"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 PVi_j
The NatLat attribute is read-only.
A default value of AST__BAD is used if no latitude value is available.