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[fitsbits] CRPIX clarification
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 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 NASA/GSFC Code 662 HEASARC +1-301-286-4599 (voice) Greenbelt MD 20771 +1-301-286-1684 (fax) |
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