Originally Posted By IvorMog
Originally Posted By Hamwich
Originally Posted By IvorMog

Don't be fooled into thinking that GPS always gives an accurate speed readout in a car.

It will be accurate on the straight and level but less accurate on hilly and twisty roads.

Not with all GPS but many don't take height change in to account when calculating the speed.

Think basic trigonometry.


Hmm. The Earth is comparatively smoother than a billiard ball, altitude change is not really a consideration. GPS gives an RMC message at least every second, often more frequently. RMC (Recommended Minimum Content) message is the one that's got Latitude, Longitude, and ground speed in knots and thus is the easiest message for readouts to process, so a cheapo GPS will look just for these.

Given that the ground speed of a car is unlikely to exceed say 300Kph, then in 1/3600 of an hour one will never travel more than 100 metres. What's the steepest road one would ever be likely to drive up? 1:4 / 25%? So an adjacent side of 100m and an opposite side of 25m gives a hypotenuse of 103m

So if even at the most extreme situation one could possibly imagine (screaming up a 1 in 4 hill at over 180mph) the GPS speed is only likely to be 3% out - well within the legal range. In all other situations it will give very accurate results.

BTW, if anyone wants the Python code to turn a Raspberry Pi into a GPS data logger, gissa shout. I'm working on it at the moment.




You're right, the theory is there but in all practical terms the GPS will be fine.

Should have done the trig calc before posting.

Unless of course the US Military switch off the GPS, or do we use our own satellites these days?



People get carried away about accuracy these days, just open a discussion about the "ovality of error" involving GPS for a really involved discussion.. guru
I was unfortunate enough to be involved in some early GPS evaluation trials checking and calibrating it.

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