Accuracy assessment of topographic measurements and monitoring of topographic changes using RTK-UAV in landslide area caused by 2018 Hokkaido Eastern Iburi Earthquake

We tested and verified topographic changes in a landslide area caused by the Hokkaido Eastern Iburi Earthquake in 2018. We accomplished this using real-time kinematic unmanned aerial vehicles (RTK-UAV) and structure-from-motion multi-view stereo (SfM-MVS) photogrammetry. To verify our measurement accuracy, the position coordinates of 11 validation points in the 4-ha assessment area were obtained using a ZED-F9P, dual frequency RTK global navigation satellite system (GNSS) on March 12, 2019. The maximum height difference between verification points was approximately 28 m. We moreover extracted position coordinates of validation points from a digital surface model (DSM), and an orthomosaic image was created from aerial images obtained by the RTK-UAV. Subsequently the position coordinates of validation points obtained by the two methods were compared. The analysis of topographic changes was performed on two DSMs created from the aerial images acquired on March 12 and April 23, 2019. The average position between each validation point and the model was 0.060 m ~ 0.064 m in both horizontal and vertical directions. The maximum vertical error was 0.108m. Knowing this error is important for monitoring the dynamics and stability of surface soil during plant growth. The analysis of topographical changes indicates a change of -0.1 m to +0.1 m in 86.86 % of the total assessment area. A change of -0.5 m to -0.1 m was the most frequent at 11.36%. In particular, the erosion area was confirmed at the boundary area between the forest and the landslide area. These results demonstrate that using RTK-UAV, topographic changes area measurable within an error range of 0.1 m even in sloped forests, where it is difficult to set ground control points.