The article explores the theme of collaborative construction of territorial knowledge through the use of ICTs, proposing a recent experimentation on the issue conducted in collaboration between the Politecnico di Torino and the University of Turin. The essay is divided into two parts. The first part aims to define the theoretical framework of the eParticipation practices - PPGIS in particular - and specifying which approach to participation is assumed as conceptual background. The second part focuses on the application OnToMap - Community Maps 3.0, which relies on a territorial ontology for encourage construction of collective and shared knowledge of the places. The article ends with a reflection on semantic and cartographic representation of urban space.