[fitsbits] CRPIX clarification

[William Pence]

Taking Rob's suggestion, the proposed changes to the new draft FITS
Standard are as follows:

1) Insert the following paragraph at the start of Sect. 8.1:

   Rather than store world coordinates separately for each datum,
   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}.
   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 {\em conventional} axes, see Sect. 8.5). They form the basis of
   the world coordinate formalism in FITS depicted schematically in Fig.
   8.1.

2) Remove the italics from "pixel coordinates" on p75 (they are used in
    the first mention above).

3) Change the definition of CRPIXj on p77 to:

   CRPIXj - [floating-point, indexed, default: 0.0]
     Location of the reference point in the image for axis j
     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 $(1.0, 1.0, \ldots)$.


If anyone objects to the use of the phrase "regular lattice
structure" in that first sentence (Steve Allen?) then I suggest the way
to handle this would be to add a footnote to this phrase, which
elaborates or constrains the meaning within this FITS context.
Perhaps something to the effect:

  "In the general case, one should not assume that the points in a
   FITS array are regularly spaced, or that the axes
   of the array necessarily have any physical units."

If any one feels this is necessary, then please suggest a wording for
such a footnote.

Bill Pence
--

Rob Seaman wrote:
> Perhaps simply excise "(e.g. as is done for random groups data)"?  The 
> less said about random groups, the better :-)
> 
> Rob
> -- 
> 
> On Jun 16, 2008, at 7:20 AM, Eric Greisen wrote:
> 
>> Mark Calabretta wrote:
>>> On Thu 2008/06/12 14:13:44 -0400, William Pence wrote
>>> in a message to: FITSBITS <fitsbits at nrao.edu>
>>>
>>>> 1.  Change the first sentence in section 8.1 from
>>>>
>>>>  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.
>>>>
>>>> to
>>>>
>>>>  Rather than store world coordinates separately for each datum, one
>>>>  may define rules for computing world coordinates at each point.
>>>
>>> This change is not acceptable to me.
>>>
>>> The underlying basis of the world coordinate formalism is that the
>>> data has a regular structure, unlike random groups for example.
>>> Metrics don't come into it, at least not at this level.
>>>
>>> Regards, Mark
>>>
>>
>> I did not like the former wording since random groups does have a
>> regular lattice with WCS under the same rules as images along with the
>> random parameters which need not have rules.
>>
>> Eric Greisen
>>
-- 
____________________________________________________________________
Dr. William Pence                       William.Pence at nasa.gov
NASA/GSFC Code 662       HEASARC        +1-301-286-4599 (voice)
Greenbelt MD 20771                      +1-301-286-1684 (fax)