Definition of Geographical Information Systems (GIS) GIS software can be used to manipulate and display spatial information. Burrough & McDonnell (1998, p. 11) provide a number of definitions of GIS, based upon the concepts of the toolbox, database and organizational systems. Examples of each include: Toolbox definition: ‘... a system for capturing, storing, checking, manipulating, analysing and displaying data which are spatially referenced to the earth ...’ Database definition: ‘... any manual or computer based set of procedures used to store and manipulate geographically referenced data ...’ System definition: ‘ An organized collection of computer hardware, software, geographic data and personnel designed to efficiently capture, store, update, manipulate, analyse, and display all forms of geographically referenced information ’. Organization based definition: ‘... a decision support system involving the integration of spatially referenced data in a problem solving environment ...’ In the context of terrain evaluation the ‘organization based’ definition is considered to be the most appropriate. This is because the intended output of terrain evaluation is to support a decision making process. More specifically, GIS can help the terrain evaluation process answer questions such as those in Table 1. GIS data models Present day GIS store information in both raster and vector formats. Raster data comprise ‘pixels’ or tiles that fill space, thus something is stored about everywhere. The raster data structure has its origins in remote sensing and digital image processing software. Vector data are stored as points, lines or polygons, thus something is stored only where a change occurs.