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[fitsbits] Fwd: multiple keyword occurance in header



 
 
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Old August 21st 07, 12:37 PM posted to sci.astro.fits
Walter Jaffe
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Default [fitsbits] Fwd: multiple keyword occurance in header

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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