The built-up environment modulates the association of artificial night-time lights with the urban extent

Artificial night-time light (NTL) emissions, collected by satellites, are a reliable and widely used remote proxy for urban extent. Recently, it was demonstrated that NTLs are stronger associated with the total volume of the buildings than with their total footprint area, a traditional urban extent measure. However, this finding may not be a general rule: We presume that both associations are sensitive to characteristics of the built-up environment, while the latter is known to be quite heterogeneous within and between cities. Moreover, we hypothesize that at least in specific built-up environments, NTL may even stronger correlate with another urban extent measure - the total lateral surface area of the buildings, due to its more accurate proportionality to the area of light-emitting windows. The present study uses NTLs and buildings datasets of 38 European capital cities and compares the associations of NTLs with the three aforementioned urban extent measures (the total footprint area, the total volume, and the total lateral surface area of the buildings) across different built-up environments. The results indicate that in urban areas characterized by a high density of tall buildings the strongest observed association is between NTLs and the building's footprint area. In comparison, the total volume of buildings is better related to NTLs in urban areas that host compact settings of relatively small buildings. But there is a third type of physical urban configuration, characterized by large buildings sparsely distributed in space. In this type of urban area, the strongest observed association is between NTLs and lateral surface area. We conclude that monitoring urban growth using NTLs will benefit from a preliminary assessment of the local built-up characteristics.