Deﬁnitions of geographical information systems (GIS) usually fall into one of three categories: toolbox-, database-, and organization-based deﬁnitions (Burrough and McDonnell, 2000). Geographical or spatial data are data about entities in the real world (physically represented as point, line, or area objects) that deﬁne their location and the attributes recorded at each location. Knowing where entities are located allows spatial relationships between them to be deﬁned, such as distances between points, adjacency or otherwise of areas, and the proximity of one object to another. A GIS database is distinguished from most other databases by knowing where in geographical space objects (points, lines, and polygons) are located in relation to one another since most other kinds of databases capture only entities and their attributes. Database deﬁnitions of GIS emphasize this diﬀerence: “any . . . computer based set of procedures used to store and manipulate geographically referenced data” (quoted in Burrough and McDonnell, 2000, p. 11, italics added). Geography is stored in the form of a series of discrete layers in the database (e.g., a layer for the roads, a layer for the waste sites, a layer for the forested areas, and a layer for the deprivation scores by census area), enabling diﬀerent features to be switched on or oﬀ when visualizing or mapping an area.