WHICH WAVEBANDS?

NIR : the availability of a Near InfraRed waveband is highly desirable in bathymetry modeling.
 

• A NIR waveband hardly contributes to the results because sunlight only penetrates down to a few decimeters in water.
• Nonetheless, a NIR waveband is extremely useful
  • in the preliminary search for adequate modeling parameters (dark pixel subtraction, deglinting),
  • for providing an alternate seed value for spectral K if desired,
  • and of course for precise delineation of the waterline.

• Visible : besides the NIR waveband(s), if any, the multi/hyperspectral imagery is comprised of N wavebands in the visible part of the solar spectrum, i. e. from 400 to 700 nanometers.

• Separation of colors:
  • Strictly speaking, bathymetry modeling refers to the dual estimation of the bottom's depth and spectral reflectance at one particular shallow water pixel.
  • Bathymetry modeling is only possible if the pixel considered exhibits bottom detection in at least one pair of wavebands i and j which happens to show a value of the ratio K i/K j less than ~0.8: this is the concept of separation of colors.

• Three cases : therefore, in bathymetry modeling, three cases are to be distinguished:
  • The N-BANDS case is where modeling is possible for at least three pairs of wavebands, like when using the Blue/Green/Red bands of Landsat imagery.
    • this yields a water column corrected spectral bottom reflectance
    • and a retrieved depth which may be slightly biased to the extent that the actual bottom signature is contrasted.
    • for example, for a BGRN image, the "red solution"  uses Blue and Green against Red, and the "green solution" only uses Blue against Green
    • if a Panchromatic band is available, the "PAN solution" uses Blue and Green against PAN
  • The 2-BANDS case is where modeling is possible for only one pair of wavebands, like when using the Green/Red bands of SPOT XS imagery.
    • this yields an average bottom reflectance
    • and a retrieved depth which may be severely biased to the extent that the actual bottom signature is contrasted.
  • The 1-BAND case is where modeling may not be performed in spite of the fact that the bottom is detected by at least one waveband i.
    • if the attenuation coefficient Ki is known,
    • if the bottom reflectance LBi is known over homogeneous bottoms,
    • then a good a depth may be retrieved.