Dependence of NDVI and SAVI on sun/sensor geometry and its effect on fAPAR relationships in Alfalfa

This article describes the impacts of sensor view and solar zenith angles on two vegetation indices -NDVI (normalized difference vegetation index) and SAVI (soil adjusted vegetation index). An evaluation of these geometric factors on the relationships between these VIs (vegetation indices) and fAPAR (fraction of photosynthetically active radiation absorbed by the canopy) was performed. To accomplish this, an experiment was conducted in Phoenix, Arizona, over plots of bare soil and low, medium, and high alfalfa. Reflectances were measured from 0.4 μm to 1.0 μm in nine view angles (from -40° to + 40°, in 10° steps) over varying solar zenith angles. This was done simultaneously with fAPAR measurements. Changes in view angle caused variations in the indices to be as high as 50% in relation to nadir. However, there was an opposite view angle behavior between NDVI and SAVI, with the former increasing from antisolar to forward scattering view direction. A derivative analysis of the indices showed the SAVI to exhibit a more linear relationship than NDVI with the individual bands. The relationships between both VIs and fAPAR were, in general, linear. However, view angle variations perturbed these relationships and caused an over- or underestimation of fAPAR, depending on view direction (antisolar or forward), view angle, and vegetation index (NDVI or SAVI). © 1995

BIDIRECTIONAL REFLECTANCE FACTOR OF 14 SOIL CLASSES FROM BRAZIL

The spectral reflectance of soils is required for effective use of remote sensing products. The absence of studies concerned with spectral reflectance of the soils from the tropical region in the 400 to 2500 nm spectral range is the main motivation of this research. The objective of this study was to present spectral reflectance data from different tropical soil types. This spectral characterization was done through measurements of the hi-directional reflectance factor of III selected soil samples, grouped in 14 tropical soil classes, taken from 53 sites (Sao Paulo State, Brazil). The measurements were made with a spectroradiometer operating in the 400 to 2500 nm region of the electromagnetic spectrum. Each soil sample is associated to a set of physical and chemical analyses data, with part of these published in descriptive reports of soil surveys.16111312