A technique for spatial reasoning is described which is directly compatible with the data representation techniques used by Geographical Information Systems (GIS). GIS may be thought of as specialised forms of database systems which are distinguished by their ability to handle spatial data and are widely used to aid environmental planners to make decisions and predictions. However, their application is limited by their lack of support for any spatio-temporal reasoning capability. The technique described here provides GIS with this capability. It is based on a quad tesseral addressing representation of space supported by a constraint based reasoning mechanism. The technique has been incorporated into a tesseral spatial reasoning system (SPARTA) which has been used successfully to resolve two-dimensional reasoning scenarios.