A subtense bar is a bar of known length, with targets at either end. It is usually made of a stable material such as invar. When used with a tacheometer/theodolite, it serves as a quick and convenient method of measuring distances indirectly. ... The distance error is proportional to the square of the distance.

What does Subtense mean?

: of, relating to, or constituting an object (such as a pole or rod of known length) used to ascertain a distance without actual measurement by observing the subtended angle from a given point subtense method subtense transit.

How long is a Subtense bar?

For example, most subtense bars were 2 meters long and transits/theodolites typically had angle measurement resolutions between 6" to 20". Solving Equation D-1 backwards means in a distance of 100 meters, the angle at the instrument is 1°08'45".