Urban sprawl is a typical problem that cannot be correctly understood and resolved because it relies on several different fields and concerns different actors. With traditional approaches, the whole process cannot be apprehended. This context partly explains the inefficiency of traditional planners' works, and asks for urban planning considered through the idea of collaborative projects. The CamDeus (Cellular Automata Models to Design Environmental and Urban Systems) project is an operational decision-making software giving cartographic and mathematical solutions in order to harmonize and share data and knowledge among scientists, territorial or administrative technicians or elected people. It consists in simplifying the modelling process, so that it can be understood by persons with different technical languages and different scientific qualifications. Thus, the question of urban dynamics is decomposed into three simple and operational questions associated with a particular and relatively simple model : "How many?" (how many hectares will be concerned by urban sprawl in the future?) ; "Where?" (where are the surfaces that will move to another landuse category?) ; "What?" (what is the land use category of the spaces quantified in the first step and located in the second one?). This global modelling is then relatively complex and allows the study of urban changes with efficiency. Its decomposition into three steps appears simplified enough to be simultaneously understood by all the actors concerned with urban planning. CamDeus then allows different points of view to be put together and concrete actions to be modeled for the development of urban territories in the perspective of sustainable development.