Figure 24.5 shows a schematic diagram of a subtense bar having centre at C fitted with targets at A and B. Let the separation between targets be s. A theodolite is set up at O. The bar is kept perpendicular to the line of sight OC by means of a sighting device at the centre of the bar. The horizontal angle between the two targets at the ends of the bar is measured, let it be q. The horizontal distance
----------------- Equation (24.10)
If the vertical angle of the centre of the subtence bar is b, then the difference in elevation (V) between the centre of the bar and the centre of the telescope is
V = D tan b ---------------- Equation (24.11)