Bathymetry and water column correction
LANDSAT 8 OLIP 
at Mulroy Bay, Ireland
Image courtesy of the U.S. Geological Survey
 
Using the Panchromatic band for water column correction
to derive water depth and spectral bottom signature:

Landsat 8 OLIP bandset used for this work
Purple=1Blue=2Green=3PAN=4Red=5NIR=6 and SWIR1=7

Work done july 2018
GSD 15 m
 
 
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HOW DEEP: Depends on what?

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 
  • 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.
CAUTION: locally high water volume reflectance caused by suspended 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.

 




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