Optical calibration, bathymetry, water column correction and bottom typing of shallow marine areas, using passive remote sensing imageries

Normalization of BOA reflectances

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Reflectances in %


Reflectances are dimensionless ratios.
Through the visible spectrum:


  • Reflectance of bright coral sand 
    • increases steadily from 0.28 to 0.65 
  • Reflectance of  dark red algae Sargassum
    • is stable at 0.02 from 400 to 500 nm
    • then peaks at 0.05 at 600 nm
    • then decreases to 0.02 at 670 nm
    • then rises sharply to 0.25 at 750 nm 


    Reflectance of terrigeneous sand at Mahares is ~1/3 that of coral sand

  • Reflectance of deep water exceeds that of dark seagrasses and algae below ~525 nm:


the negative bottom contrast case


Normalized reflectances in 4SM



dots: raw reflectances r in %
plain: normalized reflectances 

  • At all wavelengths in 4SM:
    • at the base of the atmosphere, 
    •  L=(DN-La)*200/(DN-La)bright beach
  • Color composite: through the normalization process,
    we typically want to  enhance
    the remote sensing image
    so that beach sands display as 
     shades of grey in any color composite.
  • Soil Line: then for all pairs of bandsthe slope of the Soil Line is 1.
  • Distortion: Because the reflectance of the reference-sand increases steadily through the visible spectrum, this normalization distorts ("morphs") the spectrum of spectrally contrasted bottom signatures. This might have consequences on the water column correction  process in 4SM.





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