The shape compactness of urban footprints

Urban population density has featured in a large body of literature on the Compact City paradigm as the key compactness attribute of cities, yet the shape compactness of urban footprints has hardly deserved a mention. This essay seeks to correct that. We review the literature on the Compact City Paradigm with a special focus on the relationship between urban form and climate change, and focus on twelve physical attributes of cities that make them more or less compact. Other things being equal, both population density and shape compactness help determine the average travel distances in cities, and hence affect their energy consumption and their greenhouse gas emissions. They also affect the length of infrastructure lines and the length of commutes. In principle, therefore, increasing either the shape compactness or the population density of cities can contribute—in different yet similar measure—to mitigating climate change. There are strong forces that push urban footprints to become more compact—that is, circular or near circular in shape—and these forces have evolved over time. There are also powerful forces that have pushed urban footprints to become less compact over time. We introduce these forces and illustrate their effects on particular cities. We then focus on a small set of metrics for measuring the shape compactness of cities. We use them to measure urban footprints obtained from satellite imagery in a stratified global sample of 200 cities in three time periods: 1990, 2000, and 2014. We find that the shape compactness of urban footprints the world over is independent of city size, area, density, and income and that, not surprisingly, it is strongly affected by topography. We also find that it has declined overall between 1990 and 2014 and explain some of the sources of this decline. We conclude the paper by assessing the ways in which the shape compactness of cities can be increased to make them better able to mitigate climate change in decades to come.