Surface Reconstruction Of Microtopographic Loess Slope Based On Point Cloud

As an important factor in studying the process of surface runoff and soil erosion,microtopography affects the generation and flow direction of runoff through the change and fluctuation of relative elevation at each point in space,and then affects the evolution of erosion types.In order to describe this change quantitatively,a more accurate DEM of microtopography is needed,which is of great significance to reveal the mechanism of soil erosion on Loess tillage slope.At present,there are few studies concerning surface reconstruction and dynamic simulation for microtopography of Loess tillage slope based on point cloud data,and the application of DEM based on a NURBS surface reconstruction is still in the exploratory stage.In this paper,high-precision point cloud data of Loess slope in different stages of microtopography water erosion development,in the condition of 60 mm/h rainfall intensity and different slopes?5,10,20 degrees?,were obtained by using a 3-D laser scanner.Data simplification,optimal movement analysis window and surface roughness calculation were explored in the process of microtopographic DEM reconstruction based on NURBS surface.At the same time,compared the accuracy of NURBS-DEMs with DEMs generated by IDW,Natural Neighbor and Kriging interpolation.As a result,the optimal DEM simulation method and simplified conditions for microtopography on Loess slope will be obtained.The main conclusions are as follows:?1?Point cloud simplification of microtopographySlope has an effect on the simplification rate of point cloud data and the time of surface reconstruction.On slopes of 5 and 10 degrees,when L_min=4 mm,the simplification rate of point cloud data is approximately 70%,and the time of surface reconstruction is 2 minutes,which does not change with the increase of L_min;on slope of 20 degree,when L_min=5 mm,the simplification rate of point cloud data is about 87%,the time of creating surfaces is 2 minutes,which does not change with the increase of L_min;when L_min rangs from 4 to 6 mm,the point cloud data show sharp characteristics of cultivation measures and erosion ditches.?2?Surface reconstruction of microtopographic Loess slopeAt any slopes,when L_min is 4-6 mm,the NURBS surfaces of microtopography are able to clearly present the cultivation measures and erosion development process;when L_min is less than 8 mm,the NURBS method has the best precision in constructing microtopographic DEMs;when L_min is larger than 9 mm,the precision decreases slightly.?3?Accuracy evaluation of surface three-dimensional shapeDuring the verification of local area accuracy,the bigger the moving analysis window is,the smaller the calculated root mean square height and skewness are in the local area.That is to say,when the moving analysis window is 16×16,the stronger the descriptive ability of error control and erosion condition is;when L_min=5 mm,the value of skewness and steepness is closer to that of L_min is 1 mm.?4?Surface roughness of microtopographyDEMs acquired from NURBS surface are able to be used to evaluate the surface roughness characteristic value of microtopography.The linear equation fitting degree between NURBS-DEMs calculated surface roughness and L_min is higher than that of Kriging-DEMs,which indicates that NURBS method is slightly better than traditional interpolation algorithm;the fitting degree of linear equation for calculating surface roughness and L_min calculated by surface line method is a little higher than that of area projection method,which shows that surface line method is not only more stable and convenient in calculating surface roughness of microtopography,the changes of surface roughness in each erosion development stages are also tend to be more distinct.The simplification of point cloud data based on octree method and DEMs created by NURBS are helpful to provide a more accurate model for quantifying water erosion process of microtopography.In this paper,we used an octree method to simplify the high-precision point cloud data of microtopography acquired before and after rainfall,used NURBS surface method to obtain DEMs of high accuracy.At the same time,DEM simulation optimization method comparison,local regional accuracy evaluation and surface roughness calculation were used to verify the advantages and disadvantages of NURBS method.So as to obtain the best DEM simulation method and simplified conditions for microtopography on Loess slope.It is of great significance to further reveal the mechanism of soil erosion on Loess tillage slope at the microtopographic level.