Normalized Difference Vegetation Index / NDVI
What is NDVI?
Normalized Difference Vegetation Index (NDVI) is a ratio of the red light to near-infrared light that TerrAvion sensors measure. Plants are uniquely efficient at reflecting light in a way that increases the value of this ratio. Higher NDVI values mean more chlorophyll is detected by the sensors. Both leaf area and photosynthetic activity increase NDVI values. Typically, these two values are conflated into a measure of “vigor,” but basically it is a measure of chlorophyll density from whatever cause.
How is NDVI data used?
As mentioned above, NDVI data correlates to plant vigor, and that is the basis for how it is most commonly used.. The differences in vigor on a field at one point or over time can show the effect of any input application, ranging from seed variations to fertilizer inputs, pesticide to growth regulators before it is visible to the naked eye. The data will show the effect that these inputs have on the growth of the plant. But is also used, for example, to calculate which amount of input is needed to optimize yield with on-field nitrogen tests. Or as the basis for creating variable rate maps to apply inputs were required for the amount that is needed. Our blog posts describe many use cases for NDVI imagery.
NDVI image data and sustainability
Soil health is at the roots for any agricultural success and therefore as well for sustainable agriculture. It is influenced by the use of cover crops, manure, and tillage practices (or the lack thereof). NDVI imagery is an ideal tool for evaluating the effectiveness of these practices.
Identifying the most productive input stack is another example of the usefulness of NDVI data for sustainable agriculture. Optimizing the inputs not only benefits the environment but growers and input sellers as well. More efficient management practices allow growers to do more with less and help input sellers advise their customers best. Read our blog post about aerial imagery and sustainability here.
Technical description of TerrAvion NDVI values
TerrAvion’s NDVI is translated to an 8-bit scale which has pixel values from 0-255, however our NDVI values do not span the whole range. 0 corresponds to the start of the useful range of NDVI values, not zero in a reflectance calculation and 255 is off the scale. The start of vigor, i.e. the sensor is seeing living growing chlorophyll is about 50 on our scale. Non-living materials will have values below 50 and plants that are growing will measure more based on how vigorous they appear to the sensor. Each NDVI value is the same vigor consistently throughout the season, this allows comparisons about photosynthetic potential between locations and through time.
TerrAvion multi-band technical specification
All channels are flat field corrected.
Channels 4, 5, 6 are normalized to channel 6.
NDVI is calculated from channels 4 and 6.
Color Infrared (CIR) is generated from channels 4, 5, and 6.
Buffer and alpha blending
Geometry buffer is the data TerrAvion puts in place on top of the geometry so that extra imagery is contained in the GeoTIFF, in case surrounding information is needed for analysis.
Alpha blending, also known as feather, is contained in the GeoTIFF, so that tiles derived from the GeoTIFF blend together well when there are overlapping fields.
I want to process my data in some other format, how do I do that?
You can roll your own scale and categorization. We provide the NDVI’s as downloadable palletized tif files that can be downloaded from the interface. These files are geo-registered in WGS 84 Web Mercator Auxiliary Sphere (EPSG: 3857) and they contain NDVI values from 0-255 in Band 1, with corresponding mapping to the NDVI function of that pixel. These files can be downloaded into GIS programs such as ArcGIS or QGIS. There you can stretch the NDVI histogram, classify, or run pixel math to unlock more information. If you are going to do this at scale, contact us about an API for the program you are using.
For more information, contact firstname.lastname@example.org