"coordinate system. This value is fixed for most projections, for instance it is PI/2 (90 degrees) for all zenithal projections. For some projections (e.g. the conics) the value is not fixed, but is specified by parameter one on the latitude axis.
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
The NatLat attribute is read-only.
A default value of AST__BAD is used if no latitude value is available.