We collect and store data to derive information and make judgments about a world of our interest. Ideally, they should indicate in a unique and certain way which possible world corresponds to the actual world [17]. Imperfection arises when this is not possible. Imprecision is a type of imperfection that is often encountered. Data are imprecise if we cannot precisely define the actual world, i.e. several worlds satisfy the data. A specific type of imprecision is vagueness [17; 22], which is the focus of this study. A concept is vague if objects exist that cannot be classified either to the concept or to its complement. Vagueness arises in the presence of borderline cases [18]. It is often present in collected spatial information, such as forest inventories, or geological, soil, and vegetation maps. Soil or vegetation classes are such that they cannot be defined sharply. The change from one class to another is gradual. This is in confliict with current geographical information systems (GIS) which assume that spatial objects are precisely defined, sharp objects, using points, lines, and polygons as representations
