Altimeter way off

AllanOlesen67 said:

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• Do the activity for some hours and wait for an altitude error to build up (best to do this on a day where a large change in barometric pressure is expected). The barometer reading should be unchanged during this.

When I put my 935 or 945 in altimeter mode, my barometer will still show changes in pressure, it just doesn't use these changes in adjusting elevation. 

Okay. Then that method isn't usable.

Yesterday I did another alpine hike and altimeter (set on auto calibration) was again way off many times. I think auto calibration works fine in rather flat areas but in alpine terrains it doesn't because DEM is often wrong there. So in the mountains you can't rely on the altimeter set on auto calibration. 

I made the experience the altimeter set on auto calibration is generally OK (but not always) if there is a steady not too steep rise when you go on a mountain. But if it is getting very steep or especially if you climb walls it is way off. But I also experienced that DEM even on summits showed a wrong altitude. I had a suunto ambit 3 before and altitude was always incredible accurate without manually calibrating during an activity 

Same here with regard to the Sunnto's My spartan was leaps and bounds ahead of this Garmin Fenix 5x+  in the altitude area and cost me half the price.

Maybe a silly point, but I didn't see any mention of ambient pressure here?  If the weather conditions are changing, this would alter the barometer reading and cause the watch to read wrongly?

On my Fenix, I haven't paid too much attention to the altimeter but my impression is that it reads completely wrongly most of the time - if I start an activity that switches on the GPS, it is much closer to reality after that for a while, but then goes wrong again.

My working assumption was that when the GPS is running, the altimiter gets calibrated by the GPS, but at other times it's just barometric and therefore it just changes with the weather.

I just got this silly reading right after ending a GPS activity and the calibration set to auto. And the graph shows it has jumped between +42 km and -33 km. A bit funny that values like that are even shown.

So maybe baro+temperature combination since I went inside where temperature dropped about 10 degrees. Still I have never seen this kind of values before.

PatrickSWarner said:

Maybe a silly point, but I didn't see any mention of ambient pressure here?  If the weather conditions are changing, this would alter the barometer reading and cause the watch to read wrongly?

What? The affects of the barometer (and different settings one can use) were mentioned a few times.