Since the introduction of electronic distance measuring devices in surveying instruments, prism monitoring has been an effective method of checking wall movement. Survey prisms are set in the wall and accurate measurements made to compare its xyz position over time. Although this method allows the coverage of large areas of suspected wall movement, there are some associated problems. These are the affect on data by temperature variations, achieving best available accuracy, maintaining the data and graphing prism movement.
Source:http://www.softrock.com.au/concepts/
Link to report of this instrument: TM