林木空间格局对大光斑激光雷达波形的影响模拟

Large Footprint Lidar Waveform Modelling of Forest Spatial Patterns

激光雷达是近年来国际上发展十分迅速的主动遥感技术,在森林参数的定量测量和反演上取得了成功应用。激光雷达具有与被动光学遥感不同的成像机理,对植被空间结构和地形的探测能力很强。大光斑激光雷达系统一般指光斑直径在8—70m、连续记录激光回波波形的激光雷达系统。由于大光斑连续回波的激光雷达的光斑尺寸通常大于林木冠幅,波形中往往包含了森林冠层和许多森林元素的信息而不仅仅是单株树的信息。对于搭载在ICESAT卫星上的GLAS而言,光斑直径为70m,因此光斑对应着一片森林,包括很多棵树,在GLAS的激光光斑内树木的空间分布会有一定变化。同时激光雷达发射的脉冲信号在激光光斑内的分布也不均匀,而是从中心到边缘呈递减的分布。因此树木空间分布模式的变化对波形会产生一定的影响。通过对几种典型的树木空间格局进行模拟(包括规则分布、均匀(随机)分布和集群分布),假定激光光斑内能量呈高斯分布,模拟了各种树木分布模式林分的激光雷达信号。从模拟结果可见,森林的空间分布模式对大光斑激光雷达波形有明显的影响,对于波形面积(AWAV)和波形半能量高度(HOME),规则分布>随机分布>团状分布。其中对于HOME而言,规则分布和随机分布十分接近,而对于AWAV而言,聚集中心的变动不太敏感。

Large footprint lidar has demonstrated its great potential for accurate estimation of many forest parameters, e.g. forest height, forest biomass and vertical structure of forest canopy. In addition to the canopy vegetation, many factors such as atmosphere, underlying surface, the shape of crown, et al. , influence the lidar waveform. The illuminating intensity of the laser beam across the lidar footprint is a Guassian distribution and reduces from 1.0 to e^-2 from the center to the edge of the footprint. So the forest stand spatial pattern plays an important role in the lidar waveform. The contribution of each tree to the lidar waveform varies with its location in a forest stand. This paper simulated the random, uniform and clumped tree distribution patterns in a stand. Then waveforms were simulated using a three dimensional lidar waveform model. The results show that the tree distribution patterns affect the lidar waveform profiles. The area（ or energy） under the waveform from vegetation（AWAV） and the height of median energy（HOME） were used to estimate the effects. Following trends have been revealed from the simulation ： for AWAV and HOME, uniform 〉 random 〉 cluster. The difference between uniform and random is not obvious. For the clumped case, the number of clusters does not have much effect on the lidar waveform.