The present paper focuses on the work conducted by the NGO Teto in São Paulo (Brazil), where a series of datasets providing geographic information on slums is being elaborated thanks to recent geospatial technologies well suited for a non-specialist, volunteered workforce. The objective here is to contribute to the present debate over potential uses and limitations of geographic data collected and elaborated by engaged citizens who are not specialists in the field, focusing on the possible social outcomes of such methods. Two case studies are presented to illustrate how new technologies facilitate citizen mapping, and how the latter may support humanitarian action in precarious settlements by providing relevant spatial information. Ultimately, this study argues for citizen-driven initiatives and participative processes as useful tools to complement authoritative data, specially in the Global South where an informational gap distinguishes the formal from the informal sectors of the cities.