Quantitative measurement of the topographic change at overhanging sea cliff with small UAV survey system

Structure from Motion (SfM) photogrammetry measurement with small Unmanned Aerial Vehicle (UAV) was applied to quantify inaccesible coastal cliff geometry. Firstly the author has experimented multiple UAV-SfM photogrammetry on a peninsular-rock surrounded on three sides by the sea and secondly topographic change volume and rate were calculated using the difference between 3D topographic data. The study site is located in the center part of the Taitosaki in the east of Chiba prefecture, Japan. The peninsular-rock named Suzumejima has circular shape with a diameter of 50 m and a height of 30 m. Aerial photography using the small UAV was conducted twice on 24 June and 31 October 2014. As a result, average erosion rate of the cliff was about 4.5 cm/4 months. The continuous monitoring in the future will make it possible to evaluate the erosion rate of the sea cliff more accurately including seasonal and/or annual variations.