航空激光雷达用于森林测量的数据处理方法研究

Processing of Airborne Laser Scanned Data for Altimetry in Forest Areas

航空激光雷达是最近十几年开发的最新测量技术。利用激光扫描器所获得的高度数据可以建立精密的地形模型,同时可以获得森林生产力的信息。无论是地形测量还是植被估算,在数据处理时都需要将来自地表和植被的信号分离开,即滤波。数据滤波的方法很多,但是都不成熟,而且处于技术保密状态。本文介绍目前主要的信号分类方法,并开发了动态阈值分离法。该方法以森林为解析对象,通过反复运算逐渐逼近,完成地表和植被的分离,从而实现了地形与森林测量的双重目的。通过引入地形因子把原地形转换为理想平地,大大减少了过度滤波或者滤波不充分的情况。本方法基本实现自动处理,容易操作。利用本方法计算的高度误差(均方差,RMS)为0.4—0.5m。本方法还适用于城市地区高程模型的建立。

Airborne laser scanning for altimetry is a new technology,which has been developed during last two decades.To derive topography from laser scanned data,off-ground elements like forest and buildings,which are treated as noise,need to be filtered out.This paper,in addition to the review of recent studies on derivation of topography from laser scanned data,is intended to develop a new algorithm by using average filter for forest areas.By iterative calculation,the surface approached the ground progressively by cutting off convexes above the reference surface.In particular,by introducing topographic factor to transform the surface model to a presumed flat terrain,over filtering or less filtering was significantly reduced.The accuracy(RMS) of the derived DTM was 0.4—0.5m.Data was acquired by Laser Bird,which is a laser radar system produced by Optech Ltd.A digital surface model(DSM) was created with all original data,from which the terrain model was constructed by extracting signals reflected from the ground.The basic procedure is as follows:(1) Creating surface model.With all random foot print data,Delaunay triangle was adopted to create a three-dimensional surface model DSM0.(2) Displaying shaded relief.The interim-results were manually monitored with a shaded relief image to determine whether to continue or stop filtering.(3) Filtering.A smoothed surface model was created by average filter,which is called temporal surface model(DSMt).Regardless the feature of topography,the size of filter window was set stable with 5m×5m.(4) Approaching the ground.By comparing DSMt with a threshold,smaller values were selected for DSMt modification,and the surface gradually came to the ground.(5) Reiterative computing.Above computations were reiteratively operated,which were ended up when either of following conditions was met,i) the difference between DSMt and threshold reaches the specified value,or ii) reiteration reaches 150 times.(6) Computing the reference surface.Complexity of topography was taken into account.A reference surface model(DSMr),which is similar to the real one in form,was introduced for computing.DSMr was created by using the filter with a larger window.Considering the relationship between DSM0 and DSMr,the rough surface was transformed to a plain one(DSMp),which was then filtered,and the topography was restored after calculation.(7) Classificating the data.With a threshold ΔH,signals from canopy or ground were separated by comparing with the original LP data.Points with LP-DSMt≤ΔH were considered as ground,which were used for DTM construction.(8) Constructing terrain model.Foot prints on ground were extracted from the original LP data to create digital terrain model.(9) Testing the accuracy.The elevations of 16 points in the study area were manually measured,which were used for accuracy evaluation.