RTK ‐ GNSS Assisted SfM Photogrammetry for Retaining Wall Monitoring Using Metashape and Pix4D

ABSTRACT Ground surfaces and retaining walls are necessary to monitor to mitigate against the risk of sediment disasters. Currently, the methods employed for monitoring of retaining walls rely heavily on visual inspection of cracks. This is time‐consuming and subjective as the results vary depending on the inspector. Hence, it is necessary to establish a more efficient and versatile monitoring method. This paper proposes a surface deformation monitoring method using Structure‐from‐Motion Photogrammetry, in which a Digital Single‐lens Reflex camera is attached to a Real Time Kinetic Global Navigation Satellite System (GNSS) to acquire images with precise position coordinates. In this study, point clouds were generated using Pix4DMapper and Agisoft Metashape and analysed in Cloud Compare. The two point clouds acquired at the same location at separate times, were aligned using the Iterative Closest Point algorithm. Change detection methods of the two point clouds were performed to visualise the surface deformation. The results showed that displacements can be seen from 5, 10, 15 and 20 mm test boards with errors of 0.28 to1.68 mm for Metashape and 0.29 mm to 2.84 mm for Pix4D at the retaining wall.