Longley et al. suggest that there are 3 key parts (or tiers) in a GIS software package: the graphical user interface (GUI); tools; and the data management system. the user interacts with the GUI which provides access to the tools which provide the processing capabilities. the data management system organises the data. In a simple desktop GIS these three compoments are installed on the same machine and we are not normally aware of the different roles. However, in a large GIS these functions may be on separate machines, linked together by a network using a client-server model. the GUIs, for example, could be provided by desktop PCs distributed throughout an organisation. These may act as clients to servers providing tools. These, in turn, may issue requests to the data servers located on other machines