The borehole extensometer measures extension (displacement) that takes place with time in a bored hole or in several bored holes in a rock mass. The single point borehole extensometer consists of a single anchor and a reference.
 
They help to accurately measure distance between the anchors with respect to the reference and monitor its relative displacement with passage of time. It is usually assumed that the anchor is in stable ground and so any change in the anchor spacing is interpreted as sag of the roof bed, movement of side wall or slope, settlement of the foundation etc.