Erosion Monitoring in Benggang Based on Control-Free Images and Nap-of-the-Object Photogrammetry Techniques  
Linting Zhong  
College of Resources and Environment, Fujian Agriculture and Forestry University, Fuzhou 350002, China; lintinghong1108163.com (L.Z. ); [EMAIL_ADDRESS] (L.L. ); [EMAIL_ADDRESS] (G.L. ); [EMAIL_ADDRESS] (X.J. ); [EMAIL_ADDRESS] (Y.Z. ); jsfsfsfsfsfsfsfsfsfsf  
oblique photogrammetry and ground control points to monitor Benggang areas dynamically. [PERSON] et al. [13] used UAV tilt photography for this purpose, analyzing spatial changes in Benggang. [PERSON] et al. [14] used UAV oblique photogrammetry to monitor changes in erosion and to extract comprehensive terrain information. However, steep collapse walls and significant drops [15] render oblique photogrammetry inapplicable due to the limited angles of overlooking imaging, particularly in monitoring collapse wall erosion. It is important to address these limitations and enhance accuracy in capturing surface details of nearly vertical collapse walls [16; 17; 18; 19]. The [PERSON] Academician team of Wuhan University launched nap-of-the-object photography [17; 18] using UAVs close to the research surface for image data acquisition, thus obtaining subcentimeter high-resolution images [19]. Through the further processing of photogrammetry software, the accurate coordinates and point cloud model of the study area are obtained, and the fine model is generated. At present, this technology has been used in Benggang erosion monitoring. Researchers like [PERSON] et al. [16] have utilized nap-of-the-object photogrammetry as an alternative to oblique photogrammetry, reducing the measurement error by up to 45.45% and improving overall elevation accuracy by 162.5%. Nap-of-the-object photogrammetry technology can be used to extract erosional geomorphology features in Benggang areas efficiently and accurately. However, it is necessary to establish multiple control points on a steep collapse wall to accurately determine Benggang topography and erosional information. The complexity and precarity of Benggang areas make this method perilous and challenging to use, particularly at larger scales, under typical field conditions. The development of control-free-image-based photogrammetry technology is essential to monitoring Benggang erosion. This technology has been applied successfully in various other fields. [PERSON] et al. [20], for instance, demonstrated its utility by employing no control points, integrating auxiliary data recorded by UAV systems, and performing fast UAV image mosaic and correction. [PERSON] et al. [21] improved orthophoto image accuracy without ground control points (GCPs) or high-precision sensors. [PERSON] et al. [22] investigated photogrammetric methods for UAVs without GCPs, comparing them with the traditional approach and verifying their effectiveness in estimating large inventories in dynamic environments. [PERSON] et al. [23] used low-cost micro-drones and structure-from-motion (SFM) photogrammetry to measure paleo-seismic offsets without GCPs. [PERSON] et al. [24] generated an automatic snow depth map without relying on GCPs. There have been several other valuable contributions to this area. [PERSON] et al. [25] obtained and processed data for a landslide-debris flow disaster model using control-free UAV photogrammetry images and quickly determined landslide shape characteristics, debris flow channel characteristics, and other related parameters from the aerial survey date. [PERSON] et al. [26] used UAV technology to conduct large-scale aerial surveys of islands and reefs without control points. UAV-based control-free image technology can significantly enhance efficiency without necessitating pre-set control points while ensuring sufficient accuracy, which may make it a highly effective approach to monitoring complex terrain changes [21; 27]. However, there have been few studies on the application of this technology in Benggang erosion monitoring.