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[fitsbits] How is NAXISn supposed to work?



 
 
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Old April 27th 09, 05:39 PM posted to sci.astro.fits
Peter Teuben
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Default [fitsbits] How is NAXISn supposed to work?

ok, gently then. The crux is the (english) language I guess:

"index along axis 1 varies most rapid'
^^^^^^ ^^^^^

wihch refers to how an array DATA(i,j,k,....) is indexed.
What they mean to say is that in a 2D example element DATA(i,j)
is followed by DATA(i+1,j) [and not DATA(i,j+1)].

This is the fortran style of indexing, as opposed to the C style
of indexing, where the last index varies most rapidly.

If I reread the sentence, I'm not confused about the values of NAXISi,
or that NAXIS1 NAXIS2.., but then again, I've been looking at, and
using this type of data for some time.

- peter



On [Mon Apr 27 17:54], RPEHLM wrote:
Hello folks,

I'm a simple sort of chap, so I have a simple question! g

Reading the IAUFWG v3 (2008 July 10) documentation for the Primary Data
Array (page 14, section 3, sub 3.3.2). I'm not understanding what is
meant by:

"Arrays of more than one dimension shall consist of a sequence such
that the index along axis 1 varies most rapidly, that along axis 2 next
most rapidly, and
those along subsequent axes progressively less rapidly, with that along
axis m, where
m is the value of NAXIS, varying least rapidly."

Does axis 1 equate to NAXIS1 and axis 2 to NAXIS2 etc.? Should the
value of NAXIS1 be greater than NAXIS2, which in turn should be greater
than NAXIS3 etc.?
Or should the value of NAXIS1 be the smallest and NAXISn the greatest?
Or something else entirely? For example, take a 100 by 200 array, would
the following be correct:

NAXIS = 2
NAXIS1 = 100
NAXIS2 = 200

or this:

NAXIS = 2
NAXIS1 = 200
NAXIS2 = 100

(Yes, I know the formatting is not exactly right g)

The documentation would seem to indicate the former, but I have seen
plenty of commercial applications implementing the later. Hence the
confusion to my poor abused grey thing.

Regards
Robin
P.S. My first posting so please be gentle!!!
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