Sri Lanka, Oct. 15 -- ra Eliya and the other a new housing estate out of Colombo. One place is filling up rapidly while the other is steadily emptying. This divergence has a name and it is not urbanisation.Sri Lanka Travel

The name is concentration. It describes the tendency of people and economic activity to cluster in a small number of favoured places while the rest of the map thins out. Urbanisation measures how many people live somewhere officially classified as a city.

Concentration measures something different, how unevenly a population is actually spread across its territory and the two do not necessarily move together. Sri Lanka shows that gap more clearly than almost any other country in the region.

Sri Lanka's population dens...