Two recent phenomena have deeply affected the know-space of the city: the failures of traditional data and the diffusion of digital traces. Indeed, urban decision-makers have lately revealed several discrepancies in relation to traditional data used in public policy, caused for example by excessively long publication delays, the insufficient coverage of topics that would otherwise be of interest for social cohesion, and the top-down process of data creation. To many of them, the exponential deluge of information available on the Internet represents a potential answer to such dissatisfaction.