Instrument
Subtense bar is a graduated bar of fixed length mounted horizontally on a tripod stand (Figure 24.4). The bar is centrally supported on a leveling head and is fitted with a sighting device at the centre. At its ends, there are targets and these are at fixed distance apart. The bar can rotate about the vertical axis in a horizontal plane. It can be fixed at any position using a clamping and its tangent screw. It is used to measure horizontal distance and difference in elevation indirectly where the terrain is rough and requirement of accuracy is not high.
Figure 24.4 Subtense bar