Another Garmin Rant (Read 1057 times)

I have a Forerunner 205 with latest software download. Yesterday I ran 20 laps on a modern, 400-meter track. Garmin showed that I had run 5.3 miles, rather than the 4.9712 (8k) that I actually did. This a staggering 30 seconds per mile at 8-minute pace! This isn’t the first time I’ve done tests such as this on routes known to be accurate. You say it only does this on curves? Well, the next time you find a race that doesn’t have curves or turns, let me know. And I say this too--if you can't trust it to measure a curve accurately how in the world can you trust it to have pinpoint accuracy (as so many people seem to) on any kind of deviation from a perfectly straight line? Something to think about for those who say “my Garmin said the course was “xx..x” miles long and then question the course length. I’ll trust a certified course ANY day over a GPS device. This is one reason I don’t really feel comforatable about a PR unless it was run a certified course or track. I love my Garmin. It’s great for giving me a ballpark figure of how far I’ve run on a strange route and keeping track of a lot of data. However, it has limits. For races I leave at home, preferring to use my trusty stopwatch and mile markers to gauge my pace. The next time I hear another "but my Garmin said..." I will..............count to 10

Well, that's 6.6% high, which is well outside of what you should expect:

GPS Position Accuracy* <10 meters.="" 50%="" typical="" gps="" velocity="" accuracy*=""></10><0.05 m/s="" *accuracy="" depends="" on="" view="" of="" the="" sky.="" 99%="" clear="" view,="" 95%=""></0.05>

Normally, the GPS data sort of self-corrects as you go -- positional inaccuracy is not cumulative, and errors don't compound the way they might with a footpod-based device. However, since you're running the same "course" over and over again, I think the errors do add up because you're not adding any new data locations. I'll bet if you ran on a certified 8k course, you would probably have much more accurate distance. Think of it this way. Let's assume you ran a perfectly straight line course that was 8k long, and you had only two GPS points -- start and finish. If the first location was off by 10m, and the last was off by 10m the other way, that's 20m/8000m = 0.25% off. However, run one lap on a track. If the GPS start location was off by 10m, and the finish by 10 the other way, thats 20/400 = 5% right there. And you're running that loop over and over again. Bottom line -- I think running on a track is the worst case for a GPS device because positional inaccurately gets magnified by the repetition and there aren't enough unique GPS location points. As for race courses, assuming clear GPS the whole way, most Garmins will measure long because the runner will most likely not run the perfect tangent lines and tightest possible curves. If a course is certified, the runner should trust the course over the GPS unless there's a gross error (like a 4:40 10th mile after averaging 7:30 the rest of the way).

Well, that's 6.6% high, which is well outside of what you should expect: Normally, the GPS data sort of self-corrects as you go -- positional inaccuracy is not cumulative, and errors don't compound the way they might with a footpod-based device. However, since you're running the same "course" over and over again, I think the errors do add up because you're not adding any new data locations. I'll bet if you ran on a certified 8k course, you would probably have much more accurate distance. Think of it this way. Let's assume you ran a perfectly straight line course that was 8k long, and you had only two GPS points -- start and finish. If the first location was off by 10m, and the last was off by 10m the other way, that's 20m/8000m = 0.25% off. However, run one lap on a track. If the GPS start location was off by 10m, and the finish by 10 the other way, thats 20/400 = 5% right there. And you're running that loop over and over again. Bottom line -- I think running on a track is the worst case for a GPS device because positional inaccurately gets magnified by the repetition and there aren't enough unique GPS location points. As for race courses, assuming clear GPS the whole way, most Garmins will measure long because the runner will most likely not run the perfect tangent lines and tightest possible curves. If a course is certified, the runner should trust the course over the GPS unless there's a gross error (like a 4:40 10th mile after averaging 7:30 the rest of the way).

Interesting. It does seem to do a better job on road courses I've checked. When in doubt, however, I'll always trust a course measured with a Jones counter over one measured with a GPS.

Mine was pretty accurate last time I ran on a 400m track, although when I looked at it on a map it was all over the place. I ran in lane 3 or 4 and it was recording .26 miles for every lap. Over 20 laps the .26 reading would have been .2 miles over 8k, which is about right according to this site (http://www.philsport.com/narf/atrack.htm#B, over 20 laps lane 1 = 8K (4.971M), lane 4 = 8.402K (5.221M)). What lane were you running in? I was also running slower than you which could had made it a little more accurate I guess.

It also depends how good of signal/how many satellites it was picking up. My handheld navigational Garmin actually tells you on acquisition what ther +/- error is according to current signal quality; the running versions don't! For example, last time I booted it up I had clear sky and an error margin of 86 feet! The time before it was only 15. It really irritates me when people wear a GPS on a certified course and then try to complain that the course is off. If you take a GPS run and overlay it onto a map using the software, the results are sometimes quite interesting. You can see that it is clearly not recording your exact route. GPS has inherent error. Depending on signal, I'm told it averages around +/- 15 FEET. This is why at running speeds the instantaneous pace function is USELESS, the distance you cover between sampling points is too close to the inherent noise generated by the error. GPS is a good guideline. Take it as that, with it's limitations, and run happy!

GPS has inherent error. Depending on signal, I'm told it averages around +/- 15 FEET. This is why at running speeds the instantaneous pace function is USELESS, the distance you cover between sampling points is too close to the inherent noise generated by the error.

LOL yeah. I have had my garmin tell me I am "currently" running 4 minute miles. I always keep the mile lap setting on and look at average lap pace. Jeff - lol Why run farther than I have to! Actually I had one race where my garmin measured 6.0 for a 10K and I got suspicious, mostly because I live there and run the course often and because I lost the 0.2 all in mile 5. I went home and gmap'd it. 6.0, I got in my car and drove it. 6.0 It was off! And then I ran it again this year and it turns out there was a short out and back segment at one turn that I had missed last year...at least it wasnt a AG award I would feel bad. Although I blame the volunteers for not directing traffic well.

I've thus far been awfully impressed with my new 305's accuracy. I ran yesterday on a very hilly, very curvy course with (certified? Trent?) mileage markers ... and it nailed each marker to within a step or two. Nice to know that it's not completely accurate, though. Because its been telling me that some of my long-time regular runs aren't quiiiiiiite as far as I've been saying. I'm just chalking that up to Garmin error and sticking with my original distances. Stupid Garmin.

JK. Mile marker 7 is off.

I was in EW not PW. Are they certified over here? How far off is 7 in PW? That's not the one your kids painted, is it? I'm thinking that's 9. Or 8.