Bathymetry and water column correction
LANDSAT 8 OLIP 
at Lee Stocking Island, Bahamas
Image courtesy of the U.S. Geological Survey
7671*7841 30 m pixel size, UTM zone 18


 
Using the Panchromatic band for water column correction
to derive water depth and spectral bottom signature:

Landsat 8 OLIP bandset used for this work
Purple=1Blue=2Green=3PAN=4Red=5NIR=6 and SWIR1=7

home
 
 
 
scene LC80120442015224LGN00, August 12th 2015
Work done september 2016



Robustness
As I don't have seatruth data, apart from the above speaker_harris_fig_01.png,
I used results obtained on 
lsiOLI_20140129 as a seatruth DTM
The result is astounding,
knowing that part of the fuzziness is caused
by slight misregistration of the PAN band.
No smoothing applied

Regression
August 8th 2015    vs   Jan 29th 2014
This is a test of the robustness of 4SM

ZR+0.70m - ZC
Jan 29th 2014    -   August 8th 2015
Discrepancies
Three specific sources of discrepancies must be considered
  • Cloud shadows: they cause over-estimation of retrieved depths. I could not find a way to avoid it.
  • Misregistration: the 30 m re-sampled PAN band is misregistrated by one pixel in both row and line. This causes fuzziness, and overestimated depths on occasion.
  • Variation of water optical properties: they cause local over -or under- estimation of retrieved depth
Robustness
The results are much more robust than I expected!
I intend to produce
a Combined Depth raster
 out of all these results.




Data and Deglinting

TOA TCC: raw image
logaritmmic enhancement
very dense and variable atmosphere

BOA TCC deglinted image
logaritmmic enhancement

No discolouration in august 2015
Some vegetation beds are not to be confused
with areas of discoloured waters

Strong discolouration in august 2015
Bottom features are seen
through
discoloured waters

Strong discolouration in august 2015
Bottom features are seen
through
discoloured waters
Such discoloured waters
should exhibit the following optical properties:
  • Increased KCOASTAL and KBLUE 
    • with KCOASTAL~=KBLUE ,
    • i.e. ratio KCOASTAL/KBLUE ~=1 
  • Decreased LwCOASTAL   and LwBLUE
--------------------------------------------------------------


BOA TCC deglinted:
low water volume reflectance
discoloured waters flow out towards the open waters
   
 
 



Calibration
16U data are scaled to allow for comfortable screen display

Calibration diagram for the whole scene
for bands Blue, Green, Red and NIR
 

Calibration diagram for the whole scene
for bands Coastal, PAN, Red, NIR
 





Ready for modeling
No Smoothing

BOA TCC: water column corrected
 

Retrieved depth
see legend

 





Bottom typing

SAM classified image
see legend for SAM

Average bottom brightness

 






















Work done in 2020: water optical properties:
mapping the ratio Kblue/Kgreen
 
UltraBlue band, BOA deglinted
  • this is mid-summer over the shallow Bahamas Bank
  • clearly shows that the water volume reflectance Lw in this band is heterogeneous and highly variable
  • this is only seen in Coastal and Blue bands, which exhibit highly variable water volume reflectance from oceanic to coastal waters, as a function of the production of yellow substances (colored dissolved organic matter)?
  Blue band, BOA deglinted
  • discolored waters with near-null Lw are seen flowing out south-eastwards away from the shallow platform
  • climax: areas with near-null remote sensing reflectance possibly represent seagrass beds 
 
 
Map of the ratio Kblue/Kgreen
  • it may be seen that these expansive shallow waters are extremely heterogeneous,
    • ranging from Jerlov's Oceanic water type IB
    • to Coastal water type 3 and beyond
Map of the ratio Kblue/Kgreen
  • blue to white tones for the range 0.55 to 0.80
  • white to red   tones for the range 0.8 0to 1.29
  • this is estimated in a very basic process, by spectral matching using a lookup table with the following three input parameters:
    • the effective two-ways spectral diffuse attenuation coefficients 2K, derived from Jerlov's optical classification of marine waters over the range 0.25 to 1.29
    • bottom depth Z over the range 0-30 m
    • normalized bottom brightness LB on a scale of 0-100
  • L=Lw+(LB-Lw)/exp(2K*Z)    
  • this is under strong influence of the water volume reflectance Lw 
    • dark blue areas are faulty, as they exceed the high red range limit which is Coastal type 3 of Jerlov
  • with a pixel-wise estimate of this ratio now available, the spectral diffuse attenuation coefficient is now used, pixel-wise, to derive the bottom's depth and spectral reflectance under improved conditions
    • although this process now needs some way to account for the spatial variations of the water volume reflectance Lw 
   



 



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