However, when using terrain models, the use of conventional aggregation techniques for buildings is problematic and may result in faulty visualizations and simulations. By simply displacing the z-coordinate of their nodes the buildings of the city models can by used in conjunction with these terrain models. Some visualizations and simulations utilizing city models may greatly benefit from the simultaneous usage with terrain models. A simple way to remove complexity is to aggregate individual buildings whilst obeying a set of well-introduced rules. Hence, it is necessary to generalize those city models in order to reduce their detail and to remove undesirable visual features. Whereas in some applications the requirements for storage consumption and processing power exceed the available resources, for visualization purposes it is not optimal to have too detailed graphics as well. However, not in each application such a high degree of detail is desirable. Those buildings are usually generated in an automated fashion by remote sensing methods and can by very detailed.

3D city models offered by digital map providers typically consist of several thousands or even millions of individual buildings.

Digital terrain model of city