A NormMap is a Mapping which normalises coordinate values using the astNorm function 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 astSimplify function will replace neighbouring pairs of forward and inverse NormMaps by a single UnitMap.
AST A Library for Handling World Coordinate Systems in Astronomy