Four Key Attributes of Urban Expansion

Urban Land Cover

Urban land cover, or urban extent, is typically measured by the total built-up area (or impervious surface) of cities, sometimes including the open spaces captured by their built-up areas and the open spaces on the urban fringe affected by urban development.

Density

Density, or more precisely average urban population density, is typically measured as the ratio of the total population of the city and the total built-up area it occupies.

Fragmentation

Fragmentation, or scattered development, is typically measured by the relative amount and the spatial structure of the open spaces that are fragmented by the noncontiguous expansion of cities into the surrounding countryside.

Compactness

Compactness, or the degree to which the city footprint approximates a circle rather than a tentacle-like shape, is typically measured by a set of compactness metrics.

While these four attributes are correlated with each other and are not statistically independent, they map and measure different spatial phenomena. Cities can expand because of their growing populations, for example, without also experiencing a decline in average density. Similarly, the density of the built-up areas of cities may increase whether fragmentation decreases or increases, and cities may become more compact while their density or fragmentation decreases or increases. The tables and graphs accompanying individual city maps in this atlas are meant to focus attention on these four individual attributes, and to see how an individual city fares in comparison to other cities in its own geographic regions and other cities in the world at large.

In order to study these attributes in a global comparative analytical framework, we have had to transform them into a set of precise metrics, metrics that could be measured on each city map in a consistent manner. The definition and the method of measurement of the various metrics associated with these four attributes are described in the following section.