ICESat-GLAS激光天顶角对反演森林冠层高度的影响

Effect of laser off-nadir angle on estimating forest canopy height

为了量化激光天顶角对ICESat-GLAS波形数据反演森林冠层高度的影响,以吉林省汪清林业局经营区为例,基于ICESat-GLAS波形数据及DEM数据,在Allouis模型和Nie模型基础上,分别引入激光天顶角,对光斑内坡度引起的高度距离(GroundExtent)进行修正,建立森林冠层高度估测模型,并通过模型对坡度的校正能力、天顶角引起的GroundExtent理论误差以及大气延迟增量三个方面讨论分析天顶角在反演森林冠层高度中的影响。结果表明:天顶角的引入能够提高模型的估测精度,决定系数(R2)分别提高了6.56%、4.26%,且能更好地校正地形坡度;在外部条件相同的情况下,由天顶角(<1°)引起的GroundExtent理论误差在0.122~1.100m范围内;在天顶延迟约为2.3m时,天顶角(<1°)对大气延迟增量的影响为0.04~3.50mm。由此可知天顶角对估测森林冠层高度存在一定的影响,引入激光天顶角的冠层估测模型能更准确地反演森林冠层高度。

In order to quantify the influence of laser off-nadir angle on the ICESat-GLAS waveform data estimating the forest canopy height, the Wangqing forest area of Jilin province was taken as the study area, based on the ICESat-GLAS waveform data and DEM data, the difference in elevation caused by slope within the footprint（GroundExtent） was modified by two different off-nadir angle models on the basis of Allouis model and Nie model, and established the forest canopy height estimation model, at the same time, the influence of off-nadir angle on estimating forest canopy height was discussed through three aspects： the correction ability of the model to the slope, the theoretical error of GroundExtent and the increment of atmospheric delay caused by laser off-nadir angle. The results showed that the introduction of laser off-nadir angle can improve the estimation accuracy of the model, the R2 was increased by 6.56%, 4.26% respectively, and can better correct the terrain slope; In addition, in the case of the same external conditions, the theoretical error caused by the laser off-nadir angle（ 〈1°） was 0.122-1.100 m; and when the zenith hydrostatic delay was approximately 2.3 m, the increment of atmospheric delay caused by off-nadir angle（ 〈1°） was 0.04-3.50 mm. Therefore, the off-nadir angle has certain influence on the estimation of canopy height, the canopy estimation model with the off-nadir angle can better invert the canopy heights.