## 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.