iPod touch location services.

[Capetonian]

I was intrigued as to how my son's iPod Touch can sometimes locate itself to within a few metres of its location.

It is Wi-fi only, and does not have GPS, and I assume it does not pick up GSM signals, which would allow it to triangulate and find an accurate position, and given that an IP address will often resolve to almost anywhere in the country and hardly ever to with a few miles, so my question is how the hell does this work? I find it a bit scary!

I found this article at iPhone and iPod touch: Understanding Location Services (http://support.apple.com/kb/ht1975), and the relevant parts are here :

Location Services allows applications such as Maps, Camera, and Compass to use information from cellular, Wi-Fi1, and Global Positioning System (GPS)2 networks to determine your approximate location. This information is collected anonymously and in a form that does not personally identify you.
About location precision or accuracy

Depending on your device and available services, Location Services uses a combination of cellular, Wi-Fi, and GPS to determine your location. If you're not within a clear line of sight to GPS satellites, iPhone can determine your location using Wi-Fi3. If you're not in range of any Wi-Fi, iPhone can determine your location using cellular towers.


Notes:

1. iPod touch only uses Wi-Fi for Location Services (if a Wi-Fi network is available).

[MacBoero]

I think you'll find your answer here:
Skyhook: How It Works > FAQ (http://www.skyhookwireless.com/howitworks/faq.php)

[mixture]

Capetonian,

sometimes

Being the important keyword.

As MacBoero implies, there are a number of geolocation databases out there that attempt to map IP addresses to a location. They vary widely in their accuracy and update frequency.

Accurate geolocation of an IP address is not necessarily entirely big bad big-brother. For example one good location for accurate geolocation is for CDNs (content delivery networks) such as Akamai and Youtube who would like to be able to point you to the nearest datacentre so that you have a pleasurable viewing experience. However, of course, this doesn't need cm level accuracy, the sort of thing that might be useful for is if your using your device for navigation and are in an urban canyon.

[MacBoero]

It is interesting, Skyhook for example claim to have mapped many wifi networks using cars trolling around the streets. Google tried to do the same thing and got into trouble for it.

There must be something else in play though. I recently (well 8 months ago!)moved from one flat to another. They are about 1.5 miles apart. Because they were on the same telephone exchange I had my entire BT service transferred, number, BT Broadband and all. This was very quick, the service was still active at the old place as I was tidying up the last few things, listening to streaming radio on an iPod connected to some speakers. Then the music went dead, checked the line it was dead too. Drive up to the new place with all the Homehub and gubbin's, plugged it all in and hey presto all working in the new place.

The odd thing is the iPod Touches reported the right location in the new flat straight away, and I am certain no Skyhook or Google cars followed me up the road!:}

[Ancient Observer]

Yup. Dunno what it is, but within the area covered by my local/village BT switchboard, (whatever they are called), the touch can tell if I'm in a different place if wi-fi is on.