Description

Areal sprawl indicator describes the level of compactness of a city, as the ratio between total building floor area to the area of the convex hull of the built space. The convex hull of a set of points is the minimal convex envelope that contains those points. Computing this shape gives a fair ground to compare different cities or neighbourhoods, and a closer approximation to the actual built density.

Areal sprawl is the territorial aspect of several urban transitions. According to literature (Speck, 2013; Saelens et al. 2003.) the planning of city centres can avoid areal sprawl. If downtown is liveable, less people will tend to move to the outskirts of the city and undertake the burden of daily commute for the desired quality of their place of residence. Nature-based solutions are highly relevant from compact urban form point of view. Compactness can be also achieved with the balanced availability of green spaces and ecosystem services. In addition, unrestricted urban sprawl endangers natural environment around the city and the protective zones that mitigates the intensity of urban heat island.

Scale as linked to D2.2 framework

• City
• Neighborhood

Data source [ link to data framework]

• Municipality databases
• Open sources like Open Street Map
  • But in any case, data has to be georeferenced

Data needs

• Total floor area of buildings. If this is impossible to acquire directly it can be approximately calculated knowing the ground area of buildings and their heights.
• To assess the impact of a future project, a tool that simulates urban evolution is needed.

Expected Outputs

Calculation methods

• 
• Tools: convex hull surface calculation software or library, like: Shapely, SciPy

Links and references

• Skiena, S. S. "Convex Hull." §8.6.2 in The Algorithm Design Manual. New York: Springer-Verlag, pp. 351-354, 1997. - http://mathworld.wolfram.com/ConvexHull.html (downloaded: 2020.06.11.) 
• SPECK, Jeff (2013). Walkable City: How Downtown Can Save America, One Step at a Time. North Point Press
• Saelens et al. 2003. Environmental Correlates of Walking and Cycling: Findings From the Transportation, Urban Design, and Planning Literatures" 
• Nature4Cities, D2.1 - System of integrated multi-scale and multi thematic performance indicators for the assessment of urban challenges and NBS.
• https://www.nature4cities.eu/post/nature4cities-defined performance-indicators-to-assess-urban-challenges-and-nature based-solutions.
• Nature4Cities, D2.2 - Expert-modelling toolbox
• Nature4Cities, D2.3 – NBS database completed with urban performance data
• https://www.nature4cities.eu/post/applicability-urban-challenges and-indicators-real-case-studies
• Nature4Cities, D2.4 - Development of a simplified urban performance assessment (SUA) too