Where are cities under pressure? – An indicator for measuring the impact of building changes on urban density

Abstract

In the course of urbanisation, cities are becoming increasingly dense with high local variability. Worldwide, there is a high demand for living space, especially in larger cities, which leads to settlement pressure. It is a declared goal to further densify cities in order to avoid the consumption of ecologically valuable land. But cities that are too densely built also bring negative consequences. Therefore, it is important to understand and describe the densification processes of cities. To better describe settlement densification processes, we have developed a novel indicator that can be used to determine changes in the development density, which is of interest from a planning perspective. The indicator takes into account both the extent of the building change and the existing development. If an area is already intensively developed, a newly constructed building will result in a higher density than if that building were built in a previously undeveloped area. The indicator can also be used to measure deconstruction effects symmetrically, so that densification and de-densification processes can be analysed in parallel. In addition, various spatial entities such as cadastral parcels, grid cells or city districts can be used as reference areas for indicator calculation. In addition to an area-based analysis, we have also taken into account the number of storeys of each building, which allows us for more precise calculations. We tested the indicator in this study using the City of Hamburg as an example. Freely accessible building floor plans and parcels from 2015 and 2020, taken from the Authoritative Real Estate Cadastral Information System (ALKIS) of the City of Hamburg, served as the data basis. Furthermore, we used land use data to examine how various land use classes differ in terms of density development. The results show that extensive densification is often observed on previously built-up land, especially in the city centre, which indicates particularly high settlement pressure. Newly developed residential areas are also densely built up, but comparatively do not contribute to such high building density. On the one hand, this indicates an efficient and thus sparing use of the finite resource of land in the city. On the other hand, it points to increasingly dense development in the city centre.