Apple Inc.'s iPhones and Google Inc.'s Android smartphones regularly transmit their locations back to Apple and Google, respectively, according to data and documents analyzed by The Wall Street Journal—intensifying concerns over privacy and the widening trade in personal data.

Google and Apple are gathering location information as part of their race to build massive databases capable of pinpointing people's locations via their cellphones. These databases could help them tap the $2.9 billion market for location-based services—expected...