What is NDVI?
Normalized Difference Vegetation Index (NDVI) is a ratio of the red light to near-infrared light that our sensor sees. 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 sensor. Both leaf area and photosynthetic activity increase NDVI values. Normally, these two values are conflated into a measure of “vigor,” but basically it is a measure of chlorophyll density from whatever cause.
TerrAvion NDVI images
TerrAvion offers 10 cm and 18 cm per pixel resolution NDVI imagery to our customers. Users have a number of options for how they would like to display and view their NDVI data. In our desktop app, TerrAvion has an adjustable NDVI color map tool, as well as a variety of color maps. Both of these features allow users to customize the way their NDVI data is illustrated, allowing them to focus in on certain NDVI ranges that correlate to specific conditions or issues in their fields.
Standard and dynamic vigor map layers
TerrAvion offers two NDVI products in our desktop app, a Standard Vigor and a Dynamic Vigor map layer. The Standard Vigor map layer displays NDVI data with a fixed color scale. This means that when you look at different fields or different image dates, the color scale stays fixed. With the Standard Vigor map layer, the same colors will always represent the same NDVI values. This product is great for showing temporal changes when monitoring fields throughout the season. The downside of the Standard Vigor layer is that, at various points in the season when little vigor variation is evident, like when a crop reaches full maturity, the Standard Vigor layer doesn’t display enough detail. Below is a Standard NDVI image of early emergence in a soybean field. The brighter shades of red indicate areas of early crop emergence. While some variation is evident in the image, this image doesn’t offer much information.
The Dynamic Vigor map layer has an adjustable color scale that automatically adjusts to fit the range of NDVI values for any single image. Adjusting the range of the NDVI color map to fit only the present range of NDVI values for any single image displays data with a wider spectrum of color which results in an image that shows much more detail.
Below are the Standard Vigor layer and color map from the above example.
Now look below at the same field and the same NDVI data displayed with the Dynamic Vigor layer and the auto-adjusted color scale. You can see how much more detail is pulled out, and the emerging crop is now very evident.
TerrAvion offers the Standard and Dynamic Vigor map layers so users can view their NDVI data with fixed scale (the Standard Vigor), allowing them to analyze temporal changes in their fields while the Dynamic Vigor layer with adjusted scale is the tool to pull out great detail in any single image. The Dynamic Vigor map automatically adjusts its scale. It is important to note that the actual NDVI numeric value for a particular image will remain fixed whether Standard Vigor or Dynamic Vigor map layers are shown; the difference in appearance between the Standard Vigor and Dynamic Vigor is based only on the way we are showing the NDVI value .
In addition to the Standard and Dynamic Vigor map layers, TerrAvion also offers over forty color maps to choose from. These color maps are color scales that relate different colors to the NDVI values. The different arrangements of colors in these color scales offer varying levels of contrast in an NDVI image. This is useful for users who are focusing on a certain situation in their fields that correlates to a range of NDVI values. For instance, the example below shows a soybean field with a white mold infestation. You can see how the different color maps offer distinctive perspectives on the same issue. This feature is particularly useful for users who are trying to single out a certain issue that correlates to a small range of NDVI values, making it hard to distinguish from healthy plants.
TerrAvion mobile app
Our mobile app currently offers a dynamic Vigor map layer for NDVI values that is just called Vigor. Our mobile app is available in both the Google and Apple app store.
What is your favorite color map to use and what do you use it for? Let us know in the comments below!
For more information on TerrAvion NDVI products, visit our website at www.TerrAvion.com/NDVI.