They differ in how they conceptualise, store and represent the spatial locations of objects. It should be noted that, to date, the principal applications of GIS within archaeology have been restricted to 2-dimensional models, and at best 2.5 dimensional representations. The latter are a result of the inability of currently available analysis and display tools to adequately deal with truly 3-dimensional data. Issues concerning 3-dimensional spatial representations will be discussed in detail in a forthcoming CAD Guide to Good Practice and no doubt in summary in future editions of the present guide. In the vector model, the spatial locations of features are defined on the basis of co-ordinate pairs.
Attribute data pertaining to the individual spatial features is maintained in an external database.
In dealing with vector data an important concept is that of topology . Topology, derived from geometrical mathematics, is concerned with order, contiguity and relative position rather than with actual linear dimensions. A good illustration of a topological map is that of the London Underground metro system. It is also possible to perform the same spatial modelling and interrogation processes without using stored topology, by processing the geometrical data directly, as in such GIS as ArcView and MapInfo, or by generating topology on the fly , as and when it is required. The latter is the approach taken by Intergraph, amongst other major GIS suppliers.