that the decision made will be wrong. For both
procedures (Boolean analysis and weighted linear
combination) it is a fairly simple matter to propagate
measurement error through the decision rule and
subsequently to determine the risk that a given
location will be assigned to the wrong set (i.e. the set
of selected alternatives or the set of those not to be
included). However, the continuous criteria of
weighted linear combination would appear to
express a further uncertainty that is not so easily
accommodated (see Fisher, Chapter 13). The
standardised factors of weighted linear combination
each express a perspective of suitability – the higher
the score, the more suitable. However, there is no
real threshold that can definitively allocate locations
to one of the two sets involved (areas to be chosen
and areas to be excluded). How are these
uncertainties to be accommodated in expressions of
decision risk? If these criteria really express
uncertainties, why are they combined through an
averaging process?
The surprising feature of multi-criteria evaluation
is that, despite its ubiquity in environmental
management, so little is understood of its character
in GIS. In the following sections we survey the issues
involved, and offer a perspective on a resolution
through the concept of fuzzy measures.