What is NDVI (Normalized Difference Vegetation Index)?

Normalized Difference Vegetation Index (NDVI) quantifies vegetation by measuring the difference between near-infrared (which vegetation strongly reflects) and red light (which vegetation absorbs).

NDVI always ranges from -1 to +1. But there isn’t a distinct boundary for each type of land cover. For example, when you have negative values, it’s highly likely that it’s water. On the other hand, if you have a NDVI value close to +1, there’s a high possibility that it’s dense green leaves. But when NDVI is close to zero, there isn’t green leaves and it could even be an urbanized area.

Healthy vegetation (chlorophyll) reflects more near-infrared (NIR) and green light compared to other wavelengths. But it absorbs more red and blue light.

This is why our eyes see vegetation as the color green. If you could see near-infrared, then it would be strong for vegetation too. Satellite sensors like Landsat and Sentinel-2 both have the necessary bands with NIR and red.

Overall, NDVI is a standardized way to measure healthy vegetation. When you have high NDVI values, you have healthier vegetation. When you have low NDVI, you have less or no vegetation.

What is EVI (Enhanced vegetation index)?

The enhanced vegetation index (EVI) is an 'optimized' vegetation index designed to enhance the vegetation signal with improved sensitivity in high biomass regions and improved vegetation monitoring through a de-coupling of the canopy background signal and a reduction in atmosphere influences.

Whereas the Normalized Difference Vegetation Index (NDVI) is chlorophyll sensitive, the EVI is more responsive to canopy structural variations, including leaf area index (LAI), canopy type, plant physiognomy, and canopy architecture. The two vegetation indices complement each other in global vegetation studies and improve upon the detection of vegetation changes and extraction of canopy biophysical parameters. .

Another difference between Normalized Difference Vegetation Index (NDVI) and EVI is that in the presence of snow, NDVI decreases, while EVI increases.

Most of the rangeland applications of EVI have, to date, been regional-scale to global-scale assessments of rangeland parameters. EVI has mostly been used for assessments of biomass, biophysical properties like leaf area index, quantification of evaoptranspiration or water-use efficience, or assessments of change over large areas. In addition to the citations below, SatLander includes a standard EVI component in it's web-based rangeland assessment tools.