One of the arguments in favour of anonymous mobile location tracking, nanely that it doesn't provide enough information to identify individuals, has been slapped down by a US-Belgian study. An anonymous trace of one phone's movements, plus a small amount of external data, can pick out one person out of millions.

An analysis of 1.5 million individuals' interactions with cell towers over 15 months found peoples' movements are so individual that “four spatio-temporal points are enough to uniquely identify 95% of the individuals” in their analytical dataset, even though it only used the spatial resolution given by carriers' antennas.

Once an individual's trace has been singled out from their phone's location (given by the phone network), it only takes “a few” known locations associated with that person to show who they are.

Yves-Alexandre de Montjoye at MIT in Cambridge, Massachusetts, and Université Catholique de Louvain in Belgium, lead authors of the study, notes that the four points used to identify an individual could come from a variety of sources: a person's workplace or home address, or locations revealed by their posts on Twitter.

Even decreasing the spatial resolution doesn't help much, the study found: “the uniqueness of mobility traces decays approximately as the 1/10 power of their resolution,” the researchers write in Nature Scientific Reports. “Hence, even coarse datasets provide little anonymity.”

Even coarse data (centre, right) is enough to reveal your location Source: Nature Scientific Reports

After examining the data, the researchers found that they were able to derive a single formula expressing the “uniqueness of human mobility”, and that “mobility datasets are likely to be re-identifiable using information on only a few outside locations.”

The 1.5 million users' traces, the study notes, represent roughly the same number of users of the Foursquare location service. The study would seem to put a big dent in claims by companies like shopper-tracker Path Intelligence that the information they collect “is not personally identifiable”. ®