Sand movement analysis of the Tottori Sand Dunes using aerial photogrammetry by UAVs.

The Tottori Sand Dunes, located along Japan's Sanin Coast, are a UNESCO Global Geopark and a popular tourist attraction with millions of annual visitors. Monitoring of environmental conditions including sand movement is needed for the protection and utilization of the dunes. The objectives of this paper were (i) accuracy evaluation of aerial photogrammetry using three UAVs (Un- manned Aerial Vehicles: Drones) and two types of structure from motion (SfM) software and (ii) sand movement analysis in the dunes using the best results. For each flight, about 1800 vertical photographs (side overlap: 60%, and top-bottom overlap: 80%) were taken at altitudes of approximately 100 m above ground level using three UAVs for a 1500 km² area of the dunes. The obtained images were processed using two SfM software, PhotoScan Professional (currently Metashpae: Agisoft) and Pix4Dmapper (Pix4D), for building 3D models. The position data of survey piles in the dunes were used as the GCP (ground control point) setting for image processing and model evaluation. The results revealed that the accuracy of the models was significantly affected by the difference in the SfM software but was not greatly influenced by the difference in UAVs. The models generated from PhotoScan generally curved and had larger errors at the center parts and around the edges of the models (average RMSE=1.74 m), while the models generated from Pix4D-mapper had smaller errors throughout the entire areas (average RMSE-0.38 m). From the comparison of the models obtained from Pix4Dmapper, the characteristics of sand movement such as the movement of the ridge line towards the inland area, were elucidated in the dunes from 2016 to 2018. [ABSTRACT FROM AUTHOR]