[iaufwg] Voting result on the FITS Standard

[William Thompson]

William Pence wrote:
> Dear IAU FITS Working Group members,
> 
> I am pleased to announce that all 22 members have voted and have 
> unanimously approved the draft FITS Standard, version 3.0c, as the new 
> official FITS Standard document.  However, a number of these votes of 
> approval were conditional upon adding a clarification to the document on 
> the use of the CRPIXj keyword.  This issue was first raised by William 
> Thompson in his email on 06 May 2008.
> 
> I privately asked Mark Calabretta and Eric Greisen to suggest a specific 
> change to the wording of the document to address this issue.  Below is 
> Mark's proposal:
> 
> 1) Insert the following paragraph at the start of Sect. 8.1:
> 
>    Rather than store world coordinates separately for each datum (e.g. as
>    is done for random groups data), the regular lattice structure of a
>    FITS image offers the possibility of defining rules for computing
>    world coordinates at each point.  As stated in Sect. 3.3.2 and
>    depicted in Fig. 3.1, image array data are addressed via {\em integral
>    array indices} that range in value from 1 to NAXISj on axis j.
>    Recognizing that image data values may have an extent, for example an
>    angular separation, spectral channel width or time span, and thus that
>    it may make sense to interpolate between them, these integral array
>    indices may be generalized to floating-point {\em pixel coordinates}.
>    While integral pixel coordinate values coincide with the corresponding
>    array indices, pixel coordinates are defined at all points within the
>    image lattice and outside it (except along conventional axes, see
>    Sect. 8.5).  They form the basis of the world coordinate formalism in
>    FITS depicted schematically in Fig. 8.1.

I suggest breaking up the sentence starting "While integral pixel ..." into two 
sentences:

Integral pixel coordinate values coincide with the corresponding array indices, 
while fractional pixel coordinate values lie between array indices and thus 
imply interpolation.  Pixel coordinate values are defined at all points within 
the image lattice and outside it (except along conventional axes, see Sect. 8.5).

Bill Thompson


-- 
William Thompson
NASA Goddard Space Flight Center
Code 671
Greenbelt, MD  20771
USA

301-286-2040
William.T.Thompson at nasa.gov

(Note changed email address)