Strange Altimeter Number

It means you're more than 27 million feet below sea level.  Try restarting your watch and see if it corrects it.  If not, call Garmin as something is definitely wrong. 

Thankyou for the helpful reply Restarting the watch put it back to normal.

I think the problem was that I calibrated doing the "Use DEM" option. Apparently the Digital Elevation Map for my location is a bit off. I did a calibration using GPS and it came up with 1097. Much closer.

Try the DEM calibration again as it should not be off by that much unless the DEM map is broken.

Dr Phil said:

Try the DEM calibration again

Ok, did that. Went back to crazy numbers

Went back to calibrate by GPS

That suggests there is an issue with the DEM map. You have little to lose by deleting the DEM maps from your watch then re-installing them. Connect your watch to a computer and remove any .img files with dem in the title eg gmapdem.img. If you feel more comfortable save the files to your computer before deleting them from your watch.Once they're off your watch connect to GE and update/reinstall the maps. if that has not sorted the problem, contact Garmin Support.

Thanks, but I'm not seeing any .img files with DEM in the title.

Also, not sure if it matters, my watch is a 945 LTE. Just noticed I posted this in the wrong forum.

Meanwhile, I've submitted a support case to Garmin.

Dr Phil said:

it might be worth connecting to Garmin Express and reinstalling the maps to see if that fixes it.

I connected the watch to Garmin Express and reinstalled "TopoActive Americas North 2021.20." The 3 other options were "coverage" maps. After that completed, I checked the watch for changes to the .img files and noted one updated img file and four new img files. including the previously non-existent gmapdem.img file.

Then I did a calibration using DEM and success! Thanks for your help.