关键词:地形测量 滤波 激光扫描器 数字高程模型 森林测量 信号分类 航空 激光雷达 森林 测量 数据 处理方法 研究 Forest Altimetry Data Laser Airborne 高程模型 地区 城市 均方差 高度误差 计算 操作 自动
Airborne laser scanning foraltmi etry is a new technology, which hasbeen 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 forestareas. 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 surfacemodel to a presumed flat terrain, over filtering or less filteringwas significantly reduced. The accuracy(RMS) of the derived DTM was 0·4—0·5m.Datawas 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 modelwas constructed by extracting signals reflected from the ground. The basic procedure is as follows:(1) Creating surface mode.l W ith all random foot print data, Delaunay triangle was adopted to create a three-dmi ensional surfacemodel DSM0. (2) Displaying shaded relie.f The intermi-results were manually monitored with a shaded relief mi age to determine whether to continue or stop filtering. (3) Filtering. A smoothed surface model was created by average filter, which is called temporal surfacemodel (DSMt). Regardless the feature of topography, the size of filterwindowwas setstablewith 5m×5m. (4) Approaching the ground. By comparingDSMtwith a threshold, smaller valueswere selected forDSMtmodification, and the surface gradually came to the ground. (5) Reiterative computing.Above computationswere reiteratively operated, whichwere ended upwhen eitherof following conditionswasmet,i) the difference between DSMt and threshold reaches the specified value, or ii) reiteration reaches 150 tmi es. (6) Computing the reference surface. Complexity of topographywas taken into account. A reference surface model (DSMr), which is smi ilar to the real one in form, was introduced for computing. DSMrwas created by using the filterwith a largerwindow.Considering the relationship between DSM0and DSMr, the rough surfacewas transformed to a plain one (DSMp), which was then filtered, and the topographywas restored after calculation. (7) Classificating the data. W ith a thresholdΔH,signals from canopy or groundwere separated by comparingwith the originalLP data. PointswithLP-DSMt≤ΔHwere considered as ground, whichwere used forDTM construction. (8) Constructing terrainmode.l Footprintson groundwere extracted from the originalLP data to create digital terrainmode.l (9) Testing the accuracy. The elevationsof16 points in\nthe study areaweremanuallymeasured, whichwere used for accuracy evaluation.