[iaufwg] [fitsbits] IAU FWG approval of the new FITS Standard

[Dick Shaw]

It is perhaps worthwhile to keep the abstraction of the data array (which is 
quite general) distinct from how CRPIXj is used in practice. The reference 
point on the image, corresponding to particular world coordinates as specified 
by CRVALj, really is given in units of pixels. One may argue that CRPIX in 
hindsight might not have been the best choice of name for the reference 
location, but the concept of the reference point seems clear, at least to me.

-Dick

On Tue, 27 May 2008 10:40:25 -0700
  Steve Allen <sla at ucolick.org> wrote:
> On Tue 2008-05-27T13:25:12 -0400, William Pence hath writ:
> 
>> 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.
> 
>> CRPIXj - [floating-point, indexed, default: 0.0]
>>   Location of the reference point in the image for axis j, in units
>>   of pixels, corresponding to $r_j$ in Eq. (8.1).  Note that the
>>   reference point may lie outside the image and that the first pixel
>>   in the image has pixel coordinates $(1.0, 1.0, \ldots)$.
> 
> I am not comfortable with the phrases "regular lattice structure" or
> "units of pixels".  The data array is metric free, and I think it is
> misleading to use any language that indicates it is "regular" or has
> "units".  In the absence of WCS even the concept of adjacency in the
> array does not necessarily have meaning.