Frequently Asked Question
NDRE (Normalized Difference Red Edge)
NDRE is an index that can only be formulated when the Red edge band is available in a sensor. It is sensitive to chlorophyll content in leaves (how green a leaf appears), variability in leaf area, and soil background effects. High values of NDRE represent higher levels of leaf chlorophyll content than lower values. Soil typically has the lowest values, unhealthy plants have intermediate values, and healthy plants have the highest values. Consider using NDRE if you are interested in mapping variability in fertilizer requirements or foliar Nitrogen, not necessarily Nitrogen availability in the soil.
Chlorophyll has maximum absorption in the red waveband and therefore red light does not penetrate very far past a few leaf layers. On the other hand, light in the green and red-edge edge can penetrate a leaf much more deeply than blue or red light so a pure red-edge waveband will be more sensitive to medium to high levels of chlorophyll content, and hence leaf nitrogen, than a broad waveband that encompasses blue light, red light, or a mixture of visible and NIR light (e.g. a modified single-imager camera).
NDRE is a better indicator of vegetation health/vigor than NDVI for mid to late season crops that have accumulated high levels of chlorophyll in their leaves because red-edge light is more translucent to leaves than red light and so it is less likely to be completely absorbed by a canopy. It is more suitable than NDVI for intensive management applications throughout the growing season because NDVI often loses sensitivity after plants accumulate a critical level of leaf cover or chlorophyll content.
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