According to Professional Surveyor Magazine, automatic levels work by creating a level line of sight through a telescope that appears horizontal. Automatic levels replaced earlier equipment for this purpose in the middle of the 20th century.
Professional Surveyor Magazine writes that the automatic level's compensator suspends or balances some of the instrument's optics using gravity. This compensates for the vertical axis before light reaches a person's eye. Almost all automatic levels utilize three common aspects: the damping, optical and suspension systems. The optical system typically involves a mechanical compensator adjusting light for users' eyes and may also have mirrors or prisms. This helps to alter the light reading at the reticle level for users. The suspension system supports the movable optical systems in a manner that is free of friction. Binding in the suspension makes the system come to a rest before seeking the true horizontal line. The damping system keeps the suspended component from swinging indefinitely. This typically stops the crosshair from moving before the user looks through the scope; without it, the crosshair would continuously bounce up and down. The magazine notes that damping technology is primarily magnetic but may still be pneumatic or air in certain scopes.