Direction Projector

an optical instrument in the form of a vertical optic tube used in mine surveying to transfer a direction angle from the earth’s surface to the horizon being established in an underground excavation. The design is based on the principle of the double image used in optical range finders; the double image is achieved by means of an optical wedge or biprism secured in an attachment mounted on the optic tube. Optical orientation using a direction projector involves errors from refraction of the air in the mine shaft; therefore, existing instruments guarantee the necessary precision of orientation to depths of 300 m. Optical orientation using direction projectors is being supplanted by the use of gyroscopes.