For example, scientists have developed approaches based on night-time lights-the artificial light sources visible in satellite images taken at night-to map and measure human settlements. The increasing availability of satellite imagery is now allowing researchers to use standardized approaches to define, measure, and compare urban areas globally. To derive the well-known figure that more than half the world’s population live in urban areas, the UN simply adds up individual data from each country and publishes the results in a report, World Urbanization Prospects, issued biannually. The United Nations Statistics Division’s 2018 Demographic Yearbook explains in notes on Table 6, which begin on page 120, that in the United States a place is urban if it is an agglomeration of 2,500 or more inhabitants, generally having population densities of 1,000 persons per square mile or more in Senegal, it takes an agglomeration of 10,000 to be urban. Comparing urban population levels and such related topics as urban poverty, urban carbon emissions, or urban infrastructure across countries using these definitions can be misleading, as the same settlement might be considered “urban” in one country and not in another.

Each country bases its official statistics on its own definition of “urban.” While this practice may work well for each nation’s internal purposes, it poses problems with comparisons among places.

Population density map us at night