[kag] Re: GPS accuracy?

  • From: Graeme Watson <Graeme.Watson@xxxxxx>
  • To: kag@xxxxxxxxxxxxx
  • Date: Tue, 17 Aug 2004 09:59:26 -0400

Both the 320 and the 60CS have showed the right speed for my van, so I 
suspect it may be your car.  My Honda is off the road because we cannot 
afford to insure it (damn Ontario insurance weenies), so I have not 
check it against a GPSr.  I however have previously check the Honda 
odometer against a measured kilometre, and the odometer measures a 
shorter kilometre which would translate into a higher indicated speed.  
Honda plot, or cultural calibration difference?  You are welcome to 
borrow my GPSr and check, maybe when we are racing to Ambitious Snorkeler.

Gordon Campbell wrote:

>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.
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