Jcn. Epiphanio et Ar. Huete, DEPENDENCE OF NDVI AND SAVI ON SUN SENSOR GEOMETRY AND ITS EFFECT ON FAPAR RELATIONSHIPS IN ALFALFA, Remote sensing of environment, 51(3), 1995, pp. 351-360
This article describes ihs impacts of sensor view and solar zenith ang
les on two vegetation indices - NDVI (normalized difference vegetation
index) and SAVI (soil adjusted vegetation index). An evaluation of th
ese geometric factors on the relationships between these VIs (vegetati
on indices) and fAPAR (fraction of photosynthetically active radiation
absorbed by the canopy) was performed. To accomplish this, an experim
ent was conducted in Phoenix, Arizona, over plots of bare soil and low
, medium, and high alfalfa Reflectances were measured from 0.4 mu m to
1.0 mu m in nine view angles (from - 40 degrees to + 40 degrees, in 1
0 degrees steps) over varying solar zenith angles. This was done simul
taneously with fAPAR measurements. Changes in view angle caused variat
ions 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, wit
h the former Increasing from antisolar to forward scattering view dire
ction. A derivative analysis of the indices showed the SAVI to exhibit
a more linear relationship than NDVI with the individual bands, The r
elationships between both Vis and fAPAR were, in general, linear. Howe
ver view angle variations perturbed these relationships and caused an
over- or underestimation of fAPAR, depending on view direction (antiso
lar or forward), view angle, and vegetation index (NDVI or SAVI).