This calibration does not work at all. The altimeter (at least on the Vivoactive 3) is complete garbage and does not work, and Garmin just won't admit it. In my case, it is always, always, always, several…
This doesn't seem to work for a number of people for the VA3 as shown by this thread. There seems to be an issue with either the device hardware, software or the design of the ports on the underside…
I've noticed the first km or so of a run the elevation is hugely inflated, and through the remainder the elevation basically drops to zero.
I run point to point and turn around along the same route, and have yet to see my starting elevation match the elevation at the end of my run despite being in the same location.
Overall I think the final values are more accurate than before, and match a little closer to Strava corrected values. Both of which are inferior to trainingpeaks for elevation correction, imo.
My new Vivoactive 3 (6.20 from the start) was very inaccurate in floor climbed since first GPS calibration.
I notice that performance degrade in few days till new calibration; floor descend count was always accurate.
During an activity with the altitude field works fine (3mt every floor, both climb and descend).
I suppose is a software issue.
This has been an ongoing problem with Garmin. I'm not sure they want to fix it. Biking, my VA3 seems to do fine, but running it is terrible. And I'd bet dollars to donuts it is a software issue they could address if they really wanted to. Just my two cents.
This calibration does not work at all. The altimeter (at least on the Vivoactive 3) is complete garbage and does not work, and Garmin just won't admit it. In my case, it is always, always, always, several hundred metres off (lower) in total elevation gain than the true value. Yes several hundred metres!. You call that a training watch?? I have contacted Garmin and they say the issue 'has been escalated', but cannot give me a timeline on resolution... that was 2 weeks ago... again complete garbage.
I think you should look on Wikipedia how barometric altimeters work, and what kind of precision you consequently can expect from them, then look up the altitude precision you can expect from GPS (Galileo should be more precise, but the satellites are not all operational now). I think this way you will be able to understand that your watch is working as it can be expected.
I take it in a different way:
If Garmin aim to supply what is written in the watch description, then a SW change that will constantly calibrate the barometer via GPS should be in place, like the 945 and Fenix 5 plus has.
There are other very good options suggested here in this and other threads.