In one embodiment, characteristics of signals received from Wi-Fi access points during a traversal of a number of drivable streets within a geographic area having a radius of one or more miles are determined, the characteristics of a given signal including a unique identifier of a Wi-Fi access point that transmitted the signal, a location where the signal was observed, and an observed signal strength at the location where the signal was observed. Using the characteristics, a reference database is built including records descriptive of Wi-Fi access points in the geographic area. To determine a current location of a mobile device, an observed signal strength of one or more Wi-Fi access points observed by the mobile device from the current location is determined and a current location of the mobile device is computed using at least the observed signal strength of the one or more Wi-Fi access points observed by the mobile device in comparison to information in the reference database.