Study On Slope Rill Erosion Based On UAV Nap-of-the-object Photogrammetry

Rill erosion is one of the main forms of soil erosion.Monitoring and quantifying the morphology evolution and erosion amount of slope rill is very important to further study the mechanism and process of rill erosion.Traditional measurement methods have low efficiency,high cost and poor applicability.UAV nap-of-the-object photogrammetry technology combines efficiency and accuracy and has strong adaptability.However,its application effect on rill erosion measurement on high steep slopes is still unclear.In this study,UAV(M300 RTK UAV +Zenmuse P1 mapping camera)was used to obtain high-resolution images of rill development in slope of continuous 6 artificial simulated rainfall through nap-of-the-object photogrammetry technology.Then,DJI Intelligent Map software and Arc GIS software based on Sf M-MVS technology were used to extract and calculate index parameters such as rills erosion form and erosion amount,and multi-phase comparison was carried out to evaluate the errors and feasibility of the technical means from three aspects of positioning accuracy,model accuracy and model details.The effects of flight height,lens orientation and image control point on the measurement accuracy of rill erosion were analyzed.Finally,by extracting and comparing the morphology and process of slope rill development and evolution through high-precision DEM,the digital,visual and dynamic measurement of rill erosion was realized,in order to provide new ideas and technical means for the quantitative research of slope rill erosion evolution process.The main conclusions of this paper are as follows:(1)The accuracy of UAV nap-of-the-object photogrammetry in slope rill erosion measurement can reach millimeter level.When the UAV flies 8m,lens Angle is-60°,overlap rate is 80%～90%,and 10 image control points are arranged,the root mean square error of plane is 0.4006 cm,the root mean square error of elevation is 0.8823 cm,and the root mean square error of 3D real model geographic registration is 1.5cm.The model details are clear,the color is uniform,and there is no obvious pull and distortion.(2)The main influencing factors of UAV close photogrammetry accuracy include flight height,lens orientation,image control point,etc.The higher the flight altitude,the lower the accuracy of the model,but the amount of data acquisition and processing will decrease accordingly.The flight altitude has the greatest influence on the accuracy of trench depth identification.The lens orientation essentially affects the photo overlap rate.The better the degree of "plane" fitting with the object,the higher the photo overlap rate,the smoother and finer the model details,and the higher the accuracy.The image control point has little effect on the relative accuracy of rill model,but it is very important for multi-phase model positioning and superposition analysis.(3)The multi-phase high-precision model can effectively simulate soil erosion through "grid difference reduction" calculation and combined with soil bulk density.The relative error between the measured value and the simulated value of soil erosion was between-18% and 13%.Due to surface erosion in the early rainfall period,the erosion amount was large,but the elevation changed little,leading to a large simulation error.With the continuous increase of rainfall duration,the simulation accuracy tended to be stable,and the average error was less than 10%.(4)The multi-phase high-precision model can describe the five stages of rill development: raindrop spatter erosion,chip erosion,small drop water,intermittent rill and continuous rill.The average trench width,trench depth and plane density of rill on slope developed from 1.25 cm,0.82 cm and 0.05 to 3.27 cm,4.75 cm and 0.23 in the6 th rainfall.The maximum length and depth of the main trench were 236 cm and14.23 cm respectively.(5)By using UAV technology,the rill erosion process of slope can be monitored more accurately,so as to identify the micro-topographic changes of various parts of slope more intuitively.This method is more flexible and efficient than the traditional three-dimensional laser scanner and close-range photogrammetry,and it is more targeted and adaptable,providing new ideas and technical means for the monitoring and research of rill erosion of slope.