### User Defined Fiducial Points

When reading a FrameSetFrameSet from a FitsChanFitsChan, projection parameters PVi_0, PVi_1 and PVi_2 (for longitude axis “i”) are used to indicate a user-defined fiducial point as described in section 2.5 of paper II. This results in a shift of IWC origin being applied before the WcsMapWcsMap which converts IWC into native spherical coordinates. The values of these projection parameters, if supplied, are stored as the corresponding PVi_mPVi_m attributes of the WcsMap.

When writing a FrameSet to a FitsChan, the PV attributes of the WcsMap determine the native coordinates of the fiducial point (the fixed defaults for each projection described in paper II are used if the PV attributes of the WcsMap have not been assigned a value). The corresponding celestial coordinates are used as the CRVALi keywords and the corresponding pixel coordinates as the CRPIXj keywords.