I want to make a CAR positioning system. I wrote a program which can show me the coordinate locations on a map.

But when I get coordinates from MOBILE GPS and from CAR GPS, they both differ.

Please see image

Why do they differ?

How can I resolve this problem?

  • 1
    There is a difference of about 300+ meters. Do you know which one is correct? Are you sure that your phone's GPS is using signals from GPS satellites and not from A-GPS? Sep 8 '14 at 11:35
  • 2
    Are the timestamps the same? Maybe your mobile GPS has fever satellites in view than the car GPS and so the positional accuracy suffers. Try to take measurements from the car GPS and the mobile GPS in a flat, open field with good satellite reception (place the mobile GPS 3-4 meters next to the car) and then see if the measurements are accurate to within 5m. This way you can see if there are errors from the devices (they should be present in the best case conditions also) or if they introduced by your setup with the mobile in the car.
    – til_b
    Sep 8 '14 at 11:41
  • What chipset do you have in both of your devices?
    – Mapperz
    Sep 8 '14 at 13:55
  • CAR GPS is correct one
    – Hardik
    Sep 11 '14 at 10:04
  • is there any thing so I can convert my Mobile GPS value to CAR GPS values ?
    – Hardik
    Sep 11 '14 at 10:05

Any given GPS unit only has a certain level of accuracy, and different units will determine their position in different ways. The technical term is dilution of precision (DOP) and there are many contributions to the error: what satellites are providing the signals, what time of day is it, is it cloudy, are you near dense vegetation, is the signal bouncing off lots of tall buildings...

You can also see that the Car GPS is rounding the lat/long values off, which will decrease its positional accuracy. Read more about the effect of decimal places in this StackExchange Q&A: Measuring accuracy of latitude and longitude?

Navigation applications typically snap the vehicle location to the nearest road -- if the GPS coordinates are in a field, for example, it can be assumed that the car is actually driving next to the field. (This is not always accurate, and my GPS might easily gets confused in road-dense areas; it depends on how much DOP there is at that moment in time.)

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