Auto calibration of altimeter not during activity?

KrapfenKringel said:

That over night calibration doesn't make much sense, it would calibrate on ground level based on location but my flat is 40m higher, so that would mess things up if I get down to ground level.

Correct. That is why I always ask people if they sleep at terrain level when they have problems with this.

Anyway, in your case, the error from your sleeping place will still be much smaller than the error you will get from daily pressure variations if never calibrating. If you calibrate the altimeter on a day with normal air pressure, set it to altimeter mode and then never do any kind of manual calibration or auto calibration again, the variations in air pressure over time will cause altitude errors of up to +/- 400 meter.

So a 40 meter error is still an improvement, though it would be nice if you could somehow tell the watch that your sleeping location is x meter above terrain level.

On the Polar RS800 there was a setting to auto-calibrate to a pre-set value at the start of every activity. As most of my activities started from the same location, that was really easy. But I've grown used to manually calibrating before every activity now.

Your number is a bit exaggerated. Or at least it is not valid in each of the countries.

I just checked the year high and low pressure of 2020 for my country, and the difference was not more than 40mbar (or hPa). Which can be translated to +/-160 or +/-200 m.

Moreover I would say that the typical max change within a week like 20-25mbar. So the realistic range is around the half of it. 

Of course it can happen that you are just at the edge of this range at the time of last calibration and you face a doubled change. But not more than 200 meters.