A SphMap is a Mapping which transforms points from a 3-dimensional Cartesian coordinate system into a 2-dimensional spherical coordinate system (longitude and latitude on a unit sphere centred at the origin). It works by regarding the input coordinates as position vectors and finding their intersection with the sphere surface. The inverse transformation always produces points which are a unit distance from the origin (i.e. unit vectors).
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The spherical coordinates are longitude (positive anti-clockwise looking from the positive latitude pole) and latitude. The Cartesian coordinates are right-handed, with the x axis (axis 1) at zero longitude and latitude, and the z axis (axis 3) at the positive latitude pole.
At either pole, the longitude is set to the value of the PolarLong attribute.
If the Cartesian coordinates are all zero, then the longitude and latitude are set to the value AST__BAD.
A null Object pointer (AST__NULL) will be returned if this function is invoked with the AST error status set, or if it should fail for any reason.