Optical calibration, bathymetry, water column correction and bottom typing of shallow marine areas, using passive remote sensing imageries
A 1371*1160 IKONOS 4 bands image of Clipperton atoll
 
4SM licensekey is 103 Euros for this small image
return to 4SM Study Cases
Wavelengths at midresponse curve are 480, 551, 665 and 805 nm
Quite a challenge!
Strong sky and sun glints
Unfortunately, clouds are not masked
Confined and quiet waters inside the lagoon
Work updated in 2005

 


 
1 - NO NEED for field data, nor for atmospheric correction
2 - this is demonstrated in this website, using a variety of hyper/multi spectral data
 
Requirements are
1 - homogeneous water body and atmosphere
2 - some coverage of optically deep water
3 - some coverage of dry land
 
Problems are
1 - the precision on estimated depth is found wanting, because the noise-equivalent change in radiance  of accessible data is too high for shallow water column correction work 
2 - radiance data should be preprocessed by the provider at level 1 in order to improve S/N ratio
3 - exponential decay: the deeper/darker the bottom, the poorer the performances
 
So
I keep digging
until suitable data
become available
 


Raw image, BGR TCC
histeq enhancement



Raw image, full resolution
histeq enhancement



Deglinted image, BGR TCC
histeq enhancement
No smoothing over the deep ocean



Deglinted image, full resolution
histeq enhancement
No smoothing over the deep ocean



image_Z



image_B



image_LBS normalized


image_LBS
histeq enhancement

   

Blue vs Green
natural

Blue vs Green
linearized
   



Optical calibration
of outer slopes


Fairly straightforward, although 
2K[red]=0.38 at 665 nm is a physical nonsense,
therefore the range of computed depths is problematic
  • Wavelengths must be set so that all ratios Ki/Kj fit nicely the slopes observed in this calibration diagram
  • Kblue/Kgreen=0.762
  • Kblue/Kred and Kgreen/Kred are very easy to check
  • Quite likely WLgreen must be increased and WLred must be decreased

Optical calibration
inside the lagoon

Quite difficult, as the lagoon bottom substrates
would appear to be extremely dark

The following is adopted:
Jerlov water type Coastal 3+0.14
2Kblue~=0.54
2Kgreen~=0.38
Kblue/Kgreen~=1.42