A Case Study on Terrestrial SfM Photogrammetry Slope Change Detection Without Stable Ground Control Points

ABSTRACT: Daily remote monitoring of slope processes using terrestrial structure from motion (SfM) photogrammetry is a relatively new technique in engineering geology. In this study, a five-camera photogrammetry monitoring station was installed in the Siguas River Valley near Majes, Peru in December 2020 for long-term monitoring and study of slope processes. The study area includes a retrogressive rockslide-debris slide with areas of rockfall sourced from the head scarp, areas of water-driven erosion, and other related geomorphological processes ranging from 100 to 900 m away from the camera stations. Challenges with photo and point cloud alignment accuracy and, ultimately, change detection accuracy arise due to the constant ground movement and lack of suitable locations for ground control point (GCP) installation. In this paper, we investigate point cloud alignment methods for the purpose of change detection at sites where the distance from the photogrammetry station to the area of interest is variable and where there are no stable GCPs. Additionally, we present preliminary change detection results in relation to landslide movement rates at the site. For the tested methods that produced reasonable results, the average rate of movement for the rockslide-debris slide was estimated to be between 3 and 35 cm per month, on average. 1. INTRODUCTION Intensive cropland irrigation in dry climate regions has been linked to increased landslide occurrence, and this relationship has recently become of increased interest to the geohazard scientific community (Garcia-Chevesich et al., 2021). An example of one such area is the Majes district in southern Peru, roughly 150 km to the west of Arequipa. The Siguas River runs through the Majes district, providing drinking water for local populations, water for industrial uses, and water for cropland irrigation. Since the mid 1970’s the amount of land dedicated to irrigation-dependent agriculture has continued to increase along the Siguas River (Lacroix et al., 2020). The north bank of the Siguas River, where it is adjacent to irrigated land, has had numerous landslides in the last four decades, while the southern bank of the river, which remains mostly unirrigated, has had minimal landslide activity within the same timeframe (Graber et al., 2021).