2.1 INTRODUCTION

This chapter describes the basic methods by which a Geographical Information System represents the world. It is necessarily technical and descriptive; however, a good understanding of the underlying principles is essential to provide an understanding of the strengths and limitations of GIS and its utility in historical research. In this chapter we introduce key terms associated with GIS before proceeding to discuss the range of data models that underlie all GIS data and their utility to the historian.

As was identified in Chapter 1, a Geographical Information System can be thought of as a form of database. What makes it unique is that the GIS combines attribute data, which describe the object, with spatial data, which say where the object is located. Traditionally, attribute data would have been statistical or textual; however, more recently, images, sound, video and other multi-media formats are used. Attribute and spatial data are stored together in a GIS layer (or coverage or theme). A layer is the rough equivalent of a database table. Each layer holds information about a particular theme or topic combining both the spatial and attribute data.

Within a layer, spatial data are always represented using co-ordinates. These co-ordinates may represent a point, a line, or an area or zone, known in GIS terminology as a polygon. Points, lines and polygons represent discrete features on the Earth's surface, and a GIS based on using these features uses what is termed a vector data model.