The report, Unique in the Crowd: The privacy bounds of human mobility by Yves-Alexandre de Montjoye, César A. Hidalgo, Michel Verleysen & Vincent D. Blondel, analyzes 15 months of human mobility data for 1.5 million people, and finds that their mobility traces are sufficiently unique to easily identify the people concerned. It points out that while such locational data used to be available only to the mobile phone operators themselves, this is being changed by the widespread use of mobile phone apps.
"For example," says the report, "Apple® recently updated its privacy policy to allow sharing the spatio-temporal location of their users with 'partners and licensees'... Furthermore, it is estimated that a third of the 25B copies of applications available on Apple's App Store (SM) access a user's geographic location, and that the geo-location of ~50% of all iOS and Android traffic is available to ad networks."
The privacy issues are clear. If someone's location and movement is known, that person's behavior and habits can often be inferred. "A list of potentially sensitive professional and personal information that could be inferred about an individual knowing only his mobility trace was published recently by the Electronic Frontier Foundation," notes the report. "These include the movements of a competitor sales force, attendance of a particular church or an individual's presence in a motel or at an abortion clinic."
What is clear from this study is that "the uniqueness of human mobility traces is high." That trace can then become a mobility fingerprint. The fingerprint is still 'anonymous' until the target is identified; but the initial identification is often given away via social networks and geo-location apps. Re-identification can then be confirmed by the mobility fingerprint.
The study's lead author Mr de Montjoye told the BBC, "We've really tried hard to not frame this as a 'Big Brother' situation, as 'we know everything about you'. But we show that even if there's no name or email address it can still be personal data, so we need it to be treated accordingly."
Nevertheless, concludes the report, "These results should inform future thinking in the collection, use, and protection of mobility data. Going forward, the importance of location data will only increase and knowing the bounds of individual's privacy will be crucial in the design of both future policies and information technologies."