Built-up area use in 800 European urban areas in 2020 and radial scaling law

Abstract

Most cities in Europe, even those with a decreasing population, tend to spread out with a growing urbanization. The constant growth of cities reveals sustainability issues in the housing and transport sectors by challenging the spatial organization of cities, for example by increasing travel times (Weiss et al., 2018) and car use. Cities are major sources of pollution, even more in the context of climate change and the health of city residents. Moreover, heavy urbanization and the constant increase in built-up area create profound environmental consequences (impacts on fauna and flora, urban heat islands...).  To gain a deeper understanding of built-up land in urban areas, it is important to compare cities and identify general forms and patterns on a large scale.

We use a comparative analysis of the evolution of built-up area in the largest European cities, focusing on center-periphery organization. The center-periphery analysis constitutes the first spatial differentiation for cities (Guérois and Pumain, 2008).

We use urban scaling laws to compare cities that are difficult to match in terms of size. Urban scaling laws make it possible to transform a set of objects from one spatial scale to another without changing their structure, thus facilitating comparative analyses of cities (Batty, 2015). 

This study focuses on built-up area in 2020, in 786 European urban areas with more than 50,000 inhabitants. We use GHSL built-up data (a globally harmonized database) at a spatial resolution of 100m. The built-up area is expressed as a continuous value representing the proportion of building footprint in the total cell size. To enable comparisons across our extensive dataset, we designate the city hall as the reference geographical center. Then, concentric rings of 200 m are made around the city hall to calculate the share of built-up area in these rings. The average share of built-up land in the city center of European cities is 37.5%, irrespective of city size.

The main result of this work is that the center-periphery organization of built-up land successions occurs at the same rate if we cancel out the size effect of urban areas. Thus, the built-up area per inhabitant is generally the same for all cities and is proportional to the population. 

Up to the first 30 kilometers, there is a fairly significant decrease in the built-up area. There is a common (exponential) characteristic shape that is more or less the same in all cities. It is a fundamental characteristic of the internal structure of cities, which can be measured by a characteristic decrease distance l.

However, coastal cities have on average a characteristically low distance l. This indicates that the share of built-up land decreases rapidly away from the city center.  In contrast, polycentric cities have a high characteristic distance l, which indicates that the share of built-up land decreases more slowly away from the city center. 

There are also distinctions between countries: Turkish cities have fewer built-up areas than the European average, while Italian cities have more. Quantitative characteristics specific to the different countries are linked in particular to differences in GDP per capita, household size and housing overcrowding rates.