Theodolite is mainly used for plane positioning survey, and vertical angle is generally used to correct horizontal distance in topographic survey. The purpose of leveling is to make the vertical axis perpendicular to the ground plane, and the datum plane of the ground plane is the horizontal bubble. Only the horizontal bubble is centered in two vertical directions, which means that the vertical axis of theodolite is perpendicular to the earth's horizontal plane. In this way, no matter which angle it is rotated to, the position pointed by the vertical axis is at the predetermined centering point, so the centering error is small and does not affect the role of horizontal positioning.
If it is uneven, when the vertical shaft rotates on the horizontal dial, the ground position pointed by the vertical shaft is at different points, and the alignment error may exceed the allowable value of the specification, so it is impossible to locate the plane.