Wider urban zones: use of topology and nighttime satellite images for delimiting urban areas

In the literature on the definition of urban areas, the methodological approaches are divided into formalist (aggregation by density thresholds) and functionalist (aggregation by commuting quotas). This paper proposes a mixed approach, in which the territorial density threshold from the lower-level administrative unit is combined with the brightness of nighttime satellite imagery, intended as a proxy variable for the functional links. The objective is to attain a method for the delimitation of urban areas, to be used by various States and Regions across the world in an iterative procedure, for the delimitation of urban areas as connected topological spaces. This represents an independent method, compared to the various standards adopted by national and regional statistics bureaus, which allows comparing the infrastructural, economic, and social data of different cities in the world. Such cities are hence described in terms of the "real" dimension of the urban areas, partially correcting the bias related to the adoption of administrative perimeters as a "fact" when local authorities make decisions regarding them.