Fenix 3 tracking distance getting worse

Could you post a link to the hike with the wrong distance? Open the activity in Garmin Connect Web, make it visible to Everyone (see the padlock icon in the top right corner of the page), and post here the URL from the address line of the browser. And if you can, please post a link to the same hike recorded by the friend.

Hi, trux, thanks for looking.  I am posting links to a different hike (my friend's yesterday hike was messed up).  The hike I'm posting should be about 8.8 miles if you add up the distances from a Ntl. Geo. map.  My friend's Garmin Fenix 6 tracked it as 8.9 miles (really close, and I usually have to remind him to stop his watch when we're back at the cars).  Mine tracked it as 9.5 miles, so 0.6 miles different.  Here's the link to my tracking:

https://connect.garmin.com/modern/activity/18075333057

I'm not sure if my friend's activity can be seen by all (he's at work; I've sent him a note to check and change if needed):

https://connect.garmin.com/modern/activity/18075189479

I usually pause my watch if we are going to be stopped for an extended time (for instance, at a trail intersection at 3.77 miles on my watch, 3.4 miles on my friend's); my friend usually doesn't except when we stop for lunch.

Hope you can see something going on!

Your friend's activity is not accessible publicly. He would have to open it to Everyone in order to allow us to compare it with yours.

However, when you check the track, zooming-in, it looks like the watch partially lost the GPS signal couple of times, under the dense vegetation, and due to decreased accuracy, some extra artefacts appear on it. See below 3 of them:

That alone can add some extra distance. Some more distance seems to be added by the corrective algorithm, trying to compensate for the GPS errors, using the data from the internal accelerometer. Usually it works fine, but it assumes, the watch is properly calibrated. And if you walk often in deep forests, or in narrow valleys, the lower quality of GPS signal can skew the calibration.

You can see the difference added by the corrective algorithm when you save the activity as Course, in Garmin Connect Web. It reveal the true distance of the GPS track (without the correction). In this case the GPS track is 9.18 miles long, instead of the 9.5 miles reported by the activity. It means the correction done by the watch adds an extra 0.32 mile to the GPS track distance. 

The calibration can improve after resetting the watch to factory defaults, and recording couple of hours of outdoor GPS activities, under good GPS conditions (open space with visible sky in all directions, no dense vegetation, no mountains, buildings, or wall in the direct proximity). Newer watches use more advanced GPS technology, allowing to cope with bad GPS conditions better than it was the case with Fenix 3.

Wow!  Thank you for all that.  Just for kicks, when I hear back from my friend that he has opened the viewing on his tracking, I will let you know. Definitely the Garmin 6 must have better GPS technology because his track does not have the "extra artifacts" that are visible on mine, and for the most part we were together the entire time, under the same vegetation. So, it seems I need to try 2 things - a factory reset, and then finding a neighborhood (probably) walking route that's more open so that I can record a couple of activities. Hopefully, those 2 things will improve the calibration.  Unfortunately (or really, fortunately) we mostly hike in the Smokies or other state parks where the vegetation is predominantly dense (great for the hot summer hikes, not so great when we're wishing for sun on cold winter hikes).

I don't expect that you will find anything we don't already know, but here's the link to my friends tracking.

https://connect.garmin.com/modern/activity/18075189479