[kag] Re: GPS accuracy?

  • From: Gordon Campbell <gordon@xxxxxxxxxxx>
  • To: kag@xxxxxxxxxxxxx
  • Date: Mon, 16 Aug 2004 13:18:11 -0400

It's been consistently a 5 km/hr difference no matter what the terrain
or the speed of the car. I've been assuming that Honda had intentionally
set the speedo high to make you think you're going faster than you are.


On Mon, 2004-08-16 at 12:28, Hoover wrote:
> On Mon, 16 Aug 2004, Gordon Campbell wrote:
> > Does anyone know how reliable the speed reading on a GPS is? My sTrex
> > consistently insists that my car is travelling at exactly 5km/hr less
> > than the speedometer. hmmmm.
> I saw the same thing with Root's old car. I assumed it was a calibration 
> issue with the speedometer. I should check again in the new car. Now that 
> I think of it, this is more likely related to the GPS receiver not 
> including changes in elevation in its calculations. I think it projects 
> your position onto its mathematical model of the Earth and measures your 
> movement along that. So if you are driving up a hill, your projected 
> motion would be slower than what your speedometer says. Or maybe the model 
> is not quite parallel to the surface of the Earth in our part of the 
> world. I'm just guessing at all of this, of course.
> Hoover
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Gordon Campbell <gordon@xxxxxxxxxxx>

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