Foursquare uses proprietary technology, Pilgrim, to detect a user's location. When users opt in to always-on location sharing, Pilgrim determines a user's current location by comparing historical check-in data with the user's current GPS signal, cell tower triangulation, cellular signal strength and surrounding wifi signals.The Verge. May 2014. "Meet Swarm: Foursquare's ambitious plan to split its app in two" https://www.theverge.com/2014/5/1/5666062/foursquare-swarm-new-app
