Historic urban centers have a very specific problem, with very large demands in relation to documenting their heritage elements. Not only for its typological diversity and extent, but also by the need to determine the interactions established between the different elements, especially the dialogue between the modern city and the historic towns. We present here the experience of the Roman circus of Tarragona, which occupies four hectares of the historic center of the city, where we have used the Backpack, the Mobile Mapping solution installed on a backpack from Leica-Geosystems. This system allows access and therefore the documentation of areas which are only accessible on foot, as well as underground and inside buildings.