:
> >
> >	BLOCK HISTORY BEGIN
> >	text 1
> >	text 2
> >	text 3
> >	BLOCK HISTORY END
> >
> >to
> >
> >	HISTORY text 1
> >	HISTORY text 2
> >	HISTORY text 3
> >
> >other than gaining 8 characters per line.
> >
> Clearly I was naive in expecting that repeated keywords come mostly
> from history concatenations.  As stated below by Rob, it becomes clear
> that the main problem is a namespace problem: different input streams
> use the same keyword with different meanings.  ESO "solves" this problem
> by using a HEIRARCHal keyword structure, that is nonstandard and
> restricted to ESO applications.  I speculate on a slightly grander 
> scheme than BLOCK HISTORY, which is something like:
> NAMESPACE BEGIN 
> NAME=
> DATE=
> KEYWORD1=
> KEYWORD2=
> NAMESPACE END
> 
> Which defines a piece of the header stream with a single consistent
> use of keywords.  The convention/standard/recommendation would be
> that within one namespace, keywords must be unique.  If the
> reading program wants to distinguish between uses of the same
> keyword it can try to find the correct NAMESPACE if knows which
> one it wants, or can search on the basis of date.  If it has
> no information as to which NAMESPACE is the best, it should
> regard the keyword as undefined.
> 
> Walter
> 
> >In any event, sequential header edits is only one way duplicates can  
> >arise.  NOAO iSTB grabs raw data from a couple of dozen different  
> >instruments.  These raw data often demonstrate a wide variety of  
> >"interesting" metadata, with just one example being duplicate  
> >keywords.  I would attribute this to an interaction between various  
> >groups (who often work for different organizations):  the instrument  
> >engineers, the data acquisition programmers, the telescope control  
> >system, the data handling system - to name a few.  There often is no  
> >one final arbiter of the content of the header - and this issue is  
> >rarely at the top of the list for anybody before the data reach the  
> >archivists.  In fact, an archive is an instrumentality for asserting  
> >coherent standards - but it requires a bit more subtlety than simply  
> >telling everybody up stream that something is verboten.
> >
> >Rob
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> 
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Arnold H. Rots                                Chandra X-ray Science Center
Smithsonian Astrophysical Observatory                tel:  +1 617 496 7701
60 Garden Street, MS 67                              fax:  +1 617 495 7356
Cambridge, MA 02138                             arots at head.cfa.harvard.edu
USA                                     http://hea-www.harvard.edu/~arots/
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