I mentioned above that I switched to Modular Ultra on my Ultra and was using the Altitude bezel. This morning it was still reading a more or less correct altitude, but after this morning’s ride (between 1100 and 1400 ft) it’s now reading about 1000 ft low. I’ve verified that the pressure sensor and the GPS are working by disconnecting my phone from my watch and using the MyAltitude app. Compass shows the wrong altitude. Apple really shouldn’t claim that there’s a barometric altimeter in the watch without providing some way to calibrate it.
Altimeters typically use air pressure to determine altitude. This means those types such as in basic aircraft are calibrated by entry of the local barometric pressure. Is there any indication that the Watch receives local ambient pressure from some nearby ground based source for calibration? Or are they based on some other technology, such as 3D GPS calculations (is that such a thing?)
Yes, it is. Garmin watches usually give you three typical ways to calibrate their barometric altimeters. You can manually enter an altitude. You can attach an altitude to a point of interest and it will automatically use that altitude if you’re near enough to that point. (That’s the way I usually calibrate. I have a point of interest for “Home”, with the altitude prescribed.) Or, if you have a GPS position, you can use a “digital elevation map”, basically a map that gives the altitude (more or less correctly) for every point on the planet.
You can also use “GPS altitude”, the position computed from your GPS position in 3 dimensions, but because your watch can’t see the satellites below your feet because the Earth is in the way, GPS altitude usually has pretty big error bars. You don’t usually want to use it unless you don’t have a barometric altimeter.
I found an article from MacRumors saying that there’s some problems with weather complications in WatchOS 10. That may be related to the problem I’m seeing.
That is supposedly solved with watchOS 10.0.2 - though it’s only available for the Series 9 and Ultra 2.
If you don’t have one of the new watches, there are other workarounds for the complication bug. See If the Apple Watch Weather complication isn't working - Apple Support
And weirdly enough… This morning I rode up to 2410 ft, one of the local high points. After the ride, the altimeter is working. It’s still off by about 40ft (showing 1261 as opposed to the correct 1218), but that’s acceptable and much better than 1000 ft off. I still wish I had control over the maximum shown on the scale, but today is much better than yesterday. I checked to see if there was a software update last night, but it’s still running 10.0.1, so that isn’t it.