Height acclimatisation without being in height

BTX said:

I only had two car trips

Wasn't a car window open (even just slightly), during the trip? Opening a window while driving would cause the air pressure inside the vehicle to decrease (hence detected as if you were at higher altitude). It is called Bernoulli's principle, in physics.

That could be an explanation, but I would like to rule it out.
1. because of the autumn temperatures, the windows were closed.
2. I have driven more often and much further distances under all kinds of conditions without this effect.
3. altitude acclimatisation should be accompanied by a stay at a corresponding altitude, which has also been registered by the watch. But in the graph you can see that I wasn't even close to being above 1000 metres. I should have been much, much higher, because acclimatisation takes several days at a certain altitude. 

Yes, you are completely right. Not being familiar with Epix, I did not realize the bottom graph is the registered altitude. Could be perhaps a glitch after a firmware update, if the update happened recently on your watch. It occasionally happens that when the data structure in the firmware changes, some parameters, variables, and calibrations get filled with wrong values, unless you soft-reset the watch after the update. That is generally recommended after each firmware update (I am not advocating it, I am just telling how it is, unfortunately).

So, I would recommend soft-resetting the watch, although I am unsure whether it really resets the wrong acclimation data now, when the watch was already using and processing it since several days. And I would also recommend reporting it directly to the Support - they have tools to make a more in-depth diagnostics, or may be aware of the issue from other reports.

I've witnessed strange altitude recordings (-400meters for example) when I had for many hours closed the connection between iPhone and Garmin Epix. Once bluetooth connectivity got enabled back, everything got fixed.

Did you have phone connectivity for all the time?

https://www.garmin.com/en-US/blog/outdoor/understanding-altitude-sickness-and-tracking-your-altitude-acclimation/

Altitude Acclimation Actually Work?

When the appropriate location accesses have been granted within the Garmin Connect app, your compatible Garmin smartwatch or cycling computer will report your current altitude acclimation by displaying the elevation that your body is acclimated to, based on current training.

Furthermore, the device will provide acclimation notifications and corrections to your VO2 max estimate and training status when the altitude is above 800 meters or 2,625’. The altitude acclimation feature will also check in overnight — if in sleep mode and connected to Wi-Fi® — to ensure accuracy and that it’s up to date.

It’s worth noting that those with higher activity levels may have lower resting heart rates. And, as the linked article notes, that’s OK.

So assuming your graph is in meters based on your post you reach the minimum requirement i.e. 800m

If you look at your graph it shows that your altitude since Sunday has been over 1000m. 

If that is incorrect and you are at only 300m or so, then would suggest calibrating your altimeter (can go out sometimes with weather and other pressure related events i.e. going down water slides etc, cycling and wind driving into port etc etc.

The small white graph at the bottom is the measured height, the green above is the current state off acclimation. 

As you can see, the clock was – also with her own measurement – far, far away from heights above 800m. 

I have actually already been able to see the function working correctly during my summer holiday. There I was at ~850 - 950 metres for several days and the green graph, my acclimatisation status, slowly approached this altitude. 

This display here makes no sense. None.

In order to be acclimatised to the 1,200 m indicated there, I would have had to be above this altitude (or my watch should think and display that I am) for a period of at least one week.