Inversion Of Forest Canopy Height Using ICESat-GLAS Data Combined With Laser Pointing Angle

The Geoscience Laser Altimeter system(GLAS)mounted on the Ice,Cloud and Land Elevation Satellite(ICESat)can achieve rapid and non-destructive forest canopy height estimation,which has greatly improved the efficiency and accuracy of forest canopy height estimation.However,due to the presence of the ICESat-GLAS laser pointing angle,and the spot size is larger,the slope of the topography in the spot is undulating.The interaction between the laser pointing angle and the terrain will changes the shape of the laser spot,which in turn affects the change in GrondExtent(the elevation caused by slope within the footprint),resulting in a bias in the height estimation of the forest canopy.Therefore,it is very important to establish the GroundExtent correction model,and decrease the influence of the pointing angle and the terrain on the estimation of forest canopy height.The management area of Wangqing forest area in Jilin province was taken as the study area,the ICESat-GLAS waveform data and DEM data were processed,and the parameters of them were extracted.In order to quantify the influence of laser off-nadir angle on the ICESat-GLAS data estimating the forest canopy height,based on the Allouis(2012)model and the Nie(2015)model,the laser off-nadir angle was introduced to calculate the GroundExtent,respectively.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.Then,according to the interaction relationship between the laser pointing angle and the terrain,the GroundExtent correction model was established.Based on the GroundExtent correction model,combined with the ICESat-GLAS waveform length W,the physical model and the multiple linear regression model for inverting the forest canopy height were established.The conclusions are as follows:(1)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;the absolute value of GroundExtent theoretical error caused by laser orientation(0°-360°)was 0-1.222 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.(2)GroundExtent correction model built with off-nadir angle ?,laser orientation ?,slope S,aspect ?,having terrain slope correction ability.And the physical model of forest canopy height established by the W,FWHM and GroundExtent correction models is able to estimate the forest canopy height more easily and accurately,R2 was 0.673?which increased by 0.101 and 0.021,respectively;RMSE was 2.848 m,which was reduced by 0.411 m and 0.091 m,respectively.Achieved the calibration of the ICESat-GLAS laser pointing angle and terrain,and the change of the elevation caused by slope within the footprint due to the interaction of the laser pointing angle with the terrain,which will affect the inversion of forest canopy height is avoided.(3)Multiple regression linear model for inverting forest canopy height using W and GroundExtent modified models had the best estimation accuracy,R2 was 0.709,which increased by 0.126 and 0.038,respectively;RMSE was 2.622 m,which was reduced by 0.447 m and 0.119 m,respectively,which overcomed the limitations of the values of the physical model parameters,and improved the model's estimation accuracy.