[justinsb]

Isn't this exactly how location is always done without GPS? Look at the strengths of all the radio signals we can see, and match against known locations? Admittedly it's previously been primarily cell-towers & wi-fi, not GPS signal strength, and I thought most of the previous data had been collected by people that also had GPS, not via foursquare checkins...

If not, why does my Android phone ask me to turn on wi-fi to enhance location information?

Edit: To answer my own question, the Android source code for this used to be available. At the time, they passed the visible wi-fi SSIDs and the cell-tower IDs, but not the signal strength (they had it in both cases). Not sure why they didn't pass the strength. Maybe it made caching really hard; maybe it sufficed just to pass them in order; maybe it didn't make a big enough difference... http://www.netmite.com/android/mydroid/frameworks/base/locat...

To me, this actually raises more questions. It's so surprising to me that Google don't pass the signal strengths ("everyone knows" that is how triangulation works), that I'm thinking the real secret is that the signal strength isn't actually helpful. In other words, I'd bet Google tried what FourSquare have discovered, and rejected it. Or it's patented :-)

[vidarh]

Consider that Foursquare has extra data: Checkins.

They know that when certain location data sources provided chertain signals, the user in question checked in at a location with a known map position and known constraints to the location.

There aren't many sources of location data with that extra information.

[mutagen]

The primary purpose of WiFi to enhance location information is to bootstrap the download of current GPS orbits (the ephemeris) [1].

More advanced techniques relying on radio signal strengths, RTT time, and other observables [2] are being developed but I don't know of any in widespread use. I wouldn't be surprised to see something emerge soon though.

[1] http://en.wikipedia.org/wiki/Assisted_GPS

[2] One example: http://ws2.binghamton.edu/fowler/Improving%20WLAN-Based%20In...

[JangoSteve]

Google actually does collect wifi location data via Android devices. There's a setting that allows you to toggle this data on/off. If you turn it off, it actually disables the course location API available to Android apps (I recently encountered errors with both the Lux and the UCCW Android applications due to the fact that I had this setting disabled and those apps only had course-location built-in).

Now whether Android actually uses the wifi data to determine location, as opposed to simply collecting it and manually disabling course-location if you don't send them wifi data, I have no idea. It does seem to be related though to the new Geofencing API available in the latest versions of Android.

EDIT: Sources!

http://www.zdnet.com/blog/networking/how-google-and-everyone...

http://developer.android.com/training/location/geofencing.ht...

[bri3d]

Skyhook absolutely relies on radio signal strengths and other observables and so do Google's location services.

This can be observed in practice as Android tablets and Apple devices with no GPS or cell radio at all can still locate themselves relatively well.

http://www.skyhookwireless.com/apps-enterprise/ has the marketing-bling diagram of Skyhook's use of multiple observables to determine location; even as of 4 or so years ago when I last used their API you sent them MAC + RSS and got back location.

[andor]

Indoor positioning methods are being developed, outdoor methods are in use for years now. It's how positioning works on your laptop, e.g. in Google Maps.

http://en.wikipedia.org/wiki/Wi-Fi_positioning_system

"Commercial providers of this type of positioning service include Google,[1] Navizon, AlterGeo[2] and Skyhook Wireless."