Geodetic data and spatial photographs can be used systematically and can help us to understand more traditional sources such as directories, censuses, police archives or juridical decisions. This chapter explains the technicalities in geocoding with contemporary reference tables and the necessity, for historians, to develop what might be termed a topographical scholarship. It utilizes the geo-referencing of aerial photographs taken at the time which provide precious source material to understand the persistence of shanty towns until after WWII and the development of green spaces alongside the ring boulevard of Paris. Topographical scholarship requires in-depth knowledge of the history of streets since it is necessary to prepare the historic data by adapting street names, and sometimes numbers, to the database. Schools and recreational facilities for children replaced the cite Lesage Bullourde, although the passage Bullourde still exists.