Sighting Mark

a mark (or an object in a locality) used as a check when measuring the azimuth of astronomical or geodetic instruments. The Pulkovo type of sighting mark is extensively used. Such marks are long-focus collimating lenses with a focal distance of 100–200 m, mounted near the instrument being checked (to the north and south of it). Small circular apertures illuminated from the rear are located in special boxes at the focal points of the lenses. The directions to the apertures are ascertained using a micrometer eyepiece in the instrument. With this type of sighting mark a variation in the azimuth can be checked to an accuracy of 0.05”. [16-917^]