Wifi Localization
Wifi localization work more or less like localization with GPS. Instead of GPS satellites it uses wifi access points. Instead of GPS receivers it uses wifi modules(is found on any device with wifi).
Wifi access points can get a measurement for signal strength of the device relative the access point. These measurements are read periodically for each device. It's supposed to help with the quality control. Lower the signal strength harder it becomes to communicate with the device. So it's required to measure the load the access point is working on. Once it passes certain threshold quality control can kicks in make necessary decisions like kicking out far out devices, stop accepting new devices and such. These measurements can be accessed from a central location.
This signal strength has a relationship with the distance. It requires at least 3 access points to successfully determine a location of a wifi module. So once it find where the device is in a signal strength space it can be converted to spatial space.
But of course once it detects a location, it's relative to wifi access point network. It doesn't have mush application value. It needs to be converted to a real world location. The particular implementation I've worked with has a fingerprinting phase to achieve this.
In the beginning it marks up bunch of landmarks to cover the area where the localization needs to be done. Then for all these landmarks, using number of (in the same build/model) devices it measures the signal strength relative to each access points it can reach it to. This is possible since from the central location all the access point signal measurements can be accessed and they carry the device Wifi MAC address with each these signal measurement. Once this is completed a fingerprint map is said to be built.
Now it's possible to find the device locations compared to the used landmarks. At the central location it builds another signal strength map to each device which is visible to wifi network. Then using the previously built fingerprint map it finds the nearest land mark for each of them.Thus it can approximately say where the each device is.