Apple Watch Ultra altimetre error (watchOS 10.1)

For a true altimeter/elevation reading the barometric pressure needs to be set properly. I the bar pressure is 29.30 but the watch is showing the bar setting at 30.30 that's a 1000' error. If the bar is 30.20 and the watch says 30.10 that's a 100' error. That's why pilots are given the "altimeter setting from ATC when flying below 17999'. There are other things that affect the altitude reading like temperature but for using the watch that really won't make that much difference.