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GPS Accuracy - total distance

I got a Vivoactive 3 in October and I noticed that the pace of my runs appeared to be lower than what I was used to.

I tried running with the VA3 on one wrist and a Forerunner 210 on another, and an iPhone running Endomondo or Sports-Tracker, and the VA3's total distance was always lower by more then 7%. Using GPS+GLONASS and every minute recording did not seem to improve things.

Here is an example:

Vivoactive 3 5.71km https://connect.garmin.com/modern/activity/2344389458
Forerunner 210 6.25km https://connect.garmin.com/modern/activity/2344398396
iPhone 6.34km https://www.endomondo.com/users/24403395/workouts/1035487642

Checking minutes 5:24, 10:48, 20:38, 23:02, 32:40 on the map, it is clear that the other devices are more accurate.

Garmin were kind enough to swap my device, and in the meantime the firmware version was updated. The new results were better, but still off by about 4%-6%.

Assuming that I wouldn't get two problematic devices in a row, and since I haven't read of any widespread issues with the VA3's GPS, the only reasonable explanation was user error.

I tried several things and the one change that seemed to work was to change the orientation of the VA3, having the button on the left (while wearing it on the left hand). I've done two tests so far and the total distance discrepancy between devices is 2.6% - 3.1%. Moreover, looking at the map, it is not clear which device is more accurate, since the VA3 is now visibly more accurate in some segments (and less accurate in some other segments).

This is from today:

Vivoactive 3 7.09km https://connect.garmin.com/modern/activity/2410234674
Forerunner 210 7.31km https://connect.garmin.com/modern/activity/2410261099
iPhone 7.27km http://www.sports-tracker.com/workout/thanosmarkousis/5a4bc46dc3d639076578254c

I think that, at this point, performance is reasonable for a consumer device. I'm curious though if others have tested the VA3 together with another Garmin device and found any differences in the total distance and the accuracy of the trail on the map.