Optical calibration, bathymetry, water column correction and bottom typing of shallow marine areas, using passive remote sensing imageries

The deglinting process

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  • OVERCAST+++++++++++++++++
    • I suspect that
      • * slightly overcast skies is best (whitish sky, like in the MiddleEast)
      • * very clear deep-blue sky is worst (you get something like rainbowed discoloration) like in your Kauai image

  • WHITECAPS+++++++++++++
    • I'm not sure about whitecaps and boat wakes

  • CLOUDS/HAZE/ADJACENCY_EFFECT+++++++++++++
    • Clouds and haze,
    • and adjacency effect underr overcast skies, 
    • would appear to have spectral properties similar to skyglint

  • OBLIQUE+++++++++++++++
    • to minimize glint, I want to view from Nadir to away from the sun,
    • NOT into the sun!!!!!!!!!

 

Cloud and its shadow
at Caicos Bank

Inter-band correlation is very strong.

 Glint and Cloud at Tarawa
part of the cloud has been added
to the scatter plot


Sea-surface glint,
and haze that surrounds the cloud,
seem to exhibit very similar correlation here,
as they both mostly tell us
about Mie scattering in the atmosphere.
They both may be removed
through the same "deglinting" process.

Sea-surface glint
is fairly strong

in this wind-swept SPOT image
Inter-band correlation is reasonable.
Smoothing can greatly improve this correlation.

Deglinting at Tarawa removes

  • specular reflection at the sea surface :
    • the sky glint from the sky dome
    • the sun glint from direct sun light
  • isolated lumps of haze,
  • and the haze that surrounds the clouds
 


<== Here a rectangular window has been left untouched
in order to illustrate
the -E extraction window feature of 4SM.
Deglinting blandly transfers the noise
from the NIR band into other bands!

   
  • Upper : modeling on raw data
    • note the presence of a faint lump of haze
    • and the corresponding artifact in the form of underestimated depth and bottom brightness
Deglinting at Caicos Bank

  • Lower : modeling on deglinted data
    • see how deglinting has removed the haze (while its shadow is still there)
    • and the corresponding improvement in the final result
 
 

Deglinting at Lee Stocking Island, Bahamas (Ikonos)

 

  • CENTRAL : raw data
  • BACKDROP : smoothed-deglinted data
   

 

 

Deglinting at MahoneBay,
Nova Scottia, Canada (CASI)
  • Sunglint affect this area.
    • poor correlation between the Nir band ond other bands
    • deglinting based on the red bands over this fairly deep area.
  • Removing the sunglint also removes the skyglint.
  • The profile compares
    • RAW in blue
    • DEGLINTED in red
    • Smart-Smoothed in green
    • FINAL in black

 

Smart-Smoothing
preserves the edges
at MahoneBay, Nova Scottia, Canada (CASI)

 

  • No sunglint here
  • Smart-Smoothing preserves the edges of the dark and shallower patch
  • The profile compares
    • RAW in blue
    • DEGLINTED in red
    • Smart-Smoothed in green
    • FINAL in black
   

Deglinting of a CASI mosaic
at Gezirat Siyul Reef, Red Sea

   
 
 

This  image preparation is done
"on the fly" while modeling,
as 4SM reads the raw U8 or U16 data
and applies all the required improvements
on a pixel wise basis prior to modeling.

 

Radiometric improvements:
The Delimbing/Deglinting process
on a 512 pixels wide raw CASI flight line


from bottom to top:

  • Raw data: the limb brightening is very obvious: it must be removed prior to geocoding of the image
  • Delimbed/Derowed: note that in spite of the derowing of individual rows, there remains groups of adjacent rows which are slightly brighter or darker: this shows how important for shallow water work that the imaging instrument be optimally tuned
  • Delimbed/Derowed/Deglinted/Smart-smoothed
  • Delimbed/Derowed/Deglinted/NOT-smoothed: in spite of all efforts, there remains all sorts of noise features and also remnants of imperfect deglinting: therefore it is very important that the data be smoothed prior to modeling
     
   




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