altimeter auto calibration strange values

I do not know how exactly the algorithm works, and in fact nobody else here on the forum will be able to tell it, but I do know that the algorithm is very far from being optimal. Unfortunately, at the altitude measurement, there are several major problems:

1. The pressure is influenced equally and simultaneously by the elevation and by the barometric atmospheric changes and unless the watch gets and uses precise and up-to-date barometric pressure from a local weather station, it cannot differentiate whether the change is caused by the weather, or by the altitude change (and I am sure it does not use the meteo data for this purpose, as it could or should)
   
   
2. The GPS altitude triangulation is very inaccurate (an order more than the horizontal data). While the GPS location accuracy is within +/-15m, at the elevation it is rather in tens of meters. So using exclusively GPS for the elevation profile would lead to even bigger problems, especially knowing that the watches often drop the GPS signal, or get a wrong position easily.
   
   
3. A bit better accuracy can be achieved with the DEM (Digital Elevation Model) method, but it assumes a connected phone, available mobile data, good density of DEM data for given region, and again pretty accurate GPS position. In praxis, it is unfortunatelly often not much better than GPS. Especially if you are moving in an accidental terrain, or in urban environment, the differences may be drammatic.
   
   
4. Another problem, especially at Instinct, is the sensitivity to sweat, water, dirt, wind gusts, air crculation during fast descents, wrist flexing, and other similar problems causing huge fluctuation of the pressure measured by the sensor. There are ways how to limit the problems - there are many posts on the (old, standard) Instinct forum discussing it, and offering tips.

From what I observed, I believe that even if you activate the option "Automatic Calibration during Activity", the calibration is rather sporadic. Unfortunately, I have no idea how often it happens, but from the data (of myself and of other users) that I have seen over many months, I suspect it may either not happen at all, or only at intervals longer than tens of minutes. The reason why I think it is so, is that if you start and end the Activity on the same spot, you will practically never get the same elevation from the watch, despite practically identical GPS coordinates 

Thanks thats very much in detail!

Thank you very much for your kind and very helpful answer!

This German review tested the altimeter: https://www.pocketnavigation.de/2018/10/garmin-instinct-test/

In summary best way is to manually calibrate at start if you know exact elevation and enter this manually. From there it seems pretty reliable provided weather pressure doesn't change much.

I wonder though when navigating if Garmin leverages the provided elevation in the course points. Certainly if you use the elevation profile when navigating.

I noticed that when I manually ran calibration using GPS it changed and improved accuracy. Even though I have also turned on automatic GPS calibration. So not sure when that is really triggered despite saying automatic.

A quick update after a few days of use:

these are my settings:

auto calibration during activity OFF

auto calibration not during activity OFF

barometer watch mode: altimeter

This is what my instinct solar recorded during my yesterday hike: (manually set altitude before starting) 

Instinct: total distance: 16,58 km total elevation : 1505 meters

strava (with correction activated)    total distance 16,87 km total elevation : 1480 meters

Pretty ok I'd say....