The Spring System

INTRODUCTION First, some definitions of the term Geographic Information System (GIS) will be outlined, according to different researchers' viewpoints. Notwithstanding the fact that these definitions reflect the variety of existing criteria on the subject, they all present GIS as systems that operate on spatial data, which are prioritised in relation to the associated non-spatial attributes (Maguire et al., 1993). It is relevant to point out that this capacity to make spatial analysis is the very characteristic which differs the GIS from other types of information systems: 1) A system to apprehend, store, inspect, manipulate, analyse and show data which are spatially referenced to the Earth (Doe, 1987, Maguire et al., 1993). 2) Database spatially indexed, over which operate a set of procedures aiming at answering questions on spatial entities (Maguire et al., 1993). 3) An aid-system on decisions which includes the integration of spatially referenced data in a problemsolving environm