Your Apps Know Where You Were Last Night, and They’re Not Keeping It Secret

Dozens of companies use smartphone locations to help advertisers and even hedge funds. They say it’s anonymous, but the data shows how personal it is.

By JENNIFER VALENTINO-DeVRIESNATASHA SINGER, MICHAEL H. KELLER and AARON KROLIK

The millions of dots on the map trace highways, side streets and bike trails — each one following the path of an anonymous cellphone user.

One path tracks someone from a home outside Newark to a nearby Planned Parenthood, remaining there for more than an hour. Another represents a person who travels with the mayor of New York during the day and returns to Long Island at night.

Yet another leaves a house in upstate New York at 7 a.m. and travels to a middle school 14 miles away, staying until late afternoon each school day. Only one person makes that trip: Lisa Magrin, a 46-year-old math teacher. Her smartphone goes with her.

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