Two complimentary sets of routines are provided for calculating the transformation between pixel location in an image and the the corresponding celestial coordinates on the sky. These routines rely on a set of standard World Coordinate System (WCS) keywords in the header of the HDU which define the parameters to be used when calculating the coordinate transformation.
Both sets of routines require that a 2 step procedure be followed: first an initialization routine must be called to read the relevant WCS keywords in the header. These parameters are then passed to a pair of routines that convert from pixel to sky coordinates, or from sky to pixel coordinates. See Appendix B for the definition of the parameters used in these routines.
The first set of routines described below have the advantage that they are completely self-contained within the CFITSIO library and thus are guaranteed to be available on the system. These routines only support the most common types of map projections and WCS keyword conventions however.
The second set of routines are available in a WCS library written by Doug Mink at SAO. These routines are more powerful than the ones contained in CFITSIO itself because they support all the defined WCS map projections and they support a number of non-standard keyword conventions that have been adopted over the years by various different observatories. To use these routines, however, requires that a separate WCS library be built and installed on the system in addition to CFITSIO.