First return to zero, then turn the instrument horizontally and do 90. Note that it is impossible to hit 90 accurately at this time, but how many degrees will appear on the instrument display screen. Then turn the spiral, don't turn it too hard, just fine-tune it. Then lock the transverse screw. The large screw is used for locking, but the instrument can't move. The small screw can be fine-tuned.

Extended data:

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.

References:

Baidu encyclopedia-theodolite