Your Amazfit Verge will ask you to calibrate the built-in compass before using it for the first time. Tap the Compass interface to enter the information interface for altitude, latitude and longitude. 

The altitude and latitude/longitude information must be obtained in the outdoor open space via GPS signals. To avoid interference, stay away from magnetic fields during use.  

When you start an activity, the watch displays altitude values based on air pressure readings taken by a built-in barometric pressure sensor. It comes with auto-calibration, which means that the watch calibrates itself with reference to a built-in GPS. This will save you from calibrating it manually.