Official data on informal settlements are outdated, scarce, and sometimes nonexistent. Also, existing digital tools to produce spatial data on urban form are not prepared to deal with their degree of heterogeneity. We then propose a method to obtain, structure and analyze georeferenced data, aiming to support participatory planning of precarious settlements in Brazil. The results include mapping basic elements of urban form and also automatic extraction of urban parameters. The method proved relevant to allow not only the collaboration between team members but also the dialogue with community members, revealing its role in fostering a transformative design process.