Elevation inaccuracy & overall inaccuracy?

I've found that generally my 820 gps track is more accurate to the openstreetmap trails then my friends who use the Strava App so I always assume my recorded distance is more accurate. However, looking at your tracks though, they look very similar. I didn't see the tracks overlaying each other but I don't think the GPS accuracy is causing the difference in your case. (based on a quick look at the tracks)

I have found in my own riding that my Edge 820 rarely matches my Fenix 3 and rarely matches the strava app exactly. the tracks will vary under tree cover and the distances and elevation rarely match. I pair the Garmin Speed Controller with the Fenix and the Edge 820 which should provide the most accurate distance but they still rarely match. But the 820 always has the most accurate gps track as it lines up with the open street trails the closest. Because of this, I assume it's the most accurate for distance.

But my distances also don't match some friends using Edge 800's and Edge 520's. I thought the answer was with getting the speed sensor. But I installed this on my old 29er and a friend installed one on his 27.5+ (almost a 29er diameter) We were still not getting the same distance on every ride (although sometimes we would). I now have a 27.5+ and we both have the same Garmin speed sensor and still don't get the same distance on every ride. So the difference is not caused by the wheel size. It's possible that we run different pressures causing the wheel diameter to differ slightly??? or on some rides one of us might cut a corner or two??? But I think it just comes down to the GPS reception and device. I've just accepted now that they are not perfect.

I've found that generally my 820 gps track is more accurate to the openstreetmap trails then my friends who use the Strava App so I always assume my recorded distance is more accurate. However, looking at your tracks though, they look very similar. I didn't see the tracks overlaying each other but I don't think the GPS accuracy is causing the difference in your case. (based on a quick look at the tracks)

I have found in my own riding that my Edge 820 rarely matches my Fenix 3 and rarely matches the strava app exactly. the tracks will vary under tree cover and the distances and elevation rarely match. I pair the Garmin Speed Controller with the Fenix and the Edge 820 which should provide the most accurate distance but they still rarely match. But the 820 always has the most accurate gps track as it lines up with the open street trails the closest. Because of this, I assume it's the most accurate for distance.

But my distances also don't match some friends using Edge 800's and Edge 520's. I thought the answer was with getting the speed sensor. But I installed this on my old 29er and a friend installed one on his 27.5+ (almost a 29er diameter) We were still not getting the same distance on every ride (although sometimes we would). I now have a 27.5+ and we both have the same Garmin speed sensor and still don't get the same distance on every ride. So the difference is not caused by the wheel size. It's possible that we run different pressures causing the wheel diameter to differ slightly??? or on some rides one of us might cut a corner or two??? But I think it just comes down to the GPS reception and device. I've just accepted now that they are not perfect.

This is interesting.

I was thinking about turning my speed sensor off but i guess that won't work..

Sure no device is perfect but a difference of 2Km or 5% of the total distance is just not acceptable.
I think the problem with the distance lies with the iphone tho.

I've just calibrated my starting point from -30 to +18. Hope this helps.

Can you post the FIT file for the ride? Have you made a manual wheel size setting or just left it auto?

Firstly BramVC there's no need to quote the post directly above yours - clutters up the forum.

Secondly, there's more than enough been written and said about the accuracy and precision of consumer GPS to fill a library. It would pay to read up a bit as 5% is perfectly reasonable even under perfect conditions. Chuck in all the variables that mountain biking throws up and it becomes even more difficult to get identical readings between devices.

The 46.2 distance is from the sensor. The distance calculated from the GPS track recored in the FIT file is 43.9 which is reasonably close to the Iphone distance. I think the problem must be with the wheel size setting. I don't have a 820 but I expect the auto setting should be more accurate that it appears to be. All the Edge models I have had do better. It could have been a one time glitch or maybe there is a problem with the 820 automatic setting.

I would try using a manual setting. This is how it is done on the 520. The 820 should be the same of very similar. Your 820 needs to be connected to the sensor.

Go to Menu/Settings/Sensors, select the sensor and then select Sensor Details. Google wheel size chart to get the setting. The setting for a 700x25c is 2105

FWIW: A wheel sensor has the potential to be more accurate for total distance as the granularity of the recorded GPS track should always cause it to be less than actual depending on the tortuousness of the track.

Google wheel size chart to get the setting. The setting for a 700x25c is 2105

I find it more accurate to do a "roll out test", I put a dab of paint on the tyre, roll the bike forward until the paint leaves two marks on the ground (best not to do it on the living room carpet) and measure between the marks (sitting on the bike will compress the tyre and give an even more accurate reading).

I'll try that when I get home from work.
Thanks! ;)

Thanks guys for advice on how to configure speed sensor. I'm not convinced the auto setting has been accurate on my new 27.5+ bike. I'll try the manual set up to see if that helps.


Regarding Strava, I've also found the same where on some occasions I don't trigger all of the same segments that my friends will trigger. I've observed some of the following behaviors;

*On some occasions my friends have triggered a segment on a trail we didn't even ride.
*In once instance, we rode a trail not listed on Openstreet maps. My Garmin did not register any segments but several friends using the Strava app showed them riding a segment on a parallel trail.
*On other occasions, I have not triggered a segment that we did ride over.
*I've cut a corner on a trail causing a segment not to register on my feed.

Based on this, I think this is mostly caused by Strava using some intelligence to calculate (based on the GPS track) which segments you actually rode over. If you lose signal or if reduced GPS accuracy falsely reports your course off the segment, Strava might discard the segment to reduce mistakes or even cheating. I have found that my Edge normally shows the segments we ride on more accurately than the phone app. But it doesn't sound like this is what happened on your ride.