Wrong altitude in the morning

You get home from a hike, phone goes into flight mode overnight (or just loses connection), and by morning the altimeter has quietly recalibrated itself to the wrong elevation - even though it was reading correctly when you went to bed.

What's Happening

Garmin watches auto-calibrate the altimeter every night, correcting for barometric drift using GPS and phone-assisted elevation data. If the phone isn't reachable, the watch falls back on recent elevation history instead - including the point where you went to bed the day before. Result: it wakes up "calibrated" to your previous nights altitude instead of your actual home elevation.

How to Fix It

Calibrate manually before bed, using the Digital Elevation Model (DEM) (instructions here):

• Do it in the evening, after you're back and settled - not in the morning after the damage is done.
• Do it outside, so the watch gets a clean GPS fix.
• Do it at ground level, not upstairs - DEM data reflects terrain, not floor height.
• Make sure the watch is connected to your phone, with Garmin Connect open. The DEM lookup requires that connection to fetch elevation data.