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Might be a stupid question, but I don't know the logic behind.

  1. On a Wifi enabled device, GPS coordinates are accurate.(as expected)
  2. Disable Wifi and enable 3G gives a inaccurate coordinates (device location is same as in point 1.).

when a distance is calculated from coordinates in point 1. and 2. Google maps shows a 5min walking distance.

could someone explain the above behavior and is it possible to have same coordinates with 3G?.

b8r0
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  • Are you sure you disabled wifi and not GPS? (The icon is confusing engouh)? Because without GPS, coordinates derived from wifi are definitely *not* expected to be accurate. – Jan Hudec May 02 '11 at 09:10
  • Yes, I disable WIFI from the power control widget and enable "Use Packet data" inside Mobile network. and moreover both settings under "Location and securtiy" such as "use wirelss networks" and "Use GPS satellites" are being ticked during Wifi and 3G. – b8r0 May 02 '11 at 09:31
  • Than either a known wifi hotspot is very close, so the location is exact by coincidence, or there is no known wifi, so without 3g the system turns on GPS anyway. In either case the application does not request GPS. – Jan Hudec May 02 '11 at 10:30

2 Answers2

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Android devices have three methods of calculating coordinates:

  1. GPS: This is the "fine" location and is the only one accurate.
  2. Wifi: Using a database of hotspots google collected when doing streetview.
  3. 3G: Using database of base stations and some very rough triangualtion.

So GPS coordinates are always expected to be accurate, but wifi coordinates are not and neither are 3g. Of course the cost of GPS precision is that it needs quite a lot of energy (a few phone batteries last more than 10 hours with GPS running) while wifi and 3g need no extra energy to what wifi/3g would use anyway.

Jan Hudec
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  • Thanks for the jiffy reply, But then GPS is a battery eater in contrast with 3G, 3G gives not accurate results though. – b8r0 May 02 '11 at 09:17
  • And I would like to use 3G because of batter life and get accurate results, is there any plausible approach? – b8r0 May 02 '11 at 09:24
  • @b8r0: Indeed. I forgot to mention the energy requiremens. – Jan Hudec May 02 '11 at 10:21
  • @b8r0: No, GPS is the only method that can give you sufficiently accurate results for navigation. Without knowing what the application should be doing I can't tell whether some compromise is possible. – Jan Hudec May 02 '11 at 10:27
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3g coordinates are taken from the 3g towers location (and probably triangulated from multiple towers). As a 3g cell can span several acres around the tower, the coordinate can only give you an approximation.

Also the device may just have cached an old location (especially when location updates come in via GPS and you don't need 3g). I've seen values several dozen miles away - especially when riding on the train and the 3g network handover does not allow for an easy position fix.

Heiko Rupp
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