NDVI vs. NDRE: What’s the Difference?
NDVI vs. NDRE
Sentera’s two most popular crop index products are Normalized Difference Vegetation Index (NDVI) and Normalized Difference Red Edge (NDRE), also known simply as “Red Edge.” In this post, we’ll break down the differences in NDVI vs. NDRE.
Which One Is Right For Your Operation?
Naturally, one of the most common questions we receive is “which one should I use?” And our answer is usually “it depends.”
This post will talk through some of the differences in these two products, and factors to consider if you’re evaluating a sensor purchase.
Some Background On NDVI And NDRE
NDVI and NDRE are called “index products” because they’re both constructed from a combination of two distinct frequencies of light. NDVI is built up from a combination of visual red light and near-infrared (NIR) light. NDRE uses a combination of NIR light and a frequency band that sits on the transition region between visual red and NIR light — hence the name “Red Edge”.
NDVI and NDRE are both straightforward to measure and compute and are both relatively well-known. Lots of other mechanisms for combining band data and using other bands, such as visual blue or visual green, exist. Among other things, many of these other products attempt to address some of the same shortcomings we’ll talk about in this post. We build sensors for those too, and we’d like to acknowledge that all of these solutions exist, and we do support users who are generating these products and attempting to discover new ones. This post, however, is focused on the factors that differentiate NDVI from NDRE.
NDVI Is More Common
It has decades of history, starting with the original LANDSAT satellite system, where it was developed as a useful indicator of vegetative health.
At a simple level, NDVI correlates with chlorophyll, which in turn correlates with plant health. Most space-based and terrestrial platforms that are used for agriculture, forestry, and similar applications have visual Red and NIR detector hardware, so NDVI has become a well-known, widely-used product.
NDVI is a great general indicator of crop health, and now with drone platforms, NDVI measurements can take advantage of high-resolution imaging to establish measurements during earlier growth stages than were possible or economically feasible with satellite or manned aircraft.
But NDVI Isn’t Perfect For All Crops Or For All Growth Stages
The visual-band red content that is used in NDVI is absorbed strongly by the top of the plant canopy. This means that lower levels of the canopy don’t contribute much to an NDVI measurement. This impairs correlation of NDVI to things like leaf area index (LAI). This effect increases in plants with more layers of leaves, like tree canopies or later-stage corn.
In addition, in some permanent crops, grasses and cereal crops, or during later growth stages of certain row crops, chlorophyll content reaches a point at which NDVI “saturates” at the maximum NDVI value of 1.0. In these scenarios, variability in the crop is hard to detect with NDVI until an issue becomes severe enough to drop the NDVI value below saturation, which may be at a point at which damage has already occurred.
Red Edge Offers A Solution
Substitution of NDVI’s red band with NDRE’s red edge band provides a measurement that is not as strongly absorbed by just the topmost layers of leaves. NDRE can give better insight into permanent or later stage crops because it’s able to measure further down into the canopy.
NDRE is also less prone to saturation in the presence of dense vegetation, which might be encountered in pasture biomass estimation measurements. Therefore, NDRE can sometimes provide a better measurement of variability in an area that NDVI would simply measure as a uniform +1.0.
Making A Decision
Because so much historical data and modeling work is based on NDVI, it’s often the best place to start. If you’ve flown NDVI and have experienced issues with saturation or absorption like discussed above, NDRE might provide an answer.
If you’re dealing with thick permanent crops or other dense crops, think about starting off with NDRE right away. And, of course, using both is often the most ideal solution. A lot of our customers with crops that transition from seed to thick canopies in a single season make use of both NDVI and NDRE.
Sentera builds DJI-compatible NDVI and NDRE sensors at several price points. We offer two versions of our Single sensor that can do one or the other, and versions of our Double 4K sensors that can do both NDVI and NDRE at the same time. Some models of the Sentera Double 4K Skyport which is compatible with DJI’s M200 and M210 Enterprise equipment, can also do NDVI and NDRE simultaneously as well as capture Live NDVI video that is streamed to your device while your drone is still in the air.
With so much variability, based on crop selection and management strategies, the best resource for determining which products make the most sense for your operation is your agronomist or advisor. An expanding library of scientific literature can also offer crop and practices-specific information about remote sensing approaches, including NDVI and NDRE. And, Sentera’s scientists, engineers, and other customers can provide advice as well — please contact us.