Subtense bar

Subtense bar

Another device used in tacheometry to measure distance between the measuring station and a desired point is the subtense bar. This is a rigid rod, usually of a material insensitive to change in temperature such as invar, of fixed length (typically 2 metres (6.6 ft)). The subtense bar is mounted on a tripod over the station to which the distance is desired. It is brought to level, and a small telescope on the bar enables the bar to be oriented perpendicular to the line of sight to the angle measuring station. Since the subtense bar is always 2m. The formula for the subtense bar is: