GPS V's Trip Odometer

[Mike2006]

Hi,

How accurate should your Trip Odometer be and how accurate is a GPS at calculating distance?

If the GPS says that you have travelled 100km, what sort of tolerance would you expact on your trip odometer?

Just curious on this one.

Mike.

 

[television]

MB speedometers are more accurate than most, GPS is good within a few meters

 

[verytalldave]

Any distance measuring device that relies on the revolutions of a wheel that gets smaller as time goes on must have a fairly wide error tolerance. Thats apert from the fact that most speedos overindicate by around 5% to 10% (and therefore the associated odometer as well).
Taking the combination of those two factors, you can expect an error of anywhere between 0% to about + 15% by my estimation. Therefore its reasonable to assume an average error of about +7%.
Buts thats a guess..............
GPS is bound to be more accurate and consistent.

 

[st13phil]

How accurate should your Trip Odometer be and how accurate is a GPS at calculating distance?

Can't tell you anything about MB Odo accuracy, but I do have experience of GPS use.

There's a general view that GPS receivers (GPSR's) are highly accurate and to most intents and purposes they are but they are essentially a positioning device, not a measuring device, and rely on having at least four satellites in view to position accurately. The measurement of speed by a GPSR is accomplished by reference to two snap-shot positions and the elapsed time between the snap-shots. The distance is always a straight line between the recorded points and the position of the points relies upon the instantaneous accuracy of the GPSR when each position was recorded.

To the best of my knowledge all current generation automotive GPSR's will "anticipate" their expected position based upon the last few known positions (which establishes an expected direction and speed) and in the absence of a good satellite fix when they report their position they will use this "anticipated" position. This is how automotive GPSR's which do not have dead-reckoning facilities manage to "keep going" while you are within a tunnel or have lost satellite reception for some other reason. Needless to say, speed and distance recordings under these circumstances are suspect to say the least and is why it's possible to get very obviously high or low speed readings out of a normal automotive GPSR.

So, in summary, the GPS odometer is probably more accurate than a standard car odometer, but it will not be 100% accurate, and certainly not accurate to a matter of feet over many miles.

 

[Mike2006]

Report

Just to report back then.

I have travelled 40km with my GPS and trip computer set both to zero.

After 40km (according to GPS) the trip computer is reading 40.8km.

I make that a difference of 1km for every 50km or 2%.

Mike.