|see FAQ Which water type?
- The performance in computed depth depends on
- the particular pairs of wavebands used,
- the wavelength used to represent each waveband,
- the radiometric quality of the imagery,
- the amount of turbidity in the water column,
- the viewing geometry, illumination conditions and weather conditions,
- the experience of the practitioner.
- Wavelength dependence
- From the blue start of the visible solar spectrum at 400 nm to the red end at 700 nm,
- the penetration of light decreases from several tens of meters to less than 4 meters over bright bottoms, BUT MUCH LESS over dark bottoms
- in respect of bathymetry modeling in very clear "blue waters", whether marine or inland.
- From 700 nm to 900 nm in the NIR region, the penetration of light decreases from ~4 meters to less than one meter over bright bottoms, BUT MUCH LESS over dark bottom
- in respect of bathymetry modeling in very clear "blue waters", whether marine or inland,
- from the blue start of the visible solar spectrum at 400 nm to the red end at 700 nm.
|Waveband pair dependence
- bathymetry modeling uses at least one pair of wavebands i and j in order to allow for "un-mixing" of bottom depth versus bottom brightness influences.
- ratio Ki/Kj of effective attenuation coefficients for that pair must be less than ~0.8.
- this means that certain pairs are not suitable, depending both on their wavelength and water turbidity.
| Water turbidity dependence
CAUTION: locally high water volume reflectance caused by suspended particles
- the shallow bottom must be detected through the water column!!
- shallow water modeling is for "Case 1 waters":
- in poorly productive clear waters, the penetration of light
- decreases moderately over the blue to green wavebands,
- decreases extensively over green to yellow wavebands,
- and decreases moderately over the yellow to red wavebands.
- in highly productive clear waters, as a consequence of the buildup of the content of dissolved organic matter (yellow substances),the penetration of light
- increases extensively over blue to green wavebands,
- reaches its maximum in green to yellow wavebands,
- and decreases again over the yellow to red wavebands.
- bathymetry modeling may not be performed in "Case 2 waters", i.e. where the content of the water column is high in suspended mineral particles.
- prevents bottom detection
- mimics bottom detection, very cunningly, therefore yield fancy shoaling depth artifacts
- Radiometric noise dependence:
- Signal/Noise ratio: bathymetry modeling is only possible where the Signal/Noise ratio is significant in the image data.
- Thresholds: therefore in 4SM a threshold is applied for each waveband, below which that waveband may not be used for modeling.
- Blue wavebands have a low S/N ratio,
- mainly because of high turbidity of the atmosphere
- and also because of poor performances of sensors in the blue domain.
- "calm and clear" slightly hazy: altogether the imagery to be used is best acquired at high altitude in "calm and clear" slightly hazy weather conditions, in order to minimize difficulties generated by a high level of sun glint at the water surface.
- Imaging conditions dependence:
- near-vertical viewing: the above-mentioned radiometric noise is minimized in near-vertical viewing conditions at high altitude on a calm and clear day.
- low-altitude airborne: this means that low-altitude airborne imagery collected using a large field of view imaging system
- may happen to be unusable away from the central part of the image,
- unless corrected using an adequate radiometric pre-processing correction scheme
- actually succeeds in removing limb brightening and sky/sun glint.