Understanding GPS route

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It's one of frequently asked questions: when checking GPS tracks, especially on standing time, why position of GPS devices doesn't stand still. Something like this:

vehicles_map_gps_drift.png

You may ask, why it happens? There is solid science behind that, limits of physical world, and limits of our current level of technology.

Dilution of precision

If any GPS device tries to find its position using satellites, it needs at least three satellites to fix its position, and fourth to find height from Sea level. Usually it sees much more satellites, what makes estimating current location more exact.

But fixing GPS signal into one spot is still pretty hard. Satellites are flying 20000 km far away above us, what is pretty far. Various reasons, like atmospheric disturbances, driving between high buildings or mountains, reflected signals from glass, buildings or water make hard to calculate that exact spot. There is strong maths behind that, it's called dilution of precision. Basically, what it does, is to define circle of probability, what sets radius, into what our current location stays. Anyone, who has been using Google Maps, has seen that circle, what sometimes has smaller, then again larger radius.

vehicle_map_gps_drift_dop.png

Current position is drawn into centre of that circle, but actually it may be a little different from real one all time. Result is, that GPS position travels a little around one place all time. This picture illustrates that behaviour:

vehicles_map_dop_effect.png

It actually happens during driving time too, but usually we don't notice it, because distances between GPS points are long, but position is actually deviating from one side of road to another. In cities, where there are short distances and lots of houses or other infrastructure, what constantly change GPS conditions, it's more visible:

vehicles_map_route_good_drift.png

If you're afraid, that total distance calculated via GPS is different, it's actually minor difference compared to real distance.

Still, if real distance deviates too much from road, it may be indication, that GPS tracker does not see satellites very well. It looks like that.

vehicles_map_route_drift.png

But sometimes, there is nothing to do with it. It may look ugly, but total distance is still pretty accurate compared to real one. If it's possible, try to move GPS tracker to better spot, where it can see open sky.

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