Wrong Elevation Gain

This is a known problem (see various posts here and in the beta forums) and has been going on since firmware version 19.xx or 20.xx. Prior to that, the altimeter of my watch was working o.k. Garmin is obviously unable or unwilling to produce a fix for this bug they introduced in one of the versions mentioned above.

I have a nearly flat running course, for example, where a cheapo sport watch from another manufacturer (that I borrowed for a cross check) and my trusty old Oregon 700 tell me that I have 10 to 12 meters gained during the run, which is plausible. The fenix 6 regularly shows me something between 25 and 50 meters, which is clearly way off. Same effect on a larger scale when I go mountain or road biking in the nearby low mountain range. I'm always the "meters gained" king at the end of the day when I compare my logs with those of my buddies, regardless of whether they use other Garmin devices (two older fenixes and several Edge bike computers) or watches from other manufacturers.

I'm quite sure that you'll also notice several other consequences of the buggy altimeter software:

• The height of the location where you are drifts throughout the day, even if you don't travel.
• If you go somewhere without starting an activity on the watch and come back later (e.g. going to work in the morning and coming home in the evening), the altimeter is often way off (I live at 152 m above sea level, for example, and when I go to work (105 m above sea level) and come home in the evening, the watch tells me that I'm located somewhere between 200 and 250 m above sea level.
• During activities, I often see "jumgs" in elevation that are clearly artifacts.

Please have a look at past logs of yours for such artifacts and have a look at the altimeter often throughout the day. I'm quite sure you'll see the same behaviour of the watch.

Thank you so much for your answer on this. So no rollback to a certain software would help this?

There was a bug reported in the latest firmware and a user found a workaround. Try this: https://forums.garmin.com/beta-program/fenix-6-series/f/community-discussion/333160/here-s-a-workaround-for-the-elevation-altitude-bug-in-fenix6-25-85-gps-6-60

Downgrading your Firmware to 25.10 might be a solution. 

Hey - I think I figured out what's happening. The temperature sensor is messing up the elevation data for some reason. I've seen this happening entering very cold water or in the heat (my wrist during an activity is enough). There doesn't seem to be a problem when I just mount the watch on my bike.

Can someone with that problem reproduce this?

Anybody connected this to the temperature sensor?

Yes.

Last Sunday I did a MTB cycle ride 80 Km. After the the first half noticed that the elevation was almost doubled. It was raining and hot so I thought maybe sweat, cleaned the watch and started the second half.

During the second half samething happened. Took the watch from the wrist and put it on the backpack. It was fine. Can't tell nothing about temperature because I use TEMP sensor.   

Never happened to me before.

Fabian said:

Hey - I think I figured out what's happening. The temperature sensor is messing up the elevation data for some reason. I've seen this happening entering very cold water or in the heat (my wrist during an activity is enough). There doesn't seem to be a problem when I just mount the watch on my bike.

It is not the temperature sensor to be blamed, it is the temperature change itself that results in different pressure (and hence elevation) reading. That's why Garmin recommends warming up before starting an activity, in order to have the body & the watch at the working temperature However at longer trips, wich changing altitude, wind, and weather conditions, it is difficult to keep the temperature stable, hence there will be often deviations anyway

Read also for example here: Elevation Accuracy of Outdoor and Fitness Devices | Garmin Customer Support 

Changes in Ambient Air Temperature

Since barometric altimeters are not temperature-compensated, changes in ambient air temperature can affect altimeter accuracy. If drastic changes in ambient temperature are experienced, calibrating the altimeter will restore accuracy.

This is interesting and probably your are right. However, this then clearly is a software bug as this never happened before 25.XX and the hardware seems perfectly fine when not on the wrist. I tried this on a 5,5 h bike ride. Accurate down to the meter. I refuse to recalibrate every time (also - this didn't produce much better results) and probably will roll back to an older firmware.

Honestly - it almost feels intentional to leave these kind of bugs without fix for such long time spans. "Buy new stuff always and we will treat you well :D"

Fabian said:

However, this then clearly is a software bug as this never happened before 25.XX and the hardware seems perfectly fine when not on the wrist.

Frankly told, I do not think it is a firmware problem. This behavior was always present, and you can find many complaints about it here on the forum across the subforums for practically all watch models. Perhaps you were luckier in the past, and had a more stable temperature, or the watch was not so much exposed to the wind.

I don't know if this is relevant, but we share some hardware and software parts with FRx45 Series.

https://forums.garmin.com/sports-fitness/running-multisport/f/forerunner-945/339139/forerunner-945-software-update-13-00---20

Improvements to altitude calibration error handling (Addresses reports of sudden elevation changes during an activity).