NDVI - Vegetation Index
Normalized Difference Vegetation Index (NDVI) is calculated from the visible and near-infrared light reflected by vegetation. Healthy vegetation (left) absorbs most of the visible light that hits it, and reflects a large portion of the near-infrared light. Unhealthy or sparse vegetation (right) reflects more visible light and less near-infrared light. The numbers on the figure below are representative of actual values, but real vegetation is much more varied. Measuring Vegetation - NASA Earth Observatory
The coefficient constitutes a good measurement of the physiological activity of the plants; also, in empirical studies, it shows a good level correlation with the crops yields.
The NDVI is an index calculated based on the radiation of remote sensing reflection bands:
NIR and RED are the reflectance of the near-infrared and red bands; in the particular case of Landsat images, it corresponds to bands 4 and 3.
These bands are selected because of the spectral response of the vegetation, which has a high reflectance to the near-infrared band, while the red band is used as a reference, since it always keeps a near constant value.
The relationship between reflected, absorbed, and transmitted energy is used to determine spectral signatures of individual plants. Spectral signatures are unique to plant species. Spectral Signatures are the specific combination of reflected, absorbed, and transmitted energy at wavelengths which can uniquely identify a plant species; the following graph illustrates the different signatures for different crops
The NDVI varies between known parameters (-1, 1). This facilitates its clasification since values less than 0.2 the foliar activity is low, while values greater than 0.4 correspond to active levels of foliar activity.It has a direct relation with the quantity, quality and development of vegentation, and it correlates well with the yield of a given field in key dates.
Using NDVI for scouting maps and field monitoring
Scouting Maps allow agronomists and producers to identify problem areas, and to make timely decisions. Scouting Maps can be requested at key dates as a guidance for field visits. These maps are generated based on Landsat imagery, which have a revisit time of 16 days. Thus, there are predefined dates when NDVI for scouting may be available, provided that there are no cloud obstacles on the date of image capture.
NDVI based scout maps show variations in the field, so the user knows where to look in the field to determine where corrective or preventative measures are needed. using GeoAgro GIS or other GIS packages, users can plan their field visit locations, take it to their GPS or a printable pdf report, and accurately evaluate the reasons for in-field variability.
Since NDVI shows variation of biomass within a field and divides those differences into management zones, these maps help managers to make better scouting decisions and address those decisions with variable rate crop applications driven by variable rate prescriptions created with GeoAgro GIS.
Plant growth regulators, insecticides, herbicides, fungicides, mid-season fertilizers and harvest are valuable resources that can be effectively used to for better crop management while minimizing input costs.
Another approach of using NDVI maps is for field monitoring; this involves processing all suitable images (with no cloud cover) during the crop season, and detect possible anomalies during the season. Usually there would be between 4 and 6 images available for analysis during the season.
The following screenshots illustrate the NDVI images for different dates, for field monitoring purposes
- Remote Sensing Basics (extension.org)
- Precision Agriculture: Remote Sensing and Ground Truthing (University of Missoury Extension)