A generic procedure developed in a geographical information system for estimating spatial distribution of rainfall in an urban catchment. Five different techniques were used to estimate point values at ungauged sites as well as average values for subcatchments. Thiessen Polygon, Inverse Distance Weighted, Kriging, Trend and Spline. Basic principles of the techniques outlined. Both visual and arithmetic comparisons were established. It ws found that using spline surfaces within a GIS produced robust and accurate estimates of rainfall and enabled real time estimation of spatially distributed patterns. The use of a generic GIS for rainfall modelling enables the more accurate and sophisticated techniques to be used by a wider range of users.