For some decades already, emerging countries have faced the complex reality of fast growing metropolitan areas, and local governments have showed little success in keeping track of the rapid changes that happen in their own territories. Consequently, all around the World a large number of people is marginalized: failing to appear in the official records, they are virtually non-existent and deprived of their most basic rights, amongst which the right to the city. In order to grant a voice to these excluded citizens, it is critical to include the ‘informal’, au- to-constructed urban areas and its inhabitants into the process of town planning. Such challenge demands a re-evaluation of the very concept of city-making; more precisely, it requires an epistemological revision of spatial representation through the development of more dynamic and inclusive cartographic processes. The authors propose an overview of the experience led by NGO TETO-São Paulo, in Brazil, where socio-economically vulnerable communities are being mapped through a participative system. Based on voluntary work and using open-source software, the NGO is producing unprecedented cartographic material that complements official geo-databases and can be used as support for policy-making and projects aiming to improve living conditions in ‘informal’ settlements.