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 DeVries, Natasha Singer, Michael H. Keller & Aaron Krolik @ NYTimes.com, Dec. 10; interactive feature

[....] 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.

An app on the device gathered her location information, which was then sold without her knowledge. It recorded her whereabouts as often as every two seconds, according to a database of more than a million phones in the New York area that was reviewed by The New York Times. While Ms. Magrin’s identity was not disclosed in those records, The Times was able to easily connect her to that dot.

The app tracked her as she went to a Weight Watchers meeting and to her dermatologist’s office for a minor procedure. It followed her hiking with her dog and staying at her ex-boyfriend’s home, information she found disturbing.

“It’s the thought of people finding out those intimate details that you don’t want people to know,” said Ms. Magrin, who allowed The Times to review her location data [....]