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IMAGE WORTHINESS

For an image to be worthy of shallow water work,
a number of conditions must be met,
or the practitioner must face the consequences

 

Illumination conditions
must be constant through the whole scene

  • ILS correction of airborne flight lines must be applied
    • the main reason is that the incident sun light can vary by several percents from start to end of an airborne flight line as the sun height varies
    • another reason is to correct for transient sun light variations
    • this artifact is wavelength-dependent
  • Removal of limb brightening must be applied
    • the reason is that the path length increases markedly from vertical viewing to oblique viewing across the flight line
  • Atmospheric path radiance must be homogeneous
    • areas affected by haze must be masked out, or "deglinted"
    • if at all possible, areas affected by clouds and their shadows must be masked out.
    • atmospheric adjacency effect must be corrected for, if at all possible, but "deglinting" often achieves a good correction.

Optically deep waters must be available in the image

  • This is necessary for calibrating the glint correlations.

  • This is absolutely necessary for specifying the deep water radiance Lsw.

Phototaxis
  • Cyanobacteria tend to move up towards more light in the evening, and then to move down in the morning as illumination becomes too harsh.
  • This also depends on wind conditions.
  • Passive RS images are taken at a time when solar illumination is maximum.
  • This might be the cause of weird results in many atolls like Fakarava and  Tarawa

Sun glints (sea surface clutter) must be totally removed

  • Best is to discard images or areas which are too badly affected by sun glint
  • Sun glint and Skyglint can have distinctly different spectral properties
    • sunglint has the spectral properties of direct sun light, i.e. biased towards the yellow region of the solar spectrum
    • skyglint has the spectral properties of the sky light, i.e. usually biased towards the blue region of the solar spectrum for a clear blue sky
  • Best deglinting is achieved when the atmosphere is hazy,
    • as this strengthens the correlations among bands from NIR through BLUE regions (Mie scattering).

Resampling
should have been achieved by the nearest neighbor algorithm

Optical properties of water bodies

 




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