The following routines are included in the CFITSIO library to help calculate the transformation between pixel location in an image and the corresponding celestial coordinates on the sky. These 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.
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_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)
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)
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)