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.
Why do they differ?
How can I resolve this problem?
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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.)