Defining a city: delineating urban areas using cell-phone data

What is a city? Researchers use different criteria and datasets to define it-from population density to traffic flows. We argue there is one dataset that could serve as a proxy of the temporal and spatial connections that make cities what they are: geolocated data from the world's more than 7 billion cell-phone users. Cell-phone data are a proxy of people's presence in a given area and of their movement between areas. Combined with computational methods, these data can support city delineations that are dynamic, responding to multiple statistical and administrative requirements, and tailored to different research needs, thus accelerating ongoing work in urban science.