How does google maps interior know which floor the user is in? [closed]

Question

If you see the demo here you will notice that the application tells you which floor you are in. Does the maps app use altimeter or something?

Answer 1

The Global Positioning System provides 3D coordinates through trilateration:

http://en.wikipedia.org/wiki/Altimeter#Global_Positioning_System

While this doesn't give you the floor you are on, and isn't all that accurate, Google could hazard a guess. Even if it is right only some of the time, it would still be impressive.

Newer mobile devices like the Galaxy Nexus come with a barometer built in. This would use your location to work out air pressure at sea level, versus pressure read from the phone, and can determine altitude from that.

Answer 2

I think I could formulate an answer now.

It does not determine the floor your on, the user selects what floor they would like to see.

Indoors The google location services, can use GPS or cell tower data to show your approximate 2D position. Even with GPS or cell indoors your Floor position would be very hard to determine without some sort of local location services in the building itself.

Going Indoors with Google

That little blue spot

Some GPS info with loose figures:

Portable devices have been able to track your location for years, by using GPS. GPS is a signal that is sent by special sattalites that orbit at ~12,000mi above the earth. The GPS reciever calculates the time it takes for the radio signal to get to the GPS reciever, to triangulate its own position. With at least 3 signals from the satalites the reciever can have your approximate 2D position, using 4 satalites it can calculate your 3D position. Being 12,000mi UP or way horizontal to the GPS reciever there is less accuracy to the altitude. Some put this figure at 1.5X less accurate.

Originally created for military use , it was adopted for civillian use, and in ~2000, it was available at much greater accuracy, of ~20 meters in 2D. With improvments in the system, and improvments in the the recievers, I find that it gets as close as 10meters with at least 8 satalites to calculate from. Adding in software and averaging to the NMEA sentance data comming out of the reciever it is possible to get that as close as about 3meters on a really good day, with lots of sattalites. Making it possible to know where in the building you are. I have done that many times, and I could see on the map for example that I was in the Sporting goods section of wallmart. (prior to the new maps)

The altitude of modern GPS bounces around, and might be as close as ~30meters , there is also some known issues with altitude that I dont fully understand. On a good day with a lot of satalites and averaging, the maximum precision of the altitude Outdoors for me is about ~20meters +- With averaging you might get that down to ~10meters or ~3 floors . Averaging means you would have to stand in the same place getting multiple calculations.

Gps signals have to be recieved by the gps reciever. Indoors a good consumer reciever might get ~3-6 sats instead of ~8-12 on the first floor. The location averaged is accurate then to about ~15meters, and the altitude to at best ~20 meters or ~6 floors. It starts getting into bounces of the signal, putting the calculations off by the ammount of the bounces.

Once you get on second floors, or around concrete and steel , getting a usefull GPS signal for altitude would basically end in most places, even trying to get a location would be Iffy. having a repeater send you a signal would do a good job of telling you where the repeater was :-) Might work in high-rise buildings when your close enough to the outside to get a signal.