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

A 894*1086 Landsat ETM 4 bands image of Alacranes atoll, Gulf of Mexico
 

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Work done in 2006
A case of weird radiometric and hydrologic properties
Unfortunately, the conclusion is: try another image!

please refer to:
Liceaga -Correa M. A. and Euan-Avila J. I.,
"Assessment of coral reef bathymetry mapping using visible Landsat Thematic Mapper data",
IJRS, 2002, vol. 23, No 1, pp 3-14


Please see also Chinchorro atoll



 
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, true color composite




Raw image, false color composite
histeq enhancement



Raw image, true color composite

histeq enhancement
Zoom on some weird hydrological patterns




Raw image, NIR band

histeq enhancement
High tide, very few dry
land
Gentle swell



W-E profile across deep waters

  • high level of atmospheric path radiance

  • high level of random noise

  • hardly any coherent glint: no evidence of swell-correlated glint

  • deglinting shall be impossible


W-E profile across deep inner lagoon

  • Note the change of vertical scale

  • Lsw[3] is two DNs higher inside the lagoon than over ocean: this is weird



image Z stepped

The computed depth results are much shallower
than the depths reported in the reference paper:

  • for instance, a depth range of 13 to 16 m is reported inside the deeper lagoon, where we obtain 8 to 11 m

  • this would need a CoefZ of 1.8, which is physically impossible when applied to the RED range of the spectrum




image B

This image suggests that the turbidity of the waters inside the lagoon would increase distinctly towards the deeper areas

  • where we should expect the lagoon bottom to be uniformely very bright, as is observed in the small SE troughs

  • whereas we see the average bottom reflectance getting progressively darker when going Northwards from South in the main lagoon body


image BSC



image LBS



Calibration diagram for the full image

  • Optical water type OII+0.4

  • Wavelengths used are 478, 577, 661 and 835 nm

  • Calibration appears to be quite reliable




Calibration diagram inside the lagoon,

  • Optical water type OII+0.4

  • Wavelengths used are 478, 577, 661 and 835 nm

  • Calibration appears to be quite reliable, but many deeper pixels have radiances in excess in the red band: this might be the result of the high level of system noise

  • all shallow pixels inside the lagoon area are shown in the bi-dimensional diagrams