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NDVI Index
What is the NDVI index?
The Normalized Difference Vegetation Index (NDVI), is an index used to estimate the quantity, quality, and development of crops. It is obtained through the processing of satellite images, mathematically comparing the amount of visible red light absorbed and the near infrared light reflected by the plants.
How is NDVI calculated?
The chlorophyll pigment in a healthy plant absorbs most of the visible red light, while the cell structure of a plant reflects most of the near-infrared light. This means that a high photosynthetic activity, commonly associated with dense and vigorous vegetation (left), absorbs most of the visible light and reflects a large part of the near-infrared light. Less vigorous vegetation (right) reflects more visible and less near-infrared light.
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What is it used for?
The NDVI index has multiple applications in agriculture:
- Monitoring the crop throughout its growth cycle.
- Monitor and predict agricultural production, by estimating yields and early detection of possible anomalies during the cycle.
- Identify and quantify multiple incidents that can suffer the crops, such as failures in sowing and tillage, weeds control, attacks by pests, damage by phytotoxicity, hail, fires, etc.
- Based on the regular monitoring of the fields, it is possible to make decisions on variable rate applications of different products such as herbicides, insecticides, fungicides, fertilizers, etc, to make each process more efficient.
Symbology
The NDVI varies between -1.0 and +1.0.
- Negative values (black and brown) correspond to areas with water surfaces, artificial structures, rocks and snow.
- Bare soil generally is within the range of 0.1 to 0.2.
Plants will always have positive values between 0.2 and 1. Most healthy and dense vegetation should be above 0.5, and sparse vegetation will probably fall within 0.2 to 0.5.
However, it is only a general rule and you should always take into account the season, plant type and regional characteristics to know exactly what the NDVI values mean.
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Evolution of the NDVI of a field during a production cycle.
The sequence of images below shows how the NDVI reflects the crop changes during the entire cycle of a soybean field, from pre-planting to pre-harvest.
If you want to know more about the combination of spectral bands and the calculation of the NDVI index, you can watch this video.