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[PP1-3-4] Spectral Signature of Vegetation, Water, Soil

Vegetation, water and soil represent the most common cover types of Earth surface. Their reflectances in the VIS/NIR/SWIR spectral range, plotted against wavelength in the 0,4-2,5 micron, represent the most important (basic) spectral signatures for whatever application devoted to Earth surface study. Other spectral signatures (e.g. in emissivity) in the Thermal InfraRed range are particularly important to infer specific properties of Mineral and Rocks (ref. [PP1-3-5] Spectral Signature of Mineral and Rocks). In order to discriminate among such basic cover types, the (ref. [IP3-1-2-3]) NDVI (Normalized Difference Vegetation Index) is the most simple and powerful diagnostic tool in the VIS/NIR spectral range NDVI values ranging between the values -1 and +1, are higly positive for fully vegetated (up to NDVI=1) or partly vegetated (NDVI>0,3) targets, still positive (>0) for bare soils, negative for water bodies. Values around zero are expected for clouds thanks to their similarly high reflectances both in the NIR and VIR spectral bands (ref. [PP1-3-6] Spectral Signature of Clouds).   Vegetation. a) in the visible range most of the incomig radiation is adsorbed by the photosynthetic process, transmittance is very low. The residual reflected radiation has a small peak of reflectance around 0.5 microns which is responsible of the green colour associated to vegetation by the human vision sytem (limited to the VIS spectral range); b) in the NIR range vegetation exhibits its higher reflectance together its higher transmittance (very low absorbance) so that leaf density can be estimated thanks to the the contributes (decreasing with depth) of underlaying leaf layers; c) in the SWIR spectral range (in particular in the water bands around 1,4 and 1,9 microns) it is possible to appreciate the vegetation water content. As much it is, as more incident radiation is absorbed and less is the reflected fraction of radiation. Bare Soil. Spectral reflectance is normally increasing moving from the VIS to the SWIR spectral region. Water features around 1,4 and 1,9 microns give information on soil water content (see before). Others specific features are described in [PP1-3-5] Spectral Signature of Mineral and Rocks Water. Spectral reflectance of clean deep water is quite low reaching quickly the zero value as soon as wavelengths passe  microns. However it is important to note that such a very low reflectance is due to a very high transmittance in the VIS range and to a very high absorbance in the NIR/SWIR regions (ref. [PP2-2-5-2] Attenuation Lenght and Penetration Depth). This means that water is quite transparent in the VIS spectral range (so that, in case of shallow waters, measured reflected radiance can be significantly increased by the contribution of bottom of the sea). Water is completely opaque, instead, in the NIR/SWIR. In this spectral region, even in presence of shallow waters, the presence of suspended matter (that increases the measured reflectance both in the VIS and NIR/SWIR ranges) can be better discriminated (than in the VIS) from the contribute of the bottom of the sea that, in this spectral range, is zero.

Explanation

NDVI%3D%5Cfrac%7BR_%7BNIR%7D-R_%7BVIS%7D%7D%20%7BR_%7BNIR%7D%2B%0AR_%7BVIS%7D%7D
Normalized Difference Vegetation Index

 

Spectral signatures (in reflectance)
of Water, Vegetation and Soils

 

 

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