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


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
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

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