To understand and compare modern metropolises, city borders just don't cut it.

The edges of a city's defined borders don't really mean all that much when they bleed into yet more urbanity. The stereotype of sprawling Greater Los Angeles, for example, shows hardly any discernible distinction when crossing over the border from Los Angeles proper to neighboring Inglewood or El Segundo or Long Beach.

So when the city is more than a single city – a metropolis – how do we compare one city or urban agglomeration to the next? For now, we do it very imprecisely, by say, comparing the distinctly dense Hong Kong with radiating London. But a new set of metropolitan definitions is hoping to make city-to-city comparison a lot more sensible in an age of urban agglomerations indifferent to borderlines.

A report from the OECD proposes a new way of looking at and comparing the world's urban areas by crafting a formalized approach to defining them. In "Redefining 'Urban': A New Way to Measure Metropolitan Areas," authors Monica Brezzi, Mario Piacentrini, Konstantin Rosina, Daniel Sanchez-Serra propose the concept of "functional urban areas" that are based on population density and travel-to-work flows. Basically, it's a measurement of population and interconnected economies.

Using satellite data, the researchers identify "urban high-density clusters" where the population density is at least 1,500 people per square kilometer (or 1,000 people per square kilometer in the U.S. and Canada). They create an "urban core" by outlining the shape formed by all the contiguous urban high-density clusters, filtering out some clusters that don't meet certain population thresholds that, again, vary by the region. Finally, the core is further refined by the inclusion of any municipalities where more than half of the population lives within the urban core. This set of images explains:

Source: OECD

But not all urban areas have this simple single-core structure. For polycentric urban groupings, the researchers identify interconnected cores as those with more than 15 percent of commuters traveling between each other. The researchers also define the hinterlands as places where more than 15 percent of commuters travel to an urban core from a non-core area.

The figure below shows the "functional urban areas" of Rome and Paris, with the urban core in dark blue and the hinterlands in light blue.

Source: OECD

The authors of this report argue that their definitions could help to create a standardized concept of urban areas as economic units. They've already applied this methodology to each city in 28 OECD countries with a population greater than 50,000, totaling more than 1,100 cities.

"An internationally recognized definition of urban areas as functional economic units can better guide the way city governments plan infrastructure, transportation, housing and schools, space for culture and recreation," the authors write.

Top image: OECD

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