New Fenix 7x Sapphire Solar Shows ALT of -192 After Being Accurate at 4k

Generally, the barometric pressure is a much more precise indicator of elevation than a satellite fix, so it can even detect small changes like having climbed a flight of stairs. But it is best at detecting this subtle shift over a short period of time. It cannot tell the difference between weather changing the air pressure or you slowly changing your elevation during the day. A GPS fix has the opposite problem of knowing your absolute position but having low precision, so if you look at readings in a short period of time it will look like you are bouncing around, but if you sit still it should be able to average readings and figure your elevation regardless of the weather.

Automatic calibration happens at night (I believe) when it is assumed that your location is constant and the watch can average some satellite fixes. You can also trigger altimeter calibration manually via satellite or even input a current elevation if you are at a position with a known elevation (like a trailhead, pass, or summit).

Driving long distance means you can move between different weather systems and air masses, so the apparent elevation from the barometric sensor could change a lot during your trip. You can have a similar effect from weather changes even when on foot for multiple hours.

There are options to prioritize one interpretation of pressure change versus the other, but you probably want to leave it in automatic mode if you don't know what to do. For example, if you are in a very flat area you might select "barometer only" mode so it assumes any pressure change is due to weather. Or when doing a steep mountain hike, you might select "altimeter only" mode so it assumes changes are from your climbs and descents. Of course, weather also changes rapidly in mountains, so this might not be an accurate assumption even for a long day hike. Automatic mode lets it try to guess at what is going on. I don't think Garmin publishes how their algorithm works, but we can imagine it using GPS fixes and accelerometer data to help decide when you are changing elevation.

6125703 said:

Automatic calibration happens at night (I believe) when it is assumed that your location is constant and the watch can average some satellite fixes.

No, no GPS fix is required for night calibration. Bluetooth connection between watch and phone is necessary. The Connect app running in the background determines the location of the phone, and calibrates the watch based on the DEM data.

If your watch is higher than the ground level at night (e.g. you are lying in bed on the 10th floor of a skyscraper), the nightly calibration is not useful.

Thank you for the information! That makes sense to me. I reviewed the current ALT readings from lunch back to early this morning (3 AM). It looks like around the 3am-4am time frame it calibrated back to the correct ALT for our home location. It jumped from the -190 or so to the correct ALT, so very glad of that. Again, appreciate the info =)

Hah, that's too bad. If so, the overnight calibration is totally useless to me...

I never leave my bluetooth enabled overnight and also normally have location disabled on my phone. And, I sleep upstairs in a hilly area where I doubt the DEM would precisely know the terrain either.

Will the automatic mode at least use GPS when I am getting a fix to start an outdoor activity?  My altimeter seems too consistent if it is really drifting for weeks since I don't often do a manual calibration.