🚀 go-pugleaf

From article <1dev6dINNnkp@darkstar.UCSC.EDU>, by sla@umbra.UCSC.EDU (Steve Allen):
> 

[... Does the center of pixel (1,1) have the coordinates (1.0,1.0) or (1.5,1.5)???]

> If the astronomical community will be variant in this way it will be
> essential to clearly document the definition of a pixel.  There will
> undoubtedly be some astronomical programmers who cling to the more widely
> used CG definitions of pixels, and interoperation of their software with
> other software will be difficult if these underlying assumptions are not
> made explicit in the documentation.
> _______________________________________________________________________________
> Steve Allen          | That was the equation!         |   sla@lick.ucsc.edu
> UCO/Lick Observatory | Existence!...Survival must     | If the UC were opining,
> Santa Cruz, CA 95064 | cancel out programming! -- Ruk | it wouldn't tell me.


I agree that clear documentation is crucial. Here at SAO we have always
used the "CG" definition (center of bottom corner pixel is at "0.5");
IRAF uses  mostly "1.0" but not always; and historically FITS has left
it up to the user to include a comment card to document how the
coordinate system used by CRPIX is mapped on to the data array. We've
run into awful trouble in the past by getting confused about this. I'm
happy with the "1.0" choice, but we need to make it as explicit as
possible. Make the distinction between the number of a pixel (an integer
n-tuple) and the coordinates of its center (a real n-tuple) explicit,
even if the choice of convention makes them numerically equal. 

I hope the standard will use lots of small and simple words and some
examples (like: "The first data item in the array section is considered
to be the pixel whose number is (1,1,1,...1). The number of the next
pixel is (1,1,1,...,2). The  coordinate value corresponding to the
center of the pixel numbered (1,1,...1) is (1.0,1.0,...,1.0)."


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 |  Jonathan McDowell                 |  phone : (617) 495-7176                |
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