[gnuastro-devel] [sr #109223] Change default WCS formalism from PCi_j to CDi_j

[Lee Kelvin]

URL:
  <http://savannah.gnu.org/support/?109223>

                 Summary: Change default WCS formalism from PCi_j to CDi_j
                 Project: GNU Astronomy Utilities
            Submitted by: leeskelvin
            Submitted on: Tue 17 Jan 2017 05:04:40 PM GMT
                Category: ImageWarp
                Priority: 5 - Normal
                Severity: 1 - Wish
              Item Group: Enhancement
                  Status: None
                 Privacy: Public
             Assigned to: None
        Originator Email: 
             Open/Closed: Open
         Discussion Lock: Any
        Operating System: GNU/Linux

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Details:

The FITS standard has two means by which to describe the translation from
pixel position to the adopted world coordinate system (WCS): PCi_j and CDi_j,
where i and j are the WCS and pixel coordinate axis numbers respectively.
(N.B.: there exists a third standard involving rotational information using
the header keyword CROTA, but usage of this is now deprecated).

Both the PCi_j and CDi_j formalisms provide linear transformation matrices
between pixel axes j and coordinate axes i. 

Pixel scale in the PCi_j formalism is encoded in another header keyword,
CDELTi. Pixel scale is already natively encoded in the CDi_j formalism.

ImageWarp applies the PCi_j formalism to warped (rotated) output files. This
occurs even if the input file adheres to the CDi_j formalism. I believe that
it would make more sense to adopt the CDi_j formalism for describing WCS
information rather than PCi_j, if at all possible.

The reasons for considering this may be:
1) The CDi_j formalism entirely describes the linear transformation matrix and
pixel scale, and is therefore simpler.
2) The majority of modern surveys adopt the CDi_j formalism as its native WCS
encoding. By way of example, I attach a tarball containing small 3x3 FITS
files with data produced at these facilities: VST, SDSS, Chandra, Wise,
Hubble, Spitzer, VLA, DSS. Each of these datasets adopts the CDi_j formalism.
3) As the majority of modern data uses the CDi_j formalism, adopting it within
ImageWarp necessitates less translation of the input data.

I'm aware that WCS manipulation within ImageWarp is handled by WCSLIB. Perhaps
there is a flag which allows ImageWarp to select the preferred output
formalism when invoking WCSLIB?

Alternatives to enforcing use of the CDi_j formalism always could be:
1) ImageWarp adopts the same formalism as the input dataset,
2) ImageWarp allows the user to specify on the command line their preferred
formalism.

Regards, 
Lee Kelvin




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File Attachments:


-------------------------------------------------------
Date: Tue 17 Jan 2017 05:04:40 PM GMT  Name: wcsexamples.tar.gz  Size: 4kB  
By: leeskelvin

<http://savannah.gnu.org/support/download.php?file_id=39496>

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