Normalized Difference Vegetation Index

Many techniques have been developed to study quantitatively and qualitatively the status of the vegetation from satellite images.

To reduce the number of parameters present in multispectral measurements to one unique parameter, the Vegetation Indexes were developed. Vegetation Indexes are combinations of spectral channels, in such a way that it reflects the contribution of vegetation depending on the spectral response of an area, minimizing the contribution of other factors such as soil, lighting, atmosphere, etc.

All photosynthetic organisms have one or more pigments capable of absorbing the visible radiation that initiate the photosynthetic reactions. Two portions of the electromagnetic spectrum, the blue (430 nm) and red (60 nm) show the amount of energy absorbed by plants. On the other hand, the near infrared region (750-1100 nm) acts inversely. High absorption in the red and blue spectral regions, with a high reflectance in the near infrared region marks the spectral signature of all vegetation, and it has long been used as a way to differentiate between surfaces with and without vegetation.

Based on the reflectance difference that green vegetation displays between the visible region (20%) and the near infrared region (60%) of the electromagnetic spectrum, in channels 2 and 4 of the MSS images of the Landsat satellites, the Normalized Difference Vegetation Index (NDVI) has been obtained.

The NDVI is a nonlinear function which varies between -1 and +1 but is undefined when RED and NIR are zero. Only the positive values correspond with vegetated zones. The negative values, generated by a higher reflectance in the visible region than in the infrared region, are due to clouds, snow, bare soil and rock.

NDVI values vary with absorption of red light by plant chlorophyll and the reflection of infrared radiation by water-filled leaf cells. It is correlated with IPAR (Intercepted Photo-synthetically Active Radiation). In most cases (but not all) IPAR and hence NDVI is correlated with photosynthesis. Because photosynthesis occurs in the green parts of plant material the NDVI is normally used to indicate green vegetation cover.