Measuring the morphology of African cities

Most parts of Africa's infrastructure are yet to be built. Where and how these new buildings are constructed matters since today's decisions will last for decades. The resulting morphology of cities has lasting implications for a city's metabolism and energy needs. Estimating and projecting these needs has always been challenging in Africa due to the lack of data. Yet, given the sweeping urbanisation expected in Africa over the next three decades, this obstacle needs to be overcome to guide cities towards a trajectory of sustainability and resiliency. Based on the location and surface of nearly 200 million buildings on the continent, we estimate the inter-building distance of almost six thousand cities. Satellite imagery enables the construction of urban form indicators to compare African cities' elongation, sprawl and emptiness. We establish the BASE model, where the mean distance between buildings is a functional relation to the number of Buildings and their average Area, as well as the Sprawl and the Elongation of its spatial arrangement. The mean distance between structures in cities -- our proxy for its energy demands related to mobility -- grows faster than the square root of its population, resulting from the combined impact of a sublinear growth in the number of buildings and a sublinear increase in building size and sprawl. We show that when a city doubles its population, it triples the energy demand related to commutes.