This is a good question. Today my watch decided that a run that's always been around 91m of elevation had 361m. Everything else very similar to last few runs. It means that a Strava climbing challenge I wasn't going to make is now possible; so I'm going to have to leave it, so as not to cheat. I thought Garmin got the elevation from map data not the watch, so I can't explain this at all.
I thought Garmin got the elevation from map data not the watch, so I can't explain this at all.
You can have both. By default the watch (better told most of Garmin models) uses a barometric altimeter. On the other hand the barometric altimeter does not always work perfectly - it needs calibration, and it can be influenced by atmospheric changes, the pressure port can be clogged by dirt or by sweat, and the wind or the wrist pressing against the port can influence the reading too. In such case, you can enable the Elevation Correction on the concerned activity page in Garmin Connect Web (not in the phone app), and the elevation profile is then recalculated using the cartographic data. On Strava you can do it too.
