FITS-WCS paper II introduces the concept of a "fiducial point" which is logical distinct from the projection reference point. It is easy to confuse the use of these two points. The fiducial point is the point which has celestial coordinates given by the CRVAL FITS keywords. The native spherical coordinates for this point default to the values of the NatLat and NatLon, but these defaults mey be over-ridden by values stored in the PVi_j keywords. Put another way, the CRVAL keywords will by default give the celestial coordinates of the projection reference point, but may refer to some other point if alternative native longitude and latitude values are provided through the PVi_j keywords.
The NatLat attribute is read-only.
AST A Library for Handling World Coordinate Systems in Astronomy