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

Abstract

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 th , 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 th and April 23 th , 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. 要旨：北海道胆振東部地震により森林域で発生した崩壊跡地を対象に，リアルタイムキネマティック-グローバルナビゲーションサ テライトシステム（RTK-GNSS）が搭載された小型 UAV（Phantom 4 RTK）と SfM 多視点ステレオ写真測量を用いた 3 次元計測の測 位精度の実証試験を行うとともに，地形変化の解析を行った．実証試験は，42,500 m 2 の崩壊跡地を対象に，2019 年 3 月 12 日に二周 波 RTK-GNSS が搭載された受信機（ZED-F9P）を使用し取得した 11 地点の座標データ（検証点間の最大高低差は 28 m）を検証点と して，RTK-UAV による空撮画像から構築した 3 次元モデルから検証点の位置座標を抽出し比較した．地形変化は，2019 年 3 月 12 日と同年 4 月 23 日の 2 時期に RTK-UAV による空撮と解析を同様の方法で実施し，数値表層モデル（DSM）の差分解析から地表面 の標高変化の把握を試みた．その結果， 各検証点とモデルの平均位置精度は， 水平・垂直方向で 0.060 m ～ 0.064 m であることがわかっ