A vegetation index is a numeric indicator that quantifies the density and health of vegetation cover and is calculated by mathematical algorithms using pixel values in satellite imagery. VIs commonly refer to spectral vegetation indices which are the spectral reflectance signatures collected by the spectral sensors revealing information about the state, biochemical composition, and structure of a leaf and canopy (Huete, 2012). There are also polarimetric indices, which use backscattering from co- and cross-polarization of SAR data. With the VIs we can map the health and density of the vegetation while correcting for disturbing effects of soil moisture, solar position, and other disturbances. VIs should be calibrated to the study area and vegetation type with field measurements. Moreover, certain VIs might not be the right choice for the type of vegetation you are studying.