Wi-Fi Location Tracking using RF Fingerprinting

In addition to Closest Access Point and Triangulation methods used find Wi-Fi device's location that I've mentioned earlier here, there is another method - RF Fingerprinting. Cisco's Wi-Fi Based Real-Time Location Tracking: Solutions and Technology white paper describes the method in detail.

The difference of RF Fingerprinting from Triangulation method lies in the database with which the possible locations are compared to. The signal levels detected from a Wi-Fi device are still found using multiple access points as in Triangulation. Then the combined signals are compared with the database that contains power levels of each point on the map. The database is usually calibrated by measuring power at different locations.

RF Fingerprinting takes into account such factors as reflection, attenuation, and multi-path, which can create uncertainty in other techniques.

Related:
GPS and Wi-Fi combination: link