In the last decade, there has been a growing awareness that the involvement of citizens in decision making can produce an immediate and positive impact on actions to be taken, as they are the real owners of knowledge about the place where they live. By collecting and geolocating data through smartphones and the Internet, citizens in fact can help decision makers both create sharable spatio-temporal information about objects and phenomena and interpret territorial dynamics. However, although such a role has been definitely recognized, the lack of a homogeneous paradigm for structuring the sensing process, managing the geo big data produced and handling services makes it difficult to exploit such a potentiality. In this paper, we describe a citizen-centric approach conceived to build territorial knowledge useful to provide decision makers with a timely and reliable picture of the status of a given territory. In particular, a visual representation of geospatial knowledge is described, which summaries context-sensitive information about a territory and its citizens, thus improving the land monitoring tasks. An information system, SAFE, is finally presented, which consists of a Web and a mobile component to manage citizen supplied data to be integrated for building reliable dynamic scenarios.