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.