A gyroscope that rotates at a high speed around its axis of symmetry within the gyroscope has two important characteristics: one is fixed-axis, that is, the direction of the gyroscope's rotation axis always points to the initial constant direction without external torque; the other is precession That is, under the action of external torque, the gyro rotation axis generates precession and moves along the shortest path to the vertical plane where the rotating axis is located until the two axes are on the same vertical plane.
The true meridian is the intersection of the plane (meridian plane) passing through the earth's rotation axis and the earth's surface, so the ground true meridian (true north direction) and the earth's rotation axis are in the same vertical plane. When the gyroscope's gyro rotates at a high speed and its axis is not in the vertical plane of the ground true meridian, the gyro rotation axis precesses under the action of the earth's rotation moment, and approaches the vertical plane where the true meridian and the Earth's rotation axis are located, so the gyro The axis of rotation can automatically indicate the true north direction.
The rotating shaft of the high-speed rotating free gyro does not stand still in the true north direction under the action of inertia, but swings left and right in the true north direction. The maximum amplitude of the east-west swing of the gyro shaft is called the reversal point. Therefore, the gyroscope and theodolite are combined, the theodolite is used to track the east-west reversal point of the cursor, and the horizontal dial reading is taken and the average value is obtained to obtain the true north direction.
If you want to get more details about gyro theodolite,pls visit https://www.ericcointernational.com/gyro-theodolite/