Study On High-precision Detection Method For Rill Morphology In Yuanmou Dry-hot Valley Region

Rill Erosion is the main mode of slope surface erosion.The morphology of rill is a very important factor that determines slope runoff and the amount of soil loss.The relations between parameters of rill morphology can be used to describe the erosion process of rill.Therefore,getting high-precision morphology parameters of rill is significant for revealing the development and evolution of rill erosion.Although some scholars have used various technology to detect the rill area terrain,few scholars have studied the mode of terrain mapping and the relationship between DEM resolution and the rill morphological parameters.This paper takes rills developed from the Yuanmou dry-hot valley region as the research object to discuss the effective technological methods for the detection of high-precision morphology parameters of rill.Through building control network of the rill area,the terrain of rills can be measured using self-made needle plate,total station,3D laser scanning and close-range photogrammetry technique,and the morphological parameters of rill can be obtained by GIS.In order to establish the best detection technology and method,analyzed the precision of elevation of different measuring methods and its effects on morphology parameters of rills,the quantitative relationship between resolution of DEM and parametric error of rill morphology.The results will offer a technological support for study the control of soil erosion in Yuanmou dry-hot valley region.The main conclusions are as follows:(1)6mm is the best scanning step for the cross section of rill.Processing the point cloud data of different scanning steps,the results showed that with increasing of the scanning step would considerably reduce the data quantity,but the parametric error of rill morphology will be different.Contrast with the parametric of rill morphology based on close-range photogrammetry,the absolute error and relative error of the width,depth and aspect ratio of the rill are minimized when the scanning stepsize is 6mm,the line length of cross section decreases with the increase of scanning step size;when the scanning step size is 4mm,a relatively high accuracy cross-section area can be obtained.Therefore,under the condition of reducing the data quantity and the parametric error and precision,the best scanning step for studying cross section of rill is 6mm.(2)Different data processing methods affect the precision of parameters of rill morphology.Polyline method,arc method and spline method are respectively used to fit the ditch edge measured by total stations and their relative errors in the length of ditch line are all small.Arc method has the smallest relative error in both the length of ditch line and rill erosion area.Filters have little effect on the width,depth and aspect ratio of rills but a remarkable impact on the area and line length of the cross section.Mode filter had the smallest effect on the cross section area and the effect of low pass filter and focus statistics on aspect ratio is relatively small.(3)Close-range photogrammetry gets relatively high-precision elevation data of rill morphology,and is the optimal technology for detecting high-precision morphology parameters of rill.Comparing the elevation values and errors of three measuring methods,respectively total station,and 3D laser scanning method and close-range photogrammetry,the last one can build up a more reliable DEM suitable for features of rill.Meanwhile,for the rill with complex terrain,should choose clockwise or counterclockwise photogrammetry mode.Different detecting methods can get different rill morphology parameters.As the scale of rill is relatively small but usually the broken point spacing of total station is large so it can be used only for getting parameters of rill in their length of ditch line,erosion area,width and depth of cross section;needle plate can be used to get high-precision morphology parameter of rill cross section,but it can't extract other morphology parameters.If rills are relatively straight and of uncomplicated groove type,and a total station can completely do the scanning,3D laser scanning is an ideal data acquisition method.However,rills are often twisted under natural conditions and cannot be scanned completely with only one station,while there is a relatively large margin of error in the splicing of scanning with multi-station,which would cause a larger margin of error in parameters of rill,thus not suitable for the study of rill morphology.For all kinds of complicated rills,close-range photogrammetry can acquire relatively precise morphology parameters of rill so it is the optimal detecting technology for studying morphology parameters of rill.(4)The best resolution of DEM for acquiring high-precision morphology parameter of rill is 5mm.Through different resolutions of DEM built by close-range photogrammetry and 3D laser scanning,extracted are ditch line,morphology parameters of rill cross section(including parameters of width,depth,line length,area and ratio of width to depth),erosion area and volume,line length of longitudinal section.The relationship has been analyzed between their relative error and the resolution of DEM to get the best resolution of DEM under different morphology parameters of rill.According to the minimum principle,5mm is the best resolution of DEM for studying rill morphology.The main points of innovation as follows:(1)A technological method has been proposed for high-precision detection of rill morphology in Yuanmou dry-hot valley region based on 3D Laser Scanning and close-range photogrammetry.Rill is considered as a micro-geomorphology.The most effective way for rills of various winding degrees is to take into considerations factors such as the quality(precision)of morphology data,efficiency and cost,and to complete photo reconstruction under close-range photogrammetry.3D Laser Scanning is for rills relatively straight while needle plate method is relatively effective for cross section study.(2)A modeling method has been proposed for DEM of rill based on close-range photogrammetry.To improve the effectiveness and precision of rill morphology study,a mathematical model needs to be built of the relationship between the resolution of DEM and rill morphological parameter error.In order to provide a scientific basis on how to reduce data acquisition quantity while guaranteeing precision,the impact has been discussed that the resolution of DEM has on morphology parameters,including width,depth,cross section area and volume of rill.