切沟形态特征无人机倾斜摄影测量

切沟是坡面土壤侵蚀最剧烈表现形式，其形态特征刻画的精确性与适用性，对切沟的调查、研究与防治具有重大意义。该研究选取黄土丘陵区小流域4个地点10条切沟共82个断面，利用地面测量验证携带多频全球导航卫星系统且内置实时差分定位技术（Global Navigation Satellite System and Real-Time Kinematic，GNSS RTK）的无人机倾斜摄影测量提取的沟长、沟宽、沟深指标的精确性并建立体积估算模型探讨沟长的测定方法。结果表明：1）与地面测量相比，沟长、沟宽的偏离度（Deviation Extent，DE）均值不大于3.70%，精度较高；沟深的DE均值不小于25.49%，误差较高。2）切沟体积与沟长之间有显著的幂函数关系，沟长选取沟头至沟口的最大汇水线长度时拟合效果最好，沟道中心线长度次之，直线长度拟合效果最差。因此，在黄土高原丘陵沟壑区，运用无人机倾斜摄影测量方法测量切沟沟长、沟宽指标具有高度可行性，在精度要求较高的沟深测量方面的应用还有一定的局限性。此外，由于野外测量的局限性，直线长度较最大汇水线长度的测量更简单便捷，当坡度小于18.2°时，测量直线距离作为切沟沟长可行。

Monitoring the morphological characteristics of gully using UAV oblique photogrammetry

Abstract: Gully erosion has been the most severe form of slope soil erosion in arid and semiarid areas. It is very necessary to accurately monitor the morphological characteristics for the prevention of gully erosion. Typical indicators can be used to describe the morphological characteristics of the gully development, including the gully length, width, depth, breadth depth ratio, and the vertical slope of the gully. Among them, the gully length has not yet been known in the stage of gully erosion so far. Alternatively, a low-cost and high-precision topographic monitoring technique, oblique photography has a wide range of applications in the detection of gully erosion in recent years. In this study, 82 sections of 10 gullies were selected from four sites in the hilly and gully region of the Loess Plateau area in China. The indexes of gully length, width and depth were extracted from the field measurements and Unmanned Aerial Vehicle (UAV) oblique photography with a real-time kinematic and global navigation satellite system (GNSS RTK). A volume estimation model was then established to determine the gully length. The results show that there were various indicators for the different accuracy and applicability of gully morphological parameters using oblique photogrammetry. Specifically, 1) compared with the field measurements. The deviation extent (DE) of gully length and gully width was less than 4%, indicating a higher accuracy than before. The mean values of DE of gully depth were greater than 25%, indicating a larger error. Therefore, the UAV oblique photography with the GNSS RTK was highly feasible to measure the index of gully length and gully width, but there are certain limitations in the application of gully depth measurement with high accuracy requirements. 2) There was a significant power function relationship between the gully volume and gully length. The volume-length model was also established to predict the development stage of the gully erosion, in terms of various parameters during simulation. The parameter values of the model show that the selected gully in the study area presented a high average development degree, particularly with the large cross-sectional area, deep erosion degree, and small development potential. The growth of gully volume was mainly controlled by the length of the gully, indicating a weak relationship with the gully width and depth. The model fitting demonstrated the best imitative effect when the gully length was selected as the horizontal projection length of the maximum catchment path, followed by the length of the gully centerline, and the linear distance from the gully head to the gully exit was the worst. Consequently, it was feasible for the line distance as the gully length during field measurements when the slope was less than 18.2°. And more importantly, the volume-lengh model should fully consider the influence of the development stage in the gully erosion.