10% Error w/ GPS (Read 189 times)

The GPS will always measure long, always.  They are not 100% accurate. But ten percent is ridiculous.  Spareribs

Well, not my experience.  the more turns in a course, the more the GPS will cut tangents that aren't there - giving you a shorter distance.  I have a regular run that I do many days that has about 40 turns  Using both Map my Run, and the mapping feature available on RA that course is measured at between 5.03 and 5.05 miles.  My GPS has never hit that mark, and usually comes in anywhere from 4.85 to 4.9.  When you look at the track on the map you can see how many of the turns are cut short.  Result is a short measure.

The GPS will always measure long, always.  They are not 100% accurate. But ten percent is ridiculous.  Spareribs

So off the top of my head, it seems that more frequent but less accurate readings will result in a long measurement; less frequent, more accurate readings on a curvy course could make your results short.

I just got a cheap Garmin Forerunner 10, my first use of GPS for running. I'm still in the playing-around-with-it mode, but contrary to what others claim, I've noticed that it doesn't always measure long, sometimes (probably 50%) it measures short. However, the amount measured long/short is generally between 0.05 - 0.1 mile. So all in all not too bad.

EDIT: BTW, I'll have to pay more attention to corners to see how that effects my reading, but another factor that I've been wondering about is when you change speed a lot during your run, how that lag time effects the reading, then you got the times you have to stop/start on  a course at intersections. I have my device set for automatic pause for when I stop, but there's always that lag time, but that probably effects time/pace more than distance...

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For something like five years I ran with only a chronometer. I live out in the country and start running from my front door, so I don't have too many different routes. I measured "mile markers" with the car odometer, which I figured was good enough, since it's the mileage on my legs that really matter.

I now have a Garmin, and it is mostly pretty consistent, sometimes a shade long, sometimes a shade short (and I am not sure how much moving laterally onto the shoulder over and over to get out of the way of traffic may affect it.) However, even when I wait until it is synched with the satellites, and start when I start running, it can show very large discrepancies for the first mile -- up to over 0.4 miles. It happens rarely, and it has happened both ways. Once I ran a slightly different course in a small parking area by a boat landing that I use as a turn-around, and it told me that I had run 0.3 extra miles, right in the middle of the run. Weird.

The worst I've ever had my Garmin 10 be off was about a mile on a half marathon course.  The signals bouncing off of tall buildings screwed with my signal.  However, I find most times mine measures short.  There's several bends I run on frequently where it loses the signal and cuts across the tangent, which shortens my distance.  If it's a city route, I'll figure it out on RA and record that distance.  If it's an off-the-trail route, I just deal with the discrepancy.