Authors

Thomas Theron, Sarah Irish, Vaishnavi Potphode

Research Question

What is the appropriate spatial unit for measuring a city? Across about ten diverse global cities, including New York City, the project tests how new open data products can delineate built-up extents, and how alternative boundaries change key urban indicators and cross-city rankings.

Background

Clear city boundaries are needed to compute and compare city-level indicators such as temperature, air quality, greenness and population. Municipal boundaries exist for only a small share of cities globally and are inconsistent and often unsuitable. The 2024 GHS-Urban Centres database uses coarse 1 km grids that miss peri-urban areas, and the wider GHS-Functional Urban Areas product is conceptually appropriate but coarse and outdated (released in 2019 from 2015 inputs).

The geospatial data landscape has changed substantially in the last two years. WSF Tracker now provides global 10 m impervious surface estimates every six months since 2016, and building products such as Overture and Google Open Buildings 2.5D offer increasingly complete footprint and height information. A reproducible, regularly updated definition of urban extent could improve how cities are compared, how climate and health risks are assessed, and how resources are targeted.

Methodology

The project involves:

• Delineating each city's built-up extent using two to three reproducible open-data methods.
• Computing temperature, greenness, air quality and population under every boundary.
• Evaluating whether each boundary produces cross-city rankings that match expert and local understanding (for example, whether City A is credibly greener, hotter or more polluted than City B) and supports useful policy judgments.
• Iterating between boundary methods and indicator results, with CIESIN, EC-JRC and World Bank SPARC practitioners advising on scalability and operational fitness.

Deliverables

• Algorithm development and design
• GitHub repository with documentation
• Policy recommendations

Data Sources

• WSF Tracker 10 m six-month global impervious surface grids
• Overture buildings and roads
• Google Open Buildings 2.5D
• GHS-Urban Centres (UCDB) and GHS-Functional Urban Areas (for comparison)
• Temperature, greenness, air quality and population data