Elevation/Altitude Issues Fenix 6s

This is probably just a normal atmospheric pressure drift due to changes in weather. The absolute differences are fairly small - within 10-15 feet, which is almost nothing. Removing auto-calibration won't help. The role of auto-calibration is actually to prevent this exact issue by continuously calibrating the barometer via GPS altitude. But for small changes like this it probably won't kick in.

I didn't have this problem with my vivoactive 3, it all startes with this fenix 6s. I know it may not be much in actual altitude but it's annoying af to see the graph all wrong thanks for the input!

Have you tried changing the barometer watch mode? It can be set to three different settings:

Altimeter: All changes in pressure is considered a change in altitude.
Barometer: All changes in pressure is considered a weather change.
Auto: Uses both the altimeter and barometer according to your movement.

https://www8.garmin.com/manuals/webhelp/fenix6-6ssport/EN-US/GUID-CDC9DDEE-C576-4E1D-B007-BC737686793A.html

https://support.garmin.com/en-GB/?faq=J2WVA0dss82BNpphPYWq56

Your second image looks ok in my oppinion. The first a bit worse but still acceptable for a 2h activity and without knowing the circumstances it is difficult to say why.

Did you pause on the top? Where there any weather changes *later* that day that may have caused change in pressure during your activity?

silentvoyager said:

This is probably just a normal atmospheric pressure drift due to changes in weather

No, it's not. As the opener said, it is something that happens all the time. I've got the same problems (but usually the other way around, i.e. I end up several meters higher), so it cannot be pressure drift. It's just a poorly working barometer.

I know I've said this many times before, but I've been using high quality barometric altimeters for at least 35 years and the Fenix 6 is as accurate as anything I've ever used before.  I use the barometer or altimeter in pure manual modes, and its as good as anything else out there.  But, I think sometimes you guys are expecting way too much accuracy out of a watch, its not a lab instrument after all.

The alti/baro is temperature compensated to a point, but it will drift a little over temperature.  I put mine in the fridge once to test this, and if I recall correctly the altitude reading changed by about 10 ft or so when the temperature changed from about mid-70s to mid-40s.  I had one of my 12 other altimeters at room temp the whole time as a control check, and after testing the Fenix I swapped them around and the other altimeter also changed by about 10 ft or so.  Perhaps temperature change could be the cause of the variation you're seeing.

I've compared the Fenix 6 to a few other baro/alt devices over thousands of feet of elevation change, and it always reads within a few feet of the others.  In baro mode, its within a few tenths of a millibar (0.1-0.2 usually) at any pressure range, sea level to mountains.  Based on that, I have no reason to believe the Fenix is inferior at measuring air pressure compared to any other device of its kind.

The one anomaly that I did notice though, was while river rafting if water splashed on it the altitude reading would bounce around a bit (10-20 ft) for a minute or so, which I think is similar to people theorizing that sweat getting into the air port holes is what explains the noisy altitude graphs they see in their .fit files after an activity.

So accurate! Yesterday I climbed a mountain with my old eTrex and the fenix. Set at the same altitude at the beginning, in the end the eTrex recorded the correct elevation of the mountain (3260 m) within 3-4 meters, while the Fenix set at 3199. Fenix was updated 2 days ago at the latest FW (19.20)

I seriously don't know what to do anymore with this watch. 

You can't say "the watch is accurate as it can be", given that much older devices by the same brand are much more accurate.