| In recent years, with spectral resolution accurate to nanometer level, hyperspectral has a unique advantage in the detection of plant life information and analytical aspects of vegetation growth status. Grasping bidirectional reflectance characteristics of the canopy reflectance spectra is significant to improve the accuracy of remote sensing applications. Many scholars at home and abroad measured and analyzed the vegetation reflectance rate with view angle, variation of solar zenith angle and the season, but they mainly focused on the meadows and forests and analysis of Systematic sensitivity on crop reflectance spectral was less involved. This paper got a lot of multi-angle hyperspectral remote sensing data based on surface automatic remote sensing platforms.This paper includes:a. This paper continuous observed multi-angel canopy reflectance spectra for winter wheat, analysed how various factors to influence canopy reflectance spectra and two commonly used vegetation index NDVI (Normalized Difference Vegetation Index) and EVI (Enhanced Vegetation Index); b. Deal with the observed spectra from surface automatic remote sensing platform and study the sensitivity to find suitable parameters for PROSAIL; c. Calculates observed values of a variety of vegetation index and physiological parameters, analyzes the relationship between them, and gives inversion method.This paper continuous observed multi-angel canopy reflectance spectra for winter wheat, analysed how various factors to influence canopy reflectance spectra and two commonly used vegetation index NDVI (Normalized Difference Vegetation Index) and EVI (Enhanced Vegetation Index).And provide the future research with a optional design for the remote sensing observation angle of vegetation indices and a theoretical reference for application of vegetation indices. The results showed: Compared with canopy reflectance spectra for wheat simulated by PROSAIL (PROPECT+SAIL)model, measured canopy reflectance spectra from the self-designed ground automatic remote sensing platforms, was found that the form of fluction for measured reflectance rate was consistent with simulated reflectance rate and also similar to results studied by Hiker T,Coops N C,et al. The above results demonstrate measured canopy reflectance spectral data is effective and available, and the method of calculation data is also prove to be reasonable and feasible. With solar radiation increasing, canopy reflectance spectra showed an increasing trend and was more obvious in response to weather conditions of sunny days than cloudy days. "Hot spot effect" and "specular reflection" can be used to explain that with changes of view angle, canopy reflectance spectra show different distributions; When direction of observation and incident of the sun was on the same side (backscatter), field of view (direct radiating portion of solar) coinciding with the direction of incidence of the sun was the strongest point of reflection; When direction of observation and incident of the sun was on the opposite side (forwardscatter), field of view (direct radiating portion of solar) coinciding with the direction of incidence of the sun was the weakest point of reflection. With the above described conditions, vegetation index NDVI and EVI also showed similar variation with canopy reflectance spectra. Influenced by the red band and near infrared band, the change trend of NDVI and EVI was very consistent. Through multi-angle relevant analysis of vegetation index NDVI and EVI, they have a highly significant linear correlation(R2> 0.72, P< 0.01). So there were no obvious advantages and disadvantages between NDVI and EVI for uniform growth status and full coverage of vegetation and they will provide the choice and design of remote sensing view angle and the application of vegetation indices with a theoretical reference. |
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