A self-leveling, rotating laser is shown in figure 12-6. In this instrument, the laser unit is mounted vertically on a platform containing two orthogonally mounted sensors that act like spirit levels and deviate from center when the platform is not level. The amount of deviation is detected electronically, and the 

Figure 12-7.Laser-level rod equipped with a laser detector. 

consequent electrical impulses drive servomotors that automatically level the base and make the axis of the laser vertical. The laser beam is emitted at an angle 90 degrees to the axis of the laser by an optical train, and the optics rotate to form a horizontal reference plane. This device also can be side-mounted so the axis of the laser is in a horizontal position, and a vertical plane can be formed by the rotating beam. An electronic sensing device, parallel to the axis of the laser, allows self-plumbing of the rotating beam. The instrument is self-leveling and self-plumbing within a range of 8 degrees. Beyond 8 degrees, it will not operate. This is a safety feature. The tolerance specified for the position of the reference plane with respect to true level or true vertical is 20 seconds of arc. Thus, in a distance of 330 feet, a deviation of 0.03 feet is possible.