  Urban planning still lacks appropriate standards to define city boundaries
across urban systems. This issue has historically been left to administrative
criteria, which can vary significantly across countries and political systems,
hindering a comparative analysis across urban systems. However, the wide use of
Information and Communication Technologies (ICT) has now allowed the
development of new quantitative approaches to unveil how social dynamics
relates to urban infrastructure. In fact, ICT provide the potential to portray
more accurate descriptions of the urban systems based on the empirical analysis
of millions of traces left by urbanites across the city. In this work, we apply
computational techniques over a large volume of mobile phone records to define
urban boundaries, through the analysis of travel patterns and the trajectory of
urban dwellers in conurbations with more than 100,000 inhabitants in Chile. We
created and analyzed the network of interconnected places inferred from
individual travel trajectories. We then ranked each place using a spectral
centrality method. This allowed to identify places of higher concurrency and
functional importance for each urban environment. Urban scaling analysis is
finally used as a diagnostic tool that allowed to distinguish urban from
non-urban spaces. The geographic assessment of our method shows a high
congruence with the current and administrative definitions of urban
agglomerations in Chile. Our results can potentially be considered as a
functional definition of the urban boundary. They also provide a practical
implementation of urban scaling and data-driven approaches on cities as complex
systems using increasingly larger non-conventional datasets.
