基于野外在线多角度光谱仪的荒漠草原植被光谱特征分析

Spectral Characteristics Analysis of Desert Steppe Vegetation Based on Field Online Multi-Angle Spectrometer

植被的光谱反射曲线特征区别于土壤、水等其他物质,密切关系于本身的生理性状表现。由于对冠层植被进行俯视与侧视时的入目效果不一致,实际角度原因会导致传感器获得的覆盖度信息产生误差。以内蒙古四子王旗荒漠草原植被为研究对象,利用自主设计组装的野外在线多角度光谱仪,开展草原植被多角度实时观测试验。并结合归一化植被指数(NDVI)、比值植被指数(RVI)、优化型土壤调节植被指数(OSAVI)以及光化学植被指数(PRI)共4种指数进行多维的数据比较。分析了不同传感器观测角度(SVA)和不同太阳高度角(SEA)下的草地生理性状光谱多样性三者间的关联特征。研究发现,传感器观测角越大,不同波段冠层反射率的日内变化程度越弱,表现出明显的观测角度差异性和角度敏感程度差异性,当传感器观测角(SVA)在75°附近或垂直向下观测时,植被的反射率日变化标准差较小。在固定传感器观测角度的情况下,植被的反射率与太阳高度角呈正相关关系。同样,不同植被指数的角度效应也存在差异,OSAVI指数在传感器观测角(SVA)为45°时表现最为敏感,不同月份都出现了最低值这一特征,PRI指数的最大值均出现在传感器观测角(SVA)为60°位置以下。同时发现,针对不同的太阳高度角(SEA)并基于所观测植被所处的不同生长期而选择一定传感器观测角度更有利于获得有效且精确的数据。野外在线多角度光谱仪观测结果旨在为卫星影像产品矫正、植被遥感精准监测、草原生物量的准确估算等方面提供科学数据支撑。

The spectral reflection curve characteristics of vegetation are different from other substances,such as soil and water,and are closely related to the performance of their physiological traits.The actual angle causes errors in the coverage information obtained by the sensor due to the inconsistency of the incidence effect when looking down and sideways at the canopy vegetation.In this paper,we conducted a real-time multi-angle observation experiment of grassland vegetation in the desert grassland of Siziwang Banner,Inner Mongolia,by using a field online multi-angle spectrometer designed and assembled by ourselves.The four indices,namely the Normalized Vegetation Index(NDVI),Ratio Vegetation Index(RVI),Optimized Soil Adjustment Vegetation Index(OSAVI)and Photochemical Vegetation Index(PRI),were used for multi-dimensional data comparison.We analyzed the correlation characteristics between the spectral diversity of grass physiological traits at different sensor observation angles(SVA)and solar altitude angles(SEA).It was found that the greater the sensor observation angle,the weaker the intra-day variability of canopy reflectance at different wavelengths,showing obvious variability in observation angles and differences in angular sensitivity,and the standard deviation of daily variability of vegetation reflectance was smaller when the sensor observation angle(SVA)was near 75°or observed vertically downward.The reflectance of vegetation was positively correlated with the solar altitude angle for a fixed sensor observation angle.Similarly,the angular effects of different vegetation indices also differed,with the OSAVI index being most sensitive at a sensor observation angle(SVA)of 45°,with the lowest value occurring in different months and the maximum value of the PRI index occurring below a sensor observation angle(SVA)of 60°.It was also found that selecting a certain sensor observation angle for different vegetation growth stages or for different solar altitude angles(SEA)is more beneficial to obtain valid and accurate data.The analysis results of data collected from the field online multi-angle spectrometer in this paper provide scientific data support for the correction of satellite image products,accurate monitoring of vegetation remote sensing,and accurate estimation of grassland biomass.