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GPS accuracy

Posted on 2006-06-29
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Last Modified: 2008-03-17
I just got a Garmin forerunner 201 wrist-mount GPS and am wondering how accurate the distance measurements are. I took it to a university 400 meter track and did a measured mile, and it showed 1.16 miles for distance, off by 16%.  On another track, it measured 1.02 miles.  Both were on open fields with good satellite coverage. On the highway, it tracks almost exactly with mile measurements posted on the side of the road, as long as it maintains the satellite reception.  So how accurate is the 201, and GPS in general?  I know there are a lot of variables that affect accuracy, but 16% inaccuracy under ideal conditions sure seems like a lot.
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Question by:maharlika
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by:XY_GT
XY_GT earned 25 total points
ID: 17007771
The box that you got with it may have some of the accuracy specs on it.

Usually the GPS should only be out by around 5-15 meters. With different technologies such as DGPS the accuracy increases alot.

A good resource for more information about the GPS accuracy is http://en.wikipedia.org/wiki/GPS#Accuracy
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Thibault St john Cholmondeley-ffeatherstonehaugh the 2nd earned 50 total points
ID: 17007884
If you were running around a track rather than going in a straight line then the frequency of the units updating may affect the calculated distance. If it only recalculates its position once a second then it may assume a straight line between the two points, or the software may try to fit he points to a curve, but neither may be the actual journey taken.
Did you try the same track and distance walking as well as running?
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by:kbens0n
kbens0n earned 25 total points
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Also, make sure the firmware is up-to-date here:

http://www.garmin.com/support/download_details.jsp?id=437#Instruct
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by:Thibault St john Cholmondeley-ffeatherstonehaugh the 2nd
Thibault St john Cholmondeley-ffeatherstonehaugh the 2nd earned 50 total points
ID: 17008201
This quote from here doesn't look very hopeful:
http://home.socal.rr.com/diverhank/ForeRunner201.htm#_Toc98428435

"Based on reports from users, for a typical 5K run, expect distance error anywhere from 0.0 mile to 0.5 mile.   For longer runs, errors may be worse."

Not sure why the error is measured in miles and the distance in Kilos, but half a mile over 5 km is almost exactly 16%.





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