Each Region (of any class) can have an "uncertainty" which specifies
the uncertainties associated with the boundary of the Region. This
information is supplied in the form of a second Region. The uncertainty
in any point on the boundary of a Region is found by shifting the
associated "uncertainty" Region so that it is centred at the boundary
point being considered. The area covered by the shifted uncertainty
Region then represents the uncertainty in the boundary position.
The uncertainty is assumed to be the same for all points.
The uncertainty is usually specified when the Region is created, but
allows it to be changed at any time.