If this condition is true (indicated by setting UnitRadius non-zero), it implies that a CmpMap which is composed of a SphMap applied in the forward direction followed by a similar SphMap applied in the inverse direction may be simplified (e.g. by AST_SIMPLIFY) to become a UnitMap. This is because the input and output vectors will both have unit length and will therefore have the same coordinate values.
If UnitRadius is zero (the default), then although the output vector produced by the CmpMap (above) will still have unit length, the input vector may not have. This will, in general, change the coordinate values, so it prevents the pair of SphMaps being simplified.
This attribute is intended mainly for use when SphMaps are involved in a sequence of Mappings which project (e.g.) a dataset on to the celestial sphere. By regarding the celestial sphere as a unit sphere (and setting UnitRadius to be non-zero) it becomes possible to cancel the SphMaps present, along with associated sky projections, when two datasets are aligned using celestial coordinates. This often considerably improves performance.
Such a situations often arises when interpreting FITS data and is handled automatically by the FitsChan class.
The value of the UnitRadius attribute is used only to control the simplification of Mappings and has no effect on the value of the coordinates transformed by a SphMap. The lengths of the input 3-dimensional Cartesian vectors supplied are always ignored, even if UnitRadius is non-zero.
The value of this attribute may changed only if the SphMap has no more than one reference. That is, an error is reported if the SphMap has been cloned, either by including it within another object such as a CmpMap or FrameSet or by calling the AST_CLONE function.