A geographic information system is both a database system with specific capabilities for spatially referenced data, as well as a set of operations for working with the data. By splitting a GIS into subsystems, we can define it much more clearly. Firstly, it is a data input subsystem that collects and pre-processes data from various sources. It also transforms different types of spatial data e.g. from isoline symbols on a topographic map to point elevations inside the GIS. There is also a data storage and retrieval subsystem, which organises the spatial data in such a manner as to allow retrieval, updating and editing. The data manipulation and analysis subsystem performs tasks on the data, aggregates and disaggregates, performs modelling functions and estimates parameters and constraints. The final output is produced by the reporting subsystem that displays all or part of the database in tabular, graphic, or map form. (Star, Estes, 1990)

A geographic information system is superior to manual methods of handling and analysing data. While handling and analysing data that are referenced to a geographic location are key capabilities of a GIS, the power of the system is noticeable when the quantity of data involved is too large to be handled manually. There are hundreds or thousands of features to be considered, or there may be hundreds of factors associated with each feature or location. The data forms may exist as maps, tables of data, or lists of names or addresses. Such large data

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