Question 11 of 12: Geodetic Deformation-Monitoring Techniques

Question 11 (Part C): Geodetic Deformation-Monitoring Techniques (8 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Geodetic deformation-monitoring techniques determine the movement of a structure or the ground by repeatedly measuring, from stable reference points, the positions of object points, and testing the coordinate changes between epochs for statistical significance. They are characterised by their use of an external reference frame (absolute movements), redundant observations, and rigorous least-squares analysis. The principal techniques are:

• Precise levelling — the most precise vertical technique (sub-millimetre over short lines); monitors settlement/heave of dams, buildings and subsidence areas.
• Total-station / theodolite surveys — precise directions and EDM distances (or polar coordination from stable pillars), and specialised methods such as alignment (collimation) surveys for dams and micro-triangulation/trilateration nets; robotic total stations give automated, continuous monitoring of many prisms.
• GNSS — continuous or campaign relative positioning, 3-D, all-weather, automated, no inter-point visibility (see Q4); strong for horizontal and regional/tectonic motion.
• Photogrammetry / terrestrial laser scanning (TLS) — dense, whole-surface deformation fields, good for complex shapes and inaccessible faces.
• InSAR (satellite radar interferometry) — wide-area, millimetre line-of-sight ground motion (subsidence, landslides) without ground instruments.
• Specialised/geotechnical geodetic sensors — precise inclinometers/tiltmeters, extensometers, hydrostatic levelling, laser alignment — often integrated with the geodetic net.

Two design philosophies underlie them. A reference (relative) network ties object points to demonstrably stable reference points outside the deforming zone; a free (dynamic) network analyses the object’s own points and identifies which have moved by congruency testing. In all cases the epochs are adjusted, the reference-point stability is verified, and displacement vectors are tested against their covariance ($F$-/congruency test) before being reported as real movement.