OBIA-Based Extraction of Artificial Terrace Damages in the Loess Plateau of China from UAV Photogrammetry

Abstract

Terraces, which are typical artificial landforms found around world, are of great importance for agricultural production and soil and water conservation. However, due to the lack of maintenance, terrace damages often occur and affect the local flow process, which will influence soil erosion. Automatic high-accuracy mapping of terrace damages is the basis of monitoring and related studies. Researchers have achieved artificial terrace damage mapping mainly via manual field investigation, but an automatic method is still lacking. In this study, given the success of high-resolution unmanned aerial vehicle (UAV) photogrammetry and object-based image analysis (OBIA) for image processing tasks, an integrated framework based on OBIA and UAV photogrammetry is proposed for terrace damage mapping. The Pujiawa terrace in the Loess Plateau of China was selected as the study area. Firstly, the segmentation process was optimised by considering the spectral features and the terrains and corresponding textures obtained from high-resolution images and digital surface models. The feature selection was implemented via correlation analysis, and the optimised segmentation parameter was achieved using the estimation of scale parameter algorithm. Then, a supervised k-nearest neighbourhood classifier was used to identify the terrace damages in the segmented objects, and additional geometric features at the object level were considered for classification. The comparison with the ground truth, as delineated by the image and field survey, showed that proposed classification can be adequately performed. The F-measures of extraction on three terrace damages were 92.07% (terrace sinkhole), 81.95% (ridge sinkhole), and 85.17% (collapse), and the Kappa coefficient was 85.34%. Finally, the potential application and spatial distribution of the terrace damages in this study were determined. We believe that this work can provide a credible framework for mapping terrace damages in the Loess Plateau of China.

Application of Unmanned Aerial Vehicle (UAV) in Terrain Mapping: Systematic Literature Review

“…Accuracy result between different UAV grade is comparable depending on selected data sampling and processing parameters. A reliable DEM accuracy can also be obtained when operating the UAV in a lower flight altitude (Fang et al, 2021). The results from (Fang et al, 2021) demonstrate that the DSMs and SDMs for snow depth mapping were acquired with ≤ 0.25 and ≤ 0.29 m accuracy when exposed under full sunlight.…”

“…A reliable DEM accuracy can also be obtained when operating the UAV in a lower flight altitude (Fang et al, 2021). The results from (Fang et al, 2021) demonstrate that the DSMs and SDMs for snow depth mapping were acquired with ≤ 0.25 and ≤ 0.29 m accuracy when exposed under full sunlight. To assesses the accuracy of DSM, elevations can be compared to ground control points collected with a differential GNSS (Vieira et al, 2021).…”

“…2 shows unstable area in active mine pit. A study by (Fang et al, 2021) utilised UAV to identify man-made terrace damage in loess plateau of China. The terrace damages were extracted using high-resolution orthophoto imagery and DSMs which will then be used to produce classification map for the damages (Fang et al, 2021).…”

“…A study by (Fang et al, 2021) utilised UAV to identify man-made terrace damage in loess plateau of China. The terrace damages were extracted using high-resolution orthophoto imagery and DSMs which will then be used to produce classification map for the damages (Fang et al, 2021). From map generation, researcher can obtain useful information such as soil leak area and decay length of the study area (Kim et al, 2019).…”

“…Flying UAV in a lower altitude will increase the image gain from the survey. A study by (Fang et al, 2021) shows that images increase from 80 to 247 when flight altitude decrease from 250m to 50m. Flying the drone over a lower altitude have also increase the orthomosaic resolution and DEM compared to higher altitude (Battulwar et al, 2020).…”