The characteristics of the Earth’s surface are translated into the spectral signature caught by multispectral instrument on satellites. In the case of the Copernicus’s Sentinel-2 satellites, the equipped Multispectral instrument can catch information in each of the 13 bands it is sensible to. Beside bands 2, 3 and 4, which are respectively for the blue, green, and red band, the MSI comes with the aerosol band, and near infrared (NIR) to short wave infrared (SWIR) bands.
Crops and vegetation in general are in green tones when seen in true color. Nature is this way because photosynthetic activities associated with vegetation need the energy of the blue and red wavelengths. Green is thus not necessary and is most reflected.
Seen things in true color and identifying vegetation based on green patterns is one thing. But what if we could exploit the information stored in the other bands? The following graph shows the absorption rate of chlorophyll and we clearly see that the green wavelengths are, as expected, the least absorbed. Notice however the very poor absorption rate in the NIR area (> 700nm).
By following the logic, can we thus pretend the more photosynthetically active a plant is, the more this gap between the red and NIR is widened? Observations confirm and the NDVI (Normalized Differenced Vegetation Index) index is a very good indicator for plants’ health, vitality, the greenness and the amount of vegetation (not the biomass). NDVI is the expression of the contrast between the reflectance of the red and the NIR reflectance. The formula is the following
NDVI = (NIR – VIS) / (NIR + VIS)
And the values it takes ranges from -1 to 1. Healthy vegetation has values from 0.3 to 0.8. Free standing water is around 0. So what about “wet vegetation”? Water content tend to darken the objects. Soils are around 0.1 and 0.2.
The following Sentinel-2 image were taken the 08/09/2016 in Belgium. From the true color perspective, one can see where the crops and the bare lands are. But is it really easy to spot the most photosynthetically active crop fields? The NDVI index gives much more information about that
The most active field, the greenest in the NDVI image, looks like any other crop field in the true color image. Some other crops on the left image are not active in the NDVI image and they should maybe be looked at.
Quadratic is currently studying problems encountered in the agro-industrial sector and develops for its partners real-time monitoring tools and decision support tools, intended to help them manage their logistics. Beside searching ways to reduce costs, the company also helps to monitor crop fields using the products of the ESA and European Union’s Copernicus (Sentinel) programme. Quadratic is member of the Copernicus World Alliance, which aims the development of synergies between organizations active in the Copernicus programme.