A worldwide model for boundaries of urban settlements

TL;DR

This paper introduces a global model for defining urban boundaries based on geographical and social dynamics, enabling consistent city analysis across different scales and challenging existing notions of city sizes.

Contribution

It presents a novel bottom-up model for urban boundary delineation that is applicable worldwide and aligns with social science laws like Zipf's law.

Findings

Largest cities are often conglomerations of smaller settlements.

The model's city definitions are robust and consistent across scales.

Contradicts recent UN reports on city size rankings.

Abstract

The shape of urban settlements plays a fundamental role in their sustainable planning. Properly defining the boundaries of cities is challenging and remains an open problem in the Science of Cities. Here, we propose a worldwide model to define urban settlements beyond their administrative boundaries through a bottom-up approach that takes into account geographical biases intrinsically associated with most societies around the world, and reflected in their different regional growing dynamics. The generality of the model allows to study the scaling laws of cities at all geographical levels: countries, continents, and the entire world. Our definition of cities is robust and holds to one of the most famous results in Social Sciences: Zipf's law. According to our results, the largest cities in the world are not in line with what was recently reported by the United Nations. For example, we find that the largest city in the world is an agglomeration of several small settlements close to each other, connecting three large settlements: Alexandria, Cairo, and Luxor. Our definition of cities opens the doors to the study of the economy of cities in a systematic way independently of arbitrary definitions that employ administrative boundaries.