GIS (Geographical Information System) users are increasingly turning to spatial decision support systems to assist them solving complex spatial problems which are not simply map overlay.

So, we have developed the GEODES (GEOgraphical Decision Expert System) system that allows users to specify easily symbolic knowledge of a geographical problem through a graphic user interface by using, on the one hand, data and knowledge spatial characteristics and, on the other hand, fuzzy knowledge.

This problem specification and geographical data are used by the system to provide a map of solutions of the problem showing favorable and less favorable areas for the problem solving. It's a fuzzy expert system using two kinds of data (raster and vector) and managing three kinds of rules (simple, multiple and gradual).

We present equally fast computation techniques for simple and parallel rules inferences by mean of rapid generalized modus ponens algorithms to decrease computation time by using only five parameters of an internal representation of fuzzy sets with a parametric function. Experimental results showing powerful and accuracy of the system for a study on a forestry spatial evolution are proposed but it can be used in many others domains such as geography, ecology, agriculture...