We conducted terrain measurements using a DJI Phantom4 RTK, a consumer-grade Unmanned Aerial Vehicle (UAV) with a built-in Real Time Kinematic-Global Navigation Satellite System (RTK-GNSS). We then compared its positioning performance and labor-saving effect to other UAV-SfM (Structure from Motion) methods by measuring the accuracy and precision of the position information and the working time. A three dimensional model was constructed from aerial photographs acquired with the RTK UAV. It was found that the average positional accuracy of the model was 118 mm horizontally, and 292 mm vertically. This was a great improvement compared to the method using single-point positioning GNSS equipped conventional UAV (about one tenth to one-hundredth of the average error), while the result was slightly inferior to the current method using Ground Control Points (GCPs) with high accuracy (the average error was about 3 to 6 times). The average precision (reproducibility) in the positional data was 69 mm horizontally and 35 mm vertically. Thus, when measuring topographical changes between multi-temporal Digital Surface Models (DSMs), only changes larger than the DSM measurement precision could be detected. In addition, it was found that the working time of the RTK-UAV method, including field measurement and data processing, was reduced to 19% of the time taken for the conventional method, including the work related to GCPs. Although attention should be paid to the accuracy and precision of the position information and the detection limit of the amount of topographical change, the employment of RTK-UAV equipment in topographic surveying was considered to be highly effective because it could be applied to difficult-to-access areas where the installation costs of GCPs were high and it would significantly reduce on-site working time.

The Hamada earthquake of 1872 was a big natural disaster in the San’in region that caused 536 casualties. Some of earlier researches suggested that the wave-cut bench at Iwami-tatamigaura, was uplifted during that event. The surface of the bench is characterized by linear arrangements of calcareous concretions which are described as oblate ellipsoids with height, major axis and minor axis of around 30, 50 and 40 cm, respectively. Tafoni (singular tafone), which are produced by salt weathering processes, develop on the surface of the calcareous concretions. Tafone depth can be described as a function of time, providing a measure of the duration during which individual concretions are exposed to salt weathering. The time of tafoni formation will therefore be equal to the exposure of the bench to subaerial conditions, because salt weathering never happens in a submarine environment. Tafoni depths were measured to calculate the period of tafoni formation, to discuss possible uplift of the bench during the 1872 Hamada Earthquake. An equation proposed by Sunamura and Aoki (2011) was used to calculate the formation period of tafoni. The tafoni depths were substituted into the equation together with physical and mechanical properties of the concretions. The relationship between altitudes of concretions and tafoni depths on their surface was also investigated to discuss local uplift or subsidence of the bench. The measurements from Iwami-tatamigaura were compared with those from a proxy site for undisturbed tafoni growth, providing a calibrated vertical profile of tafone depth from a comparable environment. Results show that the duration for tafoni formation is calculated to be ca. 5,000 ~ 7,000 years, indicating that the tafoni formation started during the Jomon transgression. The maximum rate for tafone formation is estimated to be 5.9×10^－3 mm yr^－1 at Iwami tatamigaura, resulting in the maximum tafone depth to be 0.88 mm that can be formed for 150 years since the occurrence of the 1872 Hamada Earthquake to the present moment. This depth is much smaller than the actual measured depths. The comparison of the vertical distribution of tafone depth, which is defined as the relationship between normalized tafone depth (the ratio of tafone depth to the maximum depth measured in this study) and its normalized altitude (altitude of a tafone divided by mean high water level), showed similar vertical profiles between Iwami-tatamigaura and the proxy site. These results indicate that the 1872 Hamada Earthquake caused no particular uplift or subsidence of the wave-cut bench at Iwami-tatamigaura.