PointCluster-based Morphological Analysis Of Loess Landforms

The topographical features with extensive distribution together control the basic trend and geometry of the loess surface.The orderly combination and change of the spatial relationship reflect the characteristics of the loess landform during the development.Around the topographical features and the topography of the loess landform,the predecessors carried out a wealth of research.However,these studies start from a single perspective and pay more attention to the characteristic elements themselves.The spatial relationship between different topographical features is neglected.The systematic analysis of the spatial relationship between topographic features of the loess landform is lacking.This study takes the topographic feature points with strong control and identification significance as the cut-in,analyzes the spatial relationship between the feature points,and constructs the topographic feature point cluster model.By analyzing the structural characteristics of point clusters and combining the morphological characteristics of the loess landform,the quantitative expression indexes and geologic meanings of point cluster data are proposed from different analytical perspectives.Taking the typical sample area of the Loess Plateau as the study area,the correspondence between the point cluster quantitative index and the loess landform type is analyzed,and the understanding and research on the morphological characteristics of the loess landform is realized.The main contents and conclusions of this study are as follows:1)On the basis of fully analyzing and summarizing the topographic feature points of the loess landform and its spatial relationship and constraints,the topographic feature point cluster model is proposed,and the concept and construction method of the point cluster are elaborated.The positive terrain abstraction is the top and the saddle ordered point pairs distributed along the watershed boundary line,and the negative topography is abstracted as the ordered set of the ditch head point,the runoff node and the erosion reference point along the water flow direction.Based on the structural characteristics of point clusters,the design is effective to extract and store the clusters of loess landform.The scale effect in the extraction of point cluster model is analyzed from the threshold scale and the analysis window scale.The results show that the choice of threshold and analysis window have a great influence on point cluster extraction.2)Combined with the characteristics of the topographic feature points and the structural features expressed by the point cluster model,a series of indicators are selected based on the principle of index selection and the corresponding spatial analysis framework.These indicators can be divided into three aspects:spatial distribution,spatial distance and spatial topological relationship.This study describes the calculation methods and geologic implications of each quantitative expression.At the same time,aiming at the morphological characteristics of the topography of the loess landform,the approximation index of the gully head is proposed.Finally,the feature description and analysis of the point cluster data from different angles are realized,which reflects the morphological characteristics of the loess landform in different regions and developmental periods.3)The topographic feature point clusters were extracted from the typical area of the Loess Plateau,and the differences in landform morphology were compared.On the whole,the experimental results show that different geomorphological features have different characteristics in the point cluster model.From the point of view of the number of feature points,with the development of the loess landform,the number of feature points in the cluster data shows an increasing trend.From the perspective of spatial distribution,there is a certain spatial correlation between each type of feature points,and the stability unit scale gradually decreases with the development of the landform.In the spatial distance,the point cluster spatial correlation degree shows an increasing trend.The characteristics of the positive terrain relief with the landform development,show the trend of the movement from the exit of the basin to the top,and from the boundary of the main stream to the boundary of the basin.The approximation of the gully head can better reflect the degree of the gully erosion.The high approximation is mainly concentrated in the basin boundary and the area with higher tributary density.In the spatial topological relationship,as the loess landform develops,the point cluster connectivity decreases and the gradation gradient increases,reflecting the variation in the number and length of the erosion trench branches.The similarity of the topographic feature point clusters of the similar landform type area is higher.