Control of an unmanned aerial vehicle (UAV) requires knowledge of information on its attitude, which is estimated with the help of satellite receivers, gyros, accelerometers, and magnetometers. To ensure correct measurements, all sensors are to be calibrated before the flight. However, the ground-based calibration of the magnetometer cannot ensure correct measurements of Earth's magnetic field because of the additional magnetic fields created by strong currents arising in the electric motor circuits in the course of flight. The paper addresses the problem of fine compass calibration to determine in-flight calibration parameters. Two calibration techniques are proposed and discussed. The discussion is illustrated by results of field experiments with an octocopter Triumph-F1.