Altitude is wrong

Not an iPhone issue, so moving thread to the fenix 5 Plus forum.

https://support.garmin.com/en-US/?faq=Vdz7RVePPV7wm1gO2d46TA

In my opinion the best option is to manually calibrate the altimeter directly before the activity and switch the automatic calibration during the activity off. For very long activity with weather changes the automatic calibration might be useful.

Something is clearly wrong. I've been using the watch for almost 2 years and for the last week or so it's showing some crazy numbers, spiking by thousands of meters during activity. I've always had auto calibration on and there were no problems. Hopefully it's a software issue. I can see in firmware log that in the newest beta there is something to be done. Fingers crossed it solves the issue...

• Improved the accuracy and reliability of altitude data.

I read in the forum that some people solved such issues by cleaning the watch in a surfactant solution (no Ultrasound!), but I never hat to try it. Otherwise it might be a defective barometer and I hope you still have warrenty.