3D Robotics Pixhawk Autopilot - Manual (Page 8)

SELECT FRAME TYPE (COPTERS ONLY)
CALIBRATE COMPASS
Select the options to enable the compass; to allow automatic declination
calculation; and to specify Pixhawk. Select
Live Calibration to launch the wizard,
and follow the prompts.
Show Me
videos demonstrating live calibration techniques at 3dr.com/learn .
For copter, select your frame orientation.