What’s the definition of ‘urban’?

Anyone reading this blog is likely to have heard the statistic that ‘over half of the world’s population now lives in urban areas’. This has been the standard opening line of reports and presentations about urbanization since this milestone was supposedly reached in 2008. But what does it really mean?

In everyday usage, terms related to human settlements have vague, shifting meanings. What one person might describe as a small ‘city’ might be a ‘town’ or ‘village’ for someone else; one person’s ‘megacity’ might be a cluster of cities from a different perspective. Similarly, we can usually identify areas that are clearly within a city and others that are outside it, but there is usually a peri-urban area of intermediate density that usually lies between the two, making it hard to define a clear city limit. Formal administrative boundaries may have historic or political meaning, but are rarely aligned with the physical or economic extents of the urban area.

So how can we come up with an urban equivalent of the dollar-a-day benchmark? Let’s look at how some countries use specific thresholds for defining urban areas, and how these can be adapted globally.

101 countries use minimum population thresholds as a means of identifying settlements as ‘urban’, either as the sole criteria or together with others. The most frequently used threshold values are 2,000 inhabitants (used by 23 countries), and 5,000 inhabitants (used by 21 countries), as shown in the figure below. The average of all these thresholds was just under 5,000 inhabitants. (It may strike you how low these figures are. The most frequently used minimum population, 2,000 people, could easily be accommodated within a single large office building.)

Only 9 countries use minimum population density thresholds, of which only one, Germany, uses it as a sole criterion for defining urban areas. The lowest density threshold used is 150 persons per sq. km. (Germany), and the highest is 1,500 (China and Seychelles).

The density thresholds also do not take into account the variation in the size of the areas over which the density is being averaged, which would result from varying sizes of administrative units. Even within the same country, two identical settlements may be treated differently, depending on whether they fall within large or small administrative units.

Past attempts to create a global ‘urban’ definition

Others have already recognized this issue, and attempted to craft a standard ‘urban’ definition:

• The WDR 2009 approach: One such attempt is the approach outlined by Chomitz et al and elaborated on by Uchida and Nelson, which identifies all settlements above a certain minimum population size and minimum population density that are within a certain travel time by road. This approach was used in the World Bank’s World Development Report 2009.
• The OECD approach: A similar but more elaborate approach is that of the Organisation for Economic Co-Operation and Development (OECD). The OECD methodology consists of three main steps: identifying contiguous or highly interconnected densely inhabited urban cores; grouping these into functional areas; and defining the commuting shed or ‘hinterland’ of the functional urban area. The OECD uses population size cutoffs (50,000 or 100,000 people, depending on the country) as well as population density cutoffs (1,000 or 1,500 people per sq. km.) to define the urban cores, and then selects those areas from which more than 15% of workers commute to the core as hinterlands