3.2.3 - The red edge

The red edge is a region in the red-NIR transition zone of vegetation reflectance spectrum and marks the boundary between absorption by chlorophyll in the red visible region, and scattering due to leaf internal structure in the NIR region. This transition zone is in the basis of several vegetation indices like NDVI which is the normalized difference between the reflectance in the red visible (0.6Ám) and the NIR (0.8Ám) reflectance. Also the red edge position (REP) is used to estimate the chlorophyll content of leaves or over a canopy.

Figure 3.6 – Reflectance spectrum for green grass, for the transition zone between VIS and NIR wavelengths- the Red Edge

The concept of Red Edge is on the basis of the most widely used vegetation index - the Normalized Difference Vegetation Index (NDVI). This index is the normalized difference between the NIR and the red VIS reflectance.

Where ρ NIR and ρred are the reflectance values for NIR and VIS (red) bands, respectively.

This combination of the strong absorption in the red visible and strong reflectance in the NIR is very specific of vegetation and allow us to distinguished vegetation from bare soil (see slide Figs 3.1. and 3.2.). Calculations of NDVI for a given pixel can always result in a number that ranges from -1 to +1; however, for natural surfaces NDVI values are within the 0 to +1 range. An NDVI close to 0 corresponds to no vegetation, while NDVI close to +1 (0.8 - 0.9) indicates the highest possible density of green leaves. NDVI can be computed at TOA or at the surface, i. e. using atmospherically corrected reflectances for NIR and Red bands.

Figure 3.7 – NDVI daily composite of SEVIRI full disk, June 15, 2008 (extracted from Yu,Y. et al. Development of Vegetation Products for U.S. GOES-R Satellite Mission, Presented on 4th Global Vegetation Workshop Univ. of Montana, Missoula, June 16-19 2009 )