3.2. Generate of NDVI
Initially, we tried to analyze the health of the vegetation using the
temporal evolution of the vegetation index (NDVI: difference of
normalized vegetation index), this index was proposed by Rousse and
al., 1974. It is a simple combination of spectral bands without
compensation of ground effects, according to the following formula:
NDVI = (PIR - R)/(PIR + R)
It uses the following data: channel near infrared (PIR) and channel red
(R).
The values obtained ranging between - 1 and + 1, respectively, for the
absence and the presence of vegetation. This combination of the
standardization by the sum of the two bands tends to reduce the lighting
effects or slope on vegetation (Caloz and al., 2001). The author adds
that the NDVI is not correlated linearly to biomass density, it has an
exponential relationship with the density of green vegetation and
saturated in the presence of thick vegetation.