The UnitMap is the simplest of Mappings. It is a null Mapping. Its purpose is simply to copy coordinate values, unaltered, from its input to its output and vice versa.
A UnitMap has no additional attributes beyond those of a basic Mapping. Its Nin and Nout attributes are always equal and are specified by the first argument supplied to its constructor. For example:
AstUnitMap *unitmap; ... unitmap = astUnitMap( 2, "" );
will create a UnitMap that copies 2-dimensional coordinates. Inverting a UnitMap has no effect beyond changing the value of its Invert attribute.
The main use of a UnitMap is to allow a Mapping to be supplied when one
is required (as an argument to a function, for example) but you wish
it to leave coordinate values unchanged.
AST A Library for Handling World Coordinate Systems in Astronomy