Extensometers are used to measure axial deformation in a borehole. They measure the relative displacement between one or more fixed points in a borehole and a measuring station at the entrance to the borehole. There are also various types of probe extensometers that can handle a higher number of measuring points in a borehole, but are not suitable for automatic data acquisition, like rod extensometers.

Surface-extensometer with electric displacement transducers

Instruments for measuring joint and crack displacements in principle work along the same principles as extensometers, but with a short measurement base. With the appropriate design, the displacement of joints and cracks can be measured in the three spatial dimensions.