A NormMap is a Mapping which normalises coordinate values using the AST_NORM routine of the supplied Frame. The number of inputs and outputs of a NormMap are both equal to the number of axes in the supplied Frame.
The forward and inverse transformation of a NormMap are both defined but are identical (that is, they do not form a real inverse pair in that the inverse transformation does not undo the normalisation, instead it reapplies it). However, the AST_SIMPLIFY function will replace neighbouring pairs of forward and inverse NormMaps by a single UnitMap.
AST A Library for Handling World Coordinate Systems in Astronomy