 Solution

Use a near-infrared/red ratio, or its normalized form, the normalized difference vegetation index:Useful bands are approximately $0.63$–$0.69\mu \mathrm{m}$ in red and $0.75$–$0.90\mu \mathrm{m}$ in near infrared, with exact ranges depending on the sensor. Chlorophyll absorbs strongly in red, whereas leaf structure scatters near infrared efficiently; healthy green vegetation therefore has a large near-infrared/red ratio and positive NDVI. Soil and water have different spectral contrasts.^[ref]

A single value is not a direct census of vegetation biomass or species. Soil background, canopy saturation, season, atmospheric effects and mixed pixels need consideration when mapping the distribution of vegetation.