Your phone’s ability to pinpoint your exact location and use that info to deliver services—a meal, a ride, a tip, a coupon—is reason for excitement. But this world of always-on GPS raises questions about what happens to our data. How much privacy are we willing to surrender? What can these services learn about our activities? What keeps detailed maps of our lives from being sold to the highest bidder? These have been issues as long as we’ve had cellphones, but they are more pressing than ever.