To determine the ebb and flow of city dwellers, public officials have typically resorted to looking around and counting. But now censuses can be taken every few milliseconds, as phones ping cellular networks and reveal their location.
For instance, Italian [PDF] and American [PDF] researchers generated usage maps for Milan and the borough of Manhattan in New York City using anonymized cellular data. Paolo Tagliolato, a postdoc at the Polytechnic University of Milan, says such anonymized data are effectively free and always up to date. Conversely, he says, Milan’s last comparable citywide survey was costly, involved 750 000 interviews, and hasn’t been updated since 2002.
This article originally appeared in print as “Ten Million Censuses Per Day.”