2 TRADITIONAL APPROACHES TO MCE IN GIS
In GIS, multi-criteria evaluation has most typically
been approached in one of two ways. In the first, all
criteria are converted to Boolean (i.e. logical
true/false) statements of suitability for the decision
under consideration. (The term Boolean is derived
from the name of the English mathematician, George
Boole, who first abstracted the basic laws of set
theory in the mid 1800s. It is used here to denote any
crisp spatial mapping in which areas are designated by
a simple binary number system as either belonging or
not belonging to the designated set.) In many
respects, these Boolean variables can be usefully
thought of as constraints, since they serve to delineate
areas that are not suitable for consideration. These
constraints are then combined by some combination
of intersection (logical AND) or union (logical OR)operators. This procedure dominates MCE with
vector software systems, but is also commonly used
with raster systems. For example, Figure 1 shows
how Boolean images, along with their intersection
achieved through the characteristic overlay
operation of a GIS, may be used here to find all
areas suitable for industrial development, subject to
the following criteria: suitable areas will be near to a
road (within 1 km – upper left), near to a labour
force (within 7.5 km of a town – middle left), on low
slopes (less than 5 per cent – upper right), and
greater that 2.5 km from designated wildlife reserves
(middle right). In addition, development is not
permitted in wildlife reserves (lower left).