This attribute holds the ZoomMapZoomMap scale factor, by which
coordinate values are multiplied (by the forward transformation)
or divided (by the inverse transformation). This factor is set
when a ZoomMap is created, but may later be modified. The
default value is unity.
Note that if a ZoomMap is inverted (e.g. by using astInvertastInvert),
then the reciprocal of this zoom factor will, in effect, be
used.