The Digital Tape Extensometer is a portable device used for measuring displacement between reference anchors fixed to an excavation or structure.
 
The instrument consists of a precision punched steel tape incorporating a repeatable tensioning system and dial gauge readout. The tape winds onto a reel, which incorporates a tape tensioning device and a digital LCD readout.
 
The body of the extensometer has a hook connector that connects to the first eyebolt. An identical hook on the free end of the tape connects to the opposing eye bolt. The user then tensions the instrument and records the readings from the digital LCD display. The measurements are not absolute, but relative to the previous measurement. This provides an accurate record of displacement over time.
 
These can be installed on a structure or in an excavation such as a tunnel or shaft.