| Re: Why can't car speedos be accurate? |
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Group: uk.transport · Group Profile
Author: MortimerMortimer Date: Sep 20, 2008 10:30
"Petert" googlemail.com> wrote in message
news:oqbad4tqqse5u15etr5v1sac9c4hpt2cog@4ax.com...> On Sat, 20 Sep 2008 13:09:53 +0100, "Mortimer" privacy.net> wrote:
>
> Someone explained to me once that the speed shown by a GPS receiver
> isn't necessarliy accurate as you may be driving up or down a steep
> hill - the GPS will show your speed as though you were driving along a
> flat road
Good point. Let's look at the error. On a 1:3 hill (probably the steepest to
be encountered), the angle will be arctan(1/3) = 18 degrees. For a
hypotenuse of 1 unit (the true distance that the car travels), the
horizontal distance that the GPS senses will be cos(18 degrees) = 0.95
So the GPS will read 95%% of the true speed and the error will be (1-0.95)/1
= 5%%.
I suppose a really clever GPS could calculate the gradient from the rate of
change of altitude and apply a suitable correction factor.
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