[lukeschlather]

GPS is based on triangulating (well, trilateralating) between 3-6 fixed points in space located thousands of kilometers away.

Wifi positioning is based on triangulating (trilateralating) between 3-40 fixed points within 100 meters.

Assuming you can get access to the raw data, Wifi should be extremely accurate for calculating the distance between two points in space separated by less than 30 meters. (And probably pretty accurate up to 100 meters or so.)

[kemiller]

What about the impact of walls and other objects in between? Signal can drop or jump sharply as you move in and out of shadows.

[JabavuAdams]

What about multipath? I measure a 1/2 decrease in signal strength, is it because I got 1/sqrt(2) times farther away, or because someone opened the fridge door next to the router?

[DubiousPusher]

In response to the three replies already here. I've never done this, I'm a game programmer. But I would use a big bag'O'tricks to address this. Getting unrealistically long distances out of your wifi data? Have the user enter a square footage and normalize on a logarithmic scale bounded by heuristics. Use the accelerometer not for tracking but detection like going up and down stairs, walking short vs. long distances.

[bradleyjg]

I'd think the problem with wifi is that the exact position of the base stations are hard to collect. The mass proliferation of telcom wifi networks could help if they are willing to sell the location data, but I don't see how Apple could reasonably know where the access points I see with SSID Holly_guest or TS-Dlink are located to use it as a trilateration point.

[ghaff]

Four actually. Three for position in 3D space plus one for time.