A method to calculate geometrical input parameters for urban dispersion models is presented. Two cases are considered according to the relative size of the pollutant plume compared to the building size. Applying image processing techniques to urban Digital Elevation Models (DEMs, 3-D urgan databases), simple geometrical parameters over extensive urgan areas are calculated. Some preliminary results are shown for a case study in central London. It is envisaged that the technique will be developed to support urban designers and planners to assess the performance of different urban developments in terms of pollutant dispersion.