In dams the displacement is measured in the inspection gallery. System requires a near vertical duct which is installed during construction of Dam. The plumb line will installed after full completion of construction or in original ground. It requires a vertical borehole of Ф300mm throughout the height. System basically comprises of a plumb hanged from the top of the construction with stainless steel wire. A support frame with a top plate and a hole at the Centre mounted with to travelling microscopes at right angle to each other for measurement of relative movement of plumb in X and Y direction are placed at the inspection gallery. The plumb is heavy weight of 10 to 20 kg, hanged through a wire and is submersed in a thick oil tank under the frame to dampen the vibration effect.

Two reference plumbs are hanged behind plumb wire in each direction. Initially the position of reference plumb and alignment is noted by aligning cross-wire of microscope and then the actual plumb is aligned through microscope. The difference of the two provides the zero position of plumb. Here after any change measured in X & Y direction gives the actual movement of dam or structure. An accurate measurement as close as 0.01mm is possible to measure with Vernier scale provided with travelling microscope.