Classical mathematical variables usually use numbers as values. Linguistic variables use linguistic values (linguistic terms), which are usually words. In the GIS fuzzy system, linguistic variables refer to the original raster data (map), whereas linguistic values (terms) are equivalent to the fuzzy set created over every given map according to a uniform definition. The inference process is defined by the set of fuzzy rules of the “IF antecedent THEN consequent” type, where the antecedent is the conditional part and the consequent is the conclusion part of the rule. The antecedent consists of one or more fuzzy variables and terms connected by the semantic operators (AND, OR); consequent consists of only one variable and term, for example: “IF distance_to_river is near AND elevation_above_river is low THEN risk is very high”. The full fuzzy system comprises input and output data, the fuzzy set definition, a system of rules and an inference machine whose work can be controlled by additional parameters.