Area Metrics

The key data we used for mapping and measuring urban spatial structure and urban expansion were built-up pixels and open space pixels of several types.

• The Built-Up Area is the area occupied by built-up pixels within the set of administrative boundaries defining the city.

We further distinguished between three types of built-up area pixels, depending on their degree of contiguity or clustering together:

• The Urban Area is the area occupied by built-up pixels that have a majority of built-up pixels in a circle 1-km² in area around them (the walking distance circle).
• The Suburban Area is the area occupied by built-up pixels that have 10-50 percent of built-up pixels within their walking distance circle.
• The Rural Area is the area occupied by built-up pixels that have less than 10 percent of built-up pixels within their walking distance circle.

As an example, the figure below further divides the built-up area of Bandung in 1991 to urban, suburban, and rural built-up areas. The cutoff percentages for the threefold division are somewhat arbitrary: they were chosen by experimentation to correspond to our common perceptions of what constitutes urban, rural and suburban areas in real-world cities.

Bandung, 1991: Urban, Suburban and Rural Built-up Areas

It is difficult to determine which open spaces "belong" to the city and which do not, or alternatively, which open spaces are disturbed by the city and which are not. Landscape ecology studies maintain that settlements developed near a forest or prairie affect vegetation and wildlife along their edges, often in a belt up to 100 meters in width (Brand and George 2001; Chen, Franklin and Spies 1992; Winter, Johnson and Faaborg 2000). This insight was used to distinguish urbanized open spaces from rural open spaces. We defined three types of urbanized open space and rural open space as follows:

• Fringe open space consists of all open space pixels within 100 meters of urban or suburban pixels;
• Captured open space consists of all open space clusters that are fully surrounded by built-up and fringe open space pixels and are less than 200 hectares in area.
• Exterior open space consists of all fringe open space pixels which are less than 100 meters from the open countryside.
• Urbanized Open Space consists of all fringe open spaces (including exterior open spaces) and all captured open spaces.
• Rural Open Space consists of all open spaces that not urbanized open spaces.

The figure below introduces Fringe Open Space, Captured Open Space and Exterior Open Space into the map of Bandung in 1991.

Bandung 1991: Fringe Open Space, Captured Open Space and Exterior Open Space

Finally, we define the city footprint:

• The City Footprint is the total area occupied by the built-up area of the city and its urbanized open space.

The figure below illustrates the city footprint of Bandung in 1991. Fringe open spaces (including exterior open spaces) and captured open spaces are unified into Urbanized Open Space. These City Footprint maps are the maps drawn for the global sample of 120 cities in the pages that follow.

Bandung 1991: City Footprint