What is a Vegetation Index?
A Vegetation Index (VI) is a spectral calculation that is done between two or more bands of the source data designed to enhance the contribution of vegetation properties and allow for comparisons of photosynthetic activity across your area of interest.
When the VI calculation takes place, it goes pixel by pixel through your entire dataset and runs a calculation on the spectral values of that pixel. This will assign a value representing some version of plant health for each pixel in the image. These values are then turned into the colorized map you see on screen when the calculation is complete.
VI’s allow you to see relative differences in the general plant health across your entire field. Because these are relative differences in plant health across your field it is important to apply only to the areas you are interested in. For an example if you include a gravel road in the bounds of your vegetation index run the lower side of your VI range is going to be thrown off by this non living material included in your area of interest.
The methodology behind these calculations are slightly different for each VI that we offer. Below we have reviewed each Vegetation Index applicable to Visual (RGB) data in PrecisionMapper.
Vegetation Indices for Visual Data
Visual data has 3 bands of the spectrum that can be used in the calculation of Vegetation Indices against the orthomosaic- Red, Green, and Blue.
Most plants are green because they contain a pigment called chlorophyll. Chlorophyll fuels the process of Photosynthesis in living plants by allowing them to absorb energy from light. Chlorophyll absorbs light in the red (long wavelength) and the blue (short wavelength) regions of the visible light spectrum. Green light is not absorbed but reflected, making the plant appear green.
For calculation of plant health values with Visual data the greenness of the vegetation is the most revealing property of the plant. Therefore each Vegetation Index for Visual data relies heavily on the greenness seen in the source imagery.
We have 3 vegetation indices that can be used with your Visual data.
- Visual NDVI- Visual Normalized Difference Vegetation Index
- GLI- Green Leaf Index
- VARI- Visual Atmospheric Resistance Index
|Supported Sensors||Visual (RGB)|
|Supported Resolution||20 cm/pixel or less|
|Estimated Processing Time||2 Hours or less|
|Outputs ||Georeferenced Image, PDF Map, KML|
Visual NDVI- Visual Normalized Difference Vegetation Index
The Visual NDVI algorithm is based from a vegetation index called NGRDI- Normalized Green Red Difference Index. In this algorithm the Red and Green bands of a visual image are utilized to calculate a Vegetation Index value. This vegetation index value is designed to detect differences in green canopy area. It leans heavily on the green color of a healthy plant.
The algorithm has been a tested indicator of chlorophyll content in several different crop types including Corn, Alfalfa, Soybean, and Wheat.
The difference between the green and red values of the image differentiates between plants and soil, and the sum normalizes for variations in light intensity between different images. The possible range of the Visual NDVI output is from -1.0 to 1.0
References- Tucker, C.J., 1979. Red and photographic infrared linear combinations for monitoring vegetation. Remote Sensing of Environment 8, 127–150.
GLI- Green Leaf Index
The Green Leaf Index is an algorithm developed around wheat that can distinguish living plants from soil and non-living matter in the field. Like the Visual NDVI it leans heavily on the green color of a healthy plant to calculate the index. Therefore, it can be used as an indicator of chlorophyll.
GLI = (2 x G - R - B) / (2 x G+R+B)
The difference between the GLI and the Visual NDVI is the use of the Blue band in the calculation. Results of the vegetation index will range between -1.0 and 1.0.
In most cases you can threshold the index near zero into two classes- Green leaves and non-living material. Negative values tend to be soil or non-living material. Positive values tend to be living plant material. However due to potential changes in light and environmental conditions in the field the threshold does not always lie directly at zero.
References- Louhaichi, M., Borman, M.M., Johnson, D.E., 2001. Spatially located platform and aerial photography for documentation of grazing impacts on wheat. Geocarto International 16, 65–70.
VARI- Visual Atmospheric Resistance Index
The Visual Atmospheric Resistance Index is a vegetative index that was originally designed for satellite imagery. It is found to be minimally sensitive to atmospheric effects, allowing the estimation of vegetation fraction in a wide range of atmospheric conditions.
VARI = (Green-Red)/(Green+Red-Blue)
As sunlight reaches the earth’s atmosphere it is scattered in all directions by the gasses and particles in the air. But blue light tends to scatter more than all the other colors because it travels in smaller wavelengths than the rest of the visual spectrum. Therefore, we see the sky as blue most of the time. By including the Blue band in the denominator of the VARI equation we are accounting for the effects of the atmosphere on this Vegetation Index calculation.
For each pixel VARI is calculating a ratio of green vegetation cover. Values can be anywhere from -1.0 and 1.0 in the final output.
References- Gitelson, A.A., Kaufman, Y.J., Stark, R., Rundquist, D., 2002. Novel algorithms for remote estimation of vegetation fraction. Remote Sensing of Environment 80, 76–87.