The most used vegetation index is undoubtedly the NDVI (Normalized Difference Vegetation Index): it describes the vigour level of the crop and it is calculated as the ratio between the difference and the sum of the refracted radiations in the near infrared and in the red, that is as (NIR-RED)/(NIR+RED).
The interpretation of the absolute value of the NDVI is highly informative, as it allows the immediate recognition of the areas of the farm or field that have problems. The NDVI is a simple index to interpret: its values vary between -1 and 1, and each value corresponds to a different agronomic situation, regardless of the crop.
The average NDVI of a field varies according to the variety and phenological stage of the crop.
In a field, where a crop tends to be at the same phenological stage, areas with a significantly lower than average NDVI value present problems in vegetative development, for example:
- nutritional stress,
- parasitic attacks,
- hail damage or frost.
In order to identify the potentially problematic areas in each field, the minimum and maximum NDVI values are identified. Then the interval is divided into 5 parts, each corresponding to a different level of vegetative stress: the areas of the field where the NDVI is lowest are those in which the plant is most stressed.