The convergence between the Internet and Computer Graphics has resulted in the emergence of a completely new media: Massive online environments. While still in constant evolution, mainly driven by the entertainment industry, they are no longer an anecdote aspect of the digital age: Blizzard entertainment recently reported 10 million active subscribers world-wide for the game World of Warcraft [Blizzard 2008]. That is roughly the population of Paris and its surroundings; these subscribers pay a monthly fee to visit, explore and contribute to this virtual world. Beyond entertainment, massive on-line environments have a tremendous potential for social development and educational purposes. Among these, virtual urban environments are of particular interest for applications in education, transportation, urban planning and tourism. Unfortunately, virtual exploration of cities is currently impaired by the impossibility to capture and store the many images representing surfaces at such a large scale.