Why do you use the left and right disks to observe when measuring the horizontal angle with theodolite?

The observation of the left and right disks is to adjust the angle error.

Theodolite is the main angle measuring instrument in measurement. It consists of a telescope, a horizontal dial, a vertical dial, a level and a base. When measuring, put the theodolite on a tripod, aim the center of the instrument at the ground station with a vertical ball or optical collimator, level the instrument with a level instrument, aim the measuring target with a telescope, and measure the horizontal angle and vertical angle with a level dial and a vertical dial. According to the accuracy, it is divided into precision theodolite and ordinary theodolite; According to the reading equipment can be divided into optical theodolite and vernier theodolite; According to the shafting structure, it can be divided into retest theodolite and direction theodolite. In addition, there is a coded dial theodolite, which can automatically record dial readings according to coded perforations; An automatic tracking theodolite that can continuously and automatically aim at air targets; Gyro theodolite and laser theodolite that can determine the orientation of ground points quickly and independently by using gyro orientation principle; Universal theodolite for astronomical observation with three functions: theodolite, meridian instrument and zenith instrument; Phototheodolite, who combined the camera and theodolite for ground photogrammetry.