Research On Plant Canopy Chlorophyll Content Retrieval With Multi-angle Remote Sensing Techniques

Chlorophyll content is an important characteristic parameter of plant nutrient content,plant nutrient content is closely related to the physiological state of vegetation,so chlorophyll content is often used to monitor the physiological status of vegetation.Compared with the traditional vertical detection,multi angle observation technology is helpful to improve the accuracy of physical and chemical parameters inversion.However,the vegetation index obtained from multi angle data often shows a certain angle effect.One of the main factors affecting the angle effect is the existence of specular reflection component.Therefore,in the process of chlorophyll retrieval,these uncertain factors will affect the retrieval accuracy.Based on these problems,this study combined with multi angle hyperspectral detection technology to explore the effect of angle,including the observation of zenith angle,solar zenith angle,and leaf inclination on the retrieval of chlorophyll content by vegetation index method;on this basis,combined with multi angle polarization measurement technology,this study explored the separation method of specular reflection components of plant leaf and canopy scale multi angle reflectance spectrum In order to provide methods and technical support for the subsequent retrieval of chlorophyll content from multi angle remote sensing,the influence of the retrieval on chlorophyll content was analyzed.The main contents and work of this study include:(1)Conducted indoor and outdoor,multi-angle and polarized reflectance spectroscopy experiments to obtain the research data.The study carried out including outdoor and indoor darkroom conditions,blade length scales and canopy,different observation angle and the polarization of reflected light and the polarization reflection spectrometry experiments through designing canopy multi-angle observation system and blade multi-angle polarized light observation system platform,etc.:tea canopy multi-angle reflection spectrum sampling experiment,tea tree leaf multi-angle polarization reflection spectrum experiment and tea canopy multi-angle polarization light measurement experiment.The data of physical and chemical parameters of ground vegetation which provided a solid data foundation for subsequent analysis of angle effect and determination and separation of specular reflection components,were obtained synchronously.(2)Study the effects of multi angle effects on the estimation of chlorophyll by vegetation index.Based on the simulated and measured canopy spectral reflectance spectra data,combined with coefficient of variation and correlation coefficient,the angle effect,including the observed zenith angle,solar zenith angle and leaf inclination angle,on the relationship between different vegetation indices and Canopy Chlorophyll content was analyzed.On this basis,the influence of angle effect of multi angle remote sensing data on the accuracy of estimating Canopy Chlorophyll Content by vegetation index method was analyzed.The results show that the angle effect has a significant impact on the retrieval performance of vegetation index,and the observation angle has the greatest impact.-The observation angle from-40°to 40°will seriously affect the retrieval ability of vegetation index.ND₇₀₅ and SR₇₀₅ based on the measured canopy data have high retrieval ability and relatively high angle stability(R²=0.71,RSME=49.95 mg/cm²,RRMSE=22%);Vegetation index AIVI has excellent retrieval ability when the observation angle is small(-20°to 20°),and the accuracy of estimating Canopy Chlorophyll content is the highest under vertical observation conditions(R²=0.72,RSME=49.08 mg/cm²,RRMSE=21%).(3)Study the separation of multi angle specular reflectance components at leaf level and its effect on chlorophyll retrieval by vegetation index.Based on the multi angle hemispherical reflectance spectrum data of leaves,it is determined that the specular reflection component of the hot spot observation direction(30°)is relatively high under the illumination of 30°light source.The retrieval performance of vegetation index before and after separation was compared,and the influence of specular reflection was analyzed.The inversion model was investigated through cross validation.The results show that:under vertical observation,the content of specular reflection component is small,so the influence of specular component on the retrieval ability of vegetation index is small.The content of specular reflection component in hot spot direction is more,and the retrieval ability of index is improved obviously after separation.The retrieval accuracy of the SR₇₀₅ at 30°observation angle is improved obviously after mirror reflection separation(R²=0.60,RMSE=8.08 ug/cm²,RRMSE=12.04%),RRMSE decreased by about 2%.(4)Study preliminary the separation of specular reflectance components of multi angle reflectance spectra at canopy scale and its effect on chlorophyll content in canopy.In this study,based on the simulated data,the reflectance spectrum intensities under different zenith angles are compared and analyzed.The hot spot observation direction(-30°),vertical observation direction and observation direction with strong specular reflection(30°)are determined to study the separation of specular reflection components of reflection spectrum.The results show that the reflectance spectrum value in the hot spot observation direction is the highest,but the specular reflection component is relatively low;the reflection spectrum value obtained under the condition of 30°zenith angle observation is the lowest,but the specular reflection component is relatively high.The analysis of the influence of specular reflectance component on the retrieval of canopy chlorophyll by vegetation index shows that the retrieval ability of vegetation index is greatly improved after the specular reflection component are separated under the condition of 30°zenith angle.The accuracy R²,RMSE and RRMSE of SR₇₀₅ for estimating Canopy Chlorophyll content are 0.35,26.94 ug/cm² and 12.55%respectively,after the specular reflection component are separated.The RRMSE decreases by about 4%compared with the estimated results before specular separation.