The gyroscope rotating rapidly around its symmetry axis in the gyroscope has two important characteristics. First, the gyroscope rotating axis always points to the initial constant direction under the action of no external torque.The second is the precession, that is, under the action of external torque, the gyro axis precesses, and the rotation axis of the outward torque along the shortest distance is close to the vertical plane until the two axes are in the same vertical plane.
The true meridian is the intersection of the plane that passes the earth's axis of rotation (the meridian plane) and the earth's surface, so that the true meridian of the ground (true north direction) is in the same vertical plane as the earth's axis of rotation.When the gyroscope rotates at high speed and its axis is not in the vertical plane of the true meridian of the ground, the axis of gyroscope precession is generated under the action of the moment of the earth's rotation, and it is close to the vertical plane where the true meridian and the earth's axis of rotation are located. Thus, the axis of gyroscope can automatically indicate the true north direction.
Under the action of inertia, the axis of a high-speed spinning free gyro will not stand still in the true north direction, but swing from left to right in the true north direction.The maximum amplitude of the east-west swing of gyro axis is called the reversal point.Therefore, the gyroscope is combined with the theodolite to track the cursor's east-west reversal point, read the horizontal dial reading and take its average value, so as to obtain the true north direction.