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We had an iPhone and a MacBook Air stolen. I was able to track both units at different times. As I understand it the Find My iPhone service uses GPS and cell tower triangulation to locate iPhones. iPhone location outdoors and/or in areas with good exposure to cell towers is very accurate.

For wifi only devices I believe Apple work from a database of wifi points discovered by Apple mobile devices cross referenced to GPS and cell tower triangulation data, derived from those same mobile devices. The location accuracy for this system seems to be significantly poorer.

What I don't understand is why the location for the stolen laptop is reported differently each time it connects to the internet - my laptop location has been given as 6 different addresses all within a few houses of one another. I've noticed the same behaviour for wifi only devices we own that don't leave our house.

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The answer is in your question. Wi-fi triangulation is a lot less accurate than GPS (as to be expected). –  Gerry Jan 14 '13 at 9:01
    
Thanks Gerry, you've misunderstood the question; the mobile device location is dynamic. The wi-fi only device location is supposedly read from a database of known wi-fi access points. If that indeed is the case the location won't be dynamic - or at least not as dynamic as the mobile locations - they should be reporting a single badly triangulated point that's stored in the DB. Is it possible that Apple introduce some randomising in the locato reporting r wi-fi only devices? –  user38567 Jan 14 '13 at 13:00
    
I don't believe I misunderstood the question, but that you do not understand what my comment implies. I expanded my comment to a full answer, hope it will become clearer. –  Gerry Jan 14 '13 at 14:39
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2 Answers

If a person can steal your MacBook, they most probably can and do steal Wi-Fi bandwidth. The thief is stealing bandwidth of his neighbours. This can be proven as all networks are within a few houses of each other.

As for Wi-Fi devices at your home, if you live in a particularly crowded area with lots of Wi-Fi networks, your device sees different networks at different points of time as they switch on and off, so it reports slightly different locations.

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Thanks. The MBA was stolen a couple of weeks ago and spent a week on and off at half a dozen addresses, widely spread. After not appearing for a few days it has repeatedly shown up in the same neighbourhood with a new user account name - I believe it has been sold to a "new owner" and is not being used by the thief. –  user38567 Jan 14 '13 at 13:05
    
The other problem with your answer is that, as I previously noted, I have observed similar behaviour from wi-fi devices that I still control :-) and the location info given for them changes as well, despite them being connected to just one router. Gotta be a better answer! –  user38567 Jan 14 '13 at 13:08
    
@user38567 I explained the behaviour exhibited by devices you control as well. The location doesn’t depend on the network the device is connected to. It depends on the visibility of various networks. Your question doesn’t provide me info on how congested the area around your home is. –  duci9y Jan 14 '13 at 17:19
    
The area around my home is relatively uncongested, two level detached houses, low density and lots of people still away on holiday. Mobile phone signal is notoriously weak though. The area around the stolen laptop is all single level detached houses. –  user38567 Jan 14 '13 at 22:48
    
You have an iPhone? iPhones update their Wi-Fi access point location database regularly. Weak cell reception can be a contributing factor in your observing shifty locations, as different levels of cell reception will affect the triangulated position of the phone. The phone will report this location, which shifts frequently, to Apple’s database and thus affect your observations. –  duci9y Jan 15 '13 at 11:13
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You are wrongfully assuming that an inaccurate localization always produces the exact same ("inaccurate") result. If that were to be the case, the localization would be precise, not an approximation.

While the device and access points may remain stationary, the signal strength and signal noise will vary depending on a lot of external factors from any point in time to another. Because the device uses these parameters to triangulate its position, the results will always vary to some point between measurements.

To put it in an analogy, if I have a digital thermometer that cannot show decimals and will round any measurement to a whole number, an actual temperature of 21.5 degrees will sometimes result in 21 degrees being displayed, and sometimes 22. Observing these different results does not mean the temperature actually dropped or increased by one degree.

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Thanks Gerry. You're saying that the wi-fi only devices are being located dynamically? There's not much information available on how Apple manages this, but another post on Ask Different says they maintain their own location database derived from sniffed WAPs, the implication being that location updating would not be dynamic or as dynamic. No-one seems to be able to shed any light on the specific mechanism that Apple use to locate wi-fi only devices! –  user38567 Jan 14 '13 at 22:59
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@user38567, I'm not sure you're reading what I wrote. Ofcourse it will be dynamic, as the signal strength, noise and interference vary over time as well. –  Gerry Jan 14 '13 at 23:24
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