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)