# The Azimuth Project Normalized difference vegetation index

## Normalized Difference Vegetation Index (NDVI)

The pigment in plant leaves, chlorophyll, strongly absorbs visible light (from 0.4 to 0.7 µm) for use in photosynthesis. The cell structure of the leaves, on the other hand, strongly reflects near-infrared light (from 0.7 to 1.1 µm).

Therefore, the proportion of reflected visible and near-infrared light gives an estimation of the amount of green (i.e. photosynthetically active) leaves in the vegetation.

The reflected visible and near-infrared light can be measured by remote sensing.

The NDVI is a function of the ratio of visible light and near-infrared light:

$NDVI=\frac{1-VIS/NIR}{1+VIS/NIR}.$

### NDVI and plant growth

In the late 1970s, scientists found that net photosynthesis is directly related to the amount of photosynthetically active radiation that plants absorb.

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The absorption of photosynthetically active radiation influences the measured NDVI, and thus NDVI is related to the growth of plants.

The difference between the average NDVI for a particular month of a given year (such as August 1993, above) and the average NDVI for the same month over the last 20 years is called NDVI anomaly.

In most climates, vegetation growth is limited by water so the relative density of vegetation is a good indicator of agricultural drought.

### Attention

The NDVI is a very simple satellite product obtained from only two spectral bands. Other factors, different from vegetation, can influence its value.

## Enhanced Vegetation Index (EVI)

While the EVI is calculated similarly to NDVI, it corrects for some distortions in the reflected light caused by the particles in the air as well as the ground cover below the vegetation. The EVI data product The difference between the average NDVI for a particular month of a given year (such as August 1993, above) and the average NDVI for the same month over the last 20 years is called NDVI anomaly. (Compare the August 1993 NDVI anomaly to August 1993 NDVI and Average August NDVI in North America.) In most climates, vegetation growth is limited by water so the relative density of vegetation is a good indicator of agricultural drought.also does not become saturated as easily as the NDVI when viewing rainforests and other areas of the Earth with large amounts of chlorophyll.

## Other satellite indices

A satellite index better suited for estimating water content is the Normalized difference water index.

## References

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