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Chapter 7   World Coordinate System Routines

The FITS community has adopted a set of keyword conventions that define the transformations needed to convert between pixel locations in an image and the corresponding celestial coordinates on the sky, or more generally, that define world coordinates that are to be associated with any pixel location in an n-dimensional FITS array. CFITSIO is distributed with a a few self-contained World Coordinate System (WCS) routines, however, these routines DO NOT support all the latest WCS conventions, so it is STRONGLY RECOMMENDED that software developers use a more robust external WCS library. Several recommended libraries are:

  WCSLIB -  supported by Mark Calabretta
  WCSTools - supported by Doug Mink
  AST library - developed by the U.K. Starlink project

More information about the WCS keyword conventions and links to all of these WCS libraries can be found on the FITS Support Office web site at http://fits.gsfc.nasa.gov under the WCS link.

The functions provided in these external WCS libraries will need access to the WCS keywords contained in the FITS file headers. One convenient way to pass this information to the external library is to use the fits_hdr2str routine in CFITSIO (defined below) to copy the header keywords into one long string, and then pass this string to an interface routine in the external library that will extract the necessary WCS information (e.g., the ’wcspih’ routine in the WCSLIB library and the ’astFitsChan’ and ’astPutCards’ functions in the AST library).

1

Concatenate the header keywords in the CHDU into a single long string of characters. Each 80-character fixed-length keyword record is appended to the output character string, in order, with no intervening separator or terminating characters. The last header record is terminated with a NULL character. This routine allocates memory for the returned character array, so the calling program must free the memory when finished.

There are 2 related routines: fits_hdr2str simply concatenates all the existing keywords in the header; fits_convert_hdr2str is similar, except that if the CHDU is a tile compressed image (stored in a binary table) then it will first convert that header back to that of a normal FITS image before concatenating the keywords.

Selected keywords may be excluded from the returned character string. If the second parameter (nocomments) is TRUE (nonzero) then any COMMENT, HISTORY, or blank keywords in the header will not be copied to the output string.

The ’exclist’ parameter may be used to supply a list of keywords that are to be excluded from the output character string. Wild card characters (*, ?, and #) may be used in the excluded keyword names. If no additional keywords are to be excluded, then set nexc = 0 and specify NULL for the the **exclist parameter.

  int fits_hdr2str
      (fitsfile *fptr, int nocomments, char **exclist, int nexc,
      > char **header, int *nkeys, int *status)

  int fits_convert_hdr2str / ffcnvthdr2str
      (fitsfile *fptr, int nocomments, char **exclist, int nexc,
      > char **header, int *nkeys, int *status)

2

The following CFITSIO routine is specifically designed for use in conjunction with the WCSLIB library. It is not expected that applications programmers will call this routine directly, but it is documented here for completeness. This routine extracts arrays from a binary table that contain WCS information using the -TAB table lookup convention. See the documentation provided with the WCSLIB library for more information.

  int fits_read_wcstab
       (fitsfile *fptr, int nwtb, wtbarr *wtb, int *status);

7.1   Self-contained WCS Routines

The following routines DO NOT support the more recent WCS conventions that have been approved as part of the FITS standard. Consequently, the following routines ARE NOW DEPRECATED. It is STRONGLY RECOMMENDED that software developers not use these routines, and instead use an external WCS library, as described in the previous section.

These routines are included mainly for backward compatibility with existing software. They support the following standard map projections: -SIN, -TAN, -ARC, -NCP, -GLS, -MER, and -AIT (these are the legal values for the coordtype parameter). These routines are based on similar functions in Classic AIPS. All the angular quantities are given in units of degrees.

1

Get the values of the basic set of standard FITS celestial coordinate system keywords from the header of a FITS image (i.e., the primary array or an IMAGE extension). These values may then be passed to the fits_pix_to_world and fits_world_to_pix routines that perform the coordinate transformations. If any or all of the WCS keywords are not present, then default values will be returned. If the first coordinate axis is the declination-like coordinate, then this routine will swap them so that the longitudinal-like coordinate is returned as the first axis.

The first routine (ffgics) returns the primary WCS, whereas the second routine returns the particular version of the WCS specified by the ’version’ parameter, which much be a character ranging from ’A’ to ’Z’ (or a blank character, which is equivalent to calling ffgics).

If the file uses the newer ’CDj_i’ WCS transformation matrix keywords instead of old style ’CDELTn’ and ’CROTA2’ keywords, then this routine will calculate and return the values of the equivalent old-style keywords. Note that the conversion from the new-style keywords to the old-style values is sometimes only an approximation, so if the approximation is larger than an internally defined threshold level, then CFITSIO will still return the approximate WCS keyword values, but will also return with status = APPROX_WCS_KEY, to warn the calling program that approximations have been made. It is then up to the calling program to decide whether the approximations are sufficiently accurate for the particular application, or whether more precise WCS transformations must be performed using new-style WCS keywords directly.

  int fits_read_img_coord / ffgics
      (fitsfile *fptr, > double *xrefval, double *yrefval,
       double *xrefpix, double *yrefpix, double *xinc, double *yinc,
       double *rot, char *coordtype, int *status)

  int fits_read_img_coord_version / ffgicsa
      (fitsfile *fptr, char version, > double *xrefval, double *yrefval,
       double *xrefpix, double *yrefpix, double *xinc, double *yinc,
       double *rot, char *coordtype, int *status)

2

Get the values of the standard FITS celestial coordinate system keywords from the header of a FITS table where the X and Y (or RA and DEC) coordinates are stored in 2 separate columns of the table (as in the Event List table format that is often used by high energy astrophysics missions). These values may then be passed to the fits_pix_to_world and fits_world_to_pix routines that perform the coordinate transformations.

  int fits_read_tbl_coord / ffgtcs
      (fitsfile *fptr, int xcol, int ycol, > double *xrefval,
       double *yrefval, double *xrefpix, double *yrefpix, double *xinc,
       double *yinc, double *rot, char *coordtype, int *status)

3

Calculate the celestial coordinate corresponding to the input X and Y pixel location in the image.

  int fits_pix_to_world / ffwldp
      (double xpix, double ypix, double xrefval, double yrefval,
       double xrefpix, double yrefpix, double xinc, double yinc,
       double rot, char *coordtype, > double *xpos, double *ypos,
       int *status)

4

Calculate the X and Y pixel location corresponding to the input celestial coordinate in the image.

  int fits_world_to_pix / ffxypx
      (double xpos, double ypos, double xrefval, double yrefval,
       double xrefpix, double yrefpix, double xinc, double yinc,
       double rot, char *coordtype, > double *xpix, double *ypix,
       int *status)