NDVI is an English acronym meaning Normalized Difference Vegetation Index, which could be translated as a Normalized Vegetation Index. This index reflects the quantity of so-called “photosynthetically active” biomass. It is calculated according to the formula:
Where R and PIR are respectively reflectances measured in the red and near infrared spectral bands. The higher the amount of chlorophyll (giving the green color to the leaves), the greater the PIR-R difference. The NDVI is a measure between -1 and +1. For example, the value of snow or water is close to -1 (PIR is less reflective than R) and that of plant cover is positive (Hall et al., 2002).
The NDVI index came into being with the launch of Landsat satellites in the 1970’s. Initially used to discriminate clouds and vegetation from other targets, it was quickly oriented to monitoring the evolution of vegetation during the season. This index is still today measured by satellites but since then, new vectors have emerged. One can think about drones but one should not forget planes or other machines performing measurement directly on the ground. The use of NDVI in agriculture and especially in cereal farming is now common or even routine.
Following numerous publications demonstrating the capacity of this measurement to help and support winegrowers in their technical decision-making, wine-growing areas covered by NDVI via remote sensing increases each year in the world. In 2016, 13% of the area planted in cereals and 1% of the wine-growing area were flown in France, representing 1.1 million hectares. This land area is covered mainly by satellite (85% of the volume) and drones and aircraft (15%). In the context of its growing adoption, it seems important to recall the basics of the NDVI measurement in order to help vintners in understanding better this tool and thus optimizing its use.
Satellite NDVI images from the Meris program
We will discuss in a next article what are the limits of the measurement according to the devices used for the acquisition. We will also suggest a few questions you could ask to your NDVI providers to evaluate the quality of their service.