Wi-Fi triangulation provides a middle ground between A-GPS and cell tower triangulation. Wi-Fi triangulation uses less power than GPS and can provide better accuracy than cell tower triangulation in some environments.
Wi-Fi location relies on a global location database of Wi-Fi networks and the ability of a phone device to detect Wi-Fi networks within its vicinity.
Wi-Fi triangulation works by detecting networks that are in range, measuring the signal strength of each network, and then triangulating the result using the Wi-Fi network location database.
The Wi-Fi network location database is provided by the company Navizon; currently, no public API exists for accessing the data directly.