Inaccurate altimeter, should I factory reset?

First thing I'd do is put the watch in some luke warm water with a drop of dish soap and slosh it around a bit. You may have a partially obstructed barometer port.  Also, make sure your barometer is calibrated to the correct pressure when you calibrate the altimeter. If the altitude is calibrated and the pressure reading is off a bit the watch may interpret the pressure change as a change in elevation when it corrects itself. I hope this may help. 

Thank you very much, some good advice there which i will definitely be trying. 

Hi,

After doing as you suggested - soaking my watch for 20mins then calibrating the barometer and altimeter on both mine and my wife’s watches yet again I saw a crazy increase in elevation at the start of tonight’s run.

Running along a flat path for the first half a mile saw my wife’s watch record 4ft and mine record just over 98ft. After that initial rise they both tracked on a similar level. It’s almost like there’s something wrong with my auto calibration during activity setting or something along those lines. 

In my experience, auto calibration during activity had some problems. I now do calibration at the start and that seems better. Do you and your wife have the same calibration setting? My previous 5 Plus had a faulty barometer, it was all over the place, even when sitting down. Had to return that one.

Tonight I tried calibration at the start and turned during off. Did that also on my wife’s. within the first half mile which is flat my wife recorded zero and I recorded 130ft!

Ive just completed a full reset but am not hopeful as sitting looking at my altimeter the figure is dropping from the calibrated figure of 172ft which is the same as my wife’s to less than 100ft then back up again. My wife’s however is rock solid and doesn’t change. 

The same problem has been with me for about three weeks - and it is still massive: on today's bike ride, my Fenix 6 sapphire shows a positive difference of 394 m, although in reality it should be around 250 m. That means a deviation of about 60 %. This is beyond good and bad - and meanwhile it pi...s me off!

The vertical meters in the graph are "unsteady", i.e. they go up and down nervously. The absolute values (at the start, in breaks and at the finish) are correct. But the real profile should be smooth. See screenshot (latest firmware version 8.10; no beta)

I also noticed that my Fenix 6 is showing more elevation gain compared to Edge 820.
After spending some time looking at graphs I think that watch picks excessive elevation mostly on flats and that is Garmin's problem to fix because no settings can change that behaviour IMO.