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My guess is that the trip distance is updated once a second (GPS update interval) adding the distance that you have travelled in the last second to the running total.
That's also my assumption but the distance between two GPS points one second apart will probably be calculated as the straight line distance between them. So you are in fact adding up all of the chords of arc representing the bend in the road, which will, as you say, always be less than the actual distance travelled.
However, all of the bends in the map database roads are formed from small straight lines anyway, so the route distances are effectively calculated in exactly the same way and probably also slightly underestimate the true distance of the route.
I suspect that these discrepancies are quite small, but if you are trying to measure 20 miles accurate to 1/10 of a mile (1 part in 200) then they might make a difference.
Time to get out the fifth wheel distance recorder.
Joined: Apr 04, 2006 Posts: 10118 Location: Bexhill, South Sussex, UK
Posted: Tue Dec 08, 2009 9:42 am Post subject:
linknet wrote:
That's also my assumption but the distance between two GPS points one second apart will probably be calculated as the straight line distance between them. So you are in fact adding up all of the chords of arc representing the bend in the road, which will, as you say, always be less than the actual distance travelled.
1 sec at 60 is 88ft so the length of the chord must be small compared with the circumference of a curve described by the chord that can reasonably be taken at the speed in question. So although it seems that the GPS will under read on a curvy road, I suspect that it must only be by a small percentage, probably a good bit better than the 1 in 200 of a tenth of a mile over 20 miles. Difficult to calculate as I havn't a clue what radius curve can be reasonably taken at 60MPH (or any other speed) As you say, fifth wheel needed.
According to my slightly rusty maths, so it might be wrong, and the fact that I hate imperial units, here's an example:
If you drive around a circle of 36 m radius at a tangential velocity of 14.137 m/s it will take exactly 16 s and you will experience a lateral force of approximately 0.56 G and will have travelled 226.194 m.
The GPS will have taken 16 points around the circumference which will create 16 chords each of 14.046 m giving a total of 224.744 m.
That's a difference of 1.450 m in 226.194 m, slightly more than 1 in 200.
Assuming that nobody drives continuously in small circles then the error will normally be much less than that.
Joined: Apr 04, 2006 Posts: 10118 Location: Bexhill, South Sussex, UK
Posted: Tue Dec 08, 2009 4:48 pm Post subject:
linknet wrote:
......tangental velocity of 14.137 m/s ....Assuming that nobody drives continuously in small circles then the error will normally be much less than that.
Thats about 30 MPH around a 118 ft radius . Truly a fairly tightish curve on the road I suspect (but the lateral force is pretty low by F1 standards).
That seems to have played that one out then unless you drive a F1 car
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As you say, fifth wheel needed. 
. Truly a fairly tightish curve on the road I suspect (but the lateral force is pretty low by F1 standards).
