The measurement of distance by measuring the horizontal angle subtended by the subtense bar targets is called:

Substance bar: A horizontal bar used in the subtense system of surveying by tacheometry. It is held at a distant point and its distance is calculated from its known length and the angle that it subtends at the observer's eye.

Subtense bar can measure horizontal and vertical distance in places where chaining is impossible because of undulations and rough topography. But it can measure short distance up to 200m.