Altimeter issues are wiiiiild.

Hey, fenix 6 series apologists will eat you pal. This series is a joke on many levels, not only altimeter but also gps accuracy, track, and the list goes on. Yesterday i had to wait 2 minutes for the GPS signal to turn green, i'm not even gonna mention distance accuracy and current pace accuracy, it's still bonkers since day one. All in all, there is no hope for your problem, all you can do is upgrade to fenix 7 or keep getting bashed by apologists here, be it regular users or Garmin employees.

These watches use a barometric altimeter. It cannot measure altitude directly. What it does is measuring the air pressure and estimating the altitude based in that. So you should see it more as an estimation than a measurement. Maybe with the rise of AI, this estimation will get better, but i doubt that a watch's altimeter will ever be super accurate. 

It's even started in Wikipedia:

"However, one must be aware that this type of altimeter relies on "density altitude" and its readings can vary by hundreds of feet owing to a sudden change in air pressure, such as from a cold front, without any actual change in altitude."

https://en.m.wikipedia.org/wiki/Pressure_altimeter

If you have a look the F7 owners also complaining for the same reason. Its a wack.

Mezey said:

If you have a look the F7 owners also complaining for the same reason

Hmm, I wonder why you are posting a screenshot of the fenix 6 forum then.

How much "user error" and what sort of "user error" could happen during sleep that causes an altitude inclanation of 10mtrs overnight, while i am sleeping m8. Dont think that i forgot to turn off the anti-grav engine in my bedframe. There is no user error that could cause things like this. I was thinking of getting an F7 pro, however as i am looking it now i will skip and check out other brands too.

Mezey said:

altitude inclanation of 10mtrs overnight, while i am sleeping

You know that the nightly calibration refers to the height of the ground ?

If you are higher than the ground level, you can simply deactivate the nightly calibration. I don't use it. As I said, I have no problems with the altimeter.

Yes I know. I am sleeping ground level, plus i have a three story building on the top of my place, so no gps signal. Before someone will mention bluetooth or wifi. I am running my watch dark, no wifi network set up on it, and no garmin app on my phone anymore, bt and wifi are disabled on the watch..

This is bonkers. I am in the same position as yesterday. Yesterday was 49mtr today only 42mtr something is fishy here.

Mezey said:

This is bonkers. I am in the same position as yesterday. Yesterday was 49mtr today only 42mtr something is fishy here.

This is called a weather change or pressure change.

EVERY barometric altimeter must be calibrated regularly. Pilots must do this before every start AND landing.
You should know and enter the barometric pressure at your location.

https://support.garmin.com/en-US/?faq=d7G0fSGGJ98l1ZH9dyglZ6 

https://youtu.be/gJuguwm7Eeo

https://youtu.be/Qc3tB98BYac

https://youtu.be/lRj06-MPQIY

https://youtu.be/gzM47PSaf08