Metrics

Our focus in this atlas is on the metropolitan area, typically a central city surrounded by suburbs and secondary cities that form a relatively contiguous whole. Urban land is occupied by urban uses that include all land in residential, commercial, industrial, and office use; land used for transport, parks, and public facilities; protected land, and vacant land. Land in urban use does not include cultivated lands, pasture lands, forests, farms and villages, intercity roads, and nature areas. The terms city and metropolitan area are used interchangeably.

A central problem involved in measuring the attributes of urban spatial structure concerns a precise and consistent definition of what constitutes the area of the city (Wolman et al. 2005; Parr 2007). Using the administrative area of a city is typically inadequate, both because it can be changed from one day to the next by decree and because it may include large rural areas. For example, the administrative area of Beijing in 1999,16,801 km², was 11 times larger than its built up area, 1,576 km².

Indeed, the built-up area of the city is a much more precise, consistent, and comparable measure of its area, and the analysis of satellite images now allows us to identify built-up areas by impervious surfaces (pavements, rooftops, and compacted soils). We used Landsat imagery with a 30-meter pixel resolution to classify land cover into built-up pixels, open pixels, and water in and around 120 cities. We then used these digital maps, in combination with population data for the administrative districts encompassing these cities, to calculate a set spatial metrics in a consistent manner across cities and countries using ArcGIS software.

We define each metric here and illustrate it with maps for Bandung, Indonesia. The figure below shows the administrative districts encompassing the built-up area of Bandung in 1991 (Study Area), the area occupied by water, the built-up area of the city at that time, and some area outside our classification of land cover (No Land Cover Data). The Central Business District (CBD) of the city is shown as a small star corresponding to the location of its city hall.

Bandung 1991: Study Area and Built-up Area