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


 
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
 

scene LC80120442015080LGN00, March 21rst 2016
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
March 21rst 2015    vs   Jan 29th 2014
This is a test of the robustness of 4SM

ZR+0.70m - ZC
March 21rst 2015   -   Jan 29th 2014
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

BOA TCC deglinted image
logaritmmic enhancement

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

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



Calibration must account for yellow substances
this is done by decreasing Lsw and Lw for Coastal and Blue bands
16U data are scaled to allow for comfortable screen display

Calibration diagram for the whole scene
for bands Blue, Green, Red and NIR
  • 0-10 m:  KBLUE/KGREEN=0.58
    • OIB+0.5
    • this applies to the whole platform
  • 10-30 m: KBLUE/KGREEN= from 0.58  to 0.47
    • from OIB+0.5 to OIB+0.1

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

 
  • The fuzzyness of the BPL for the pair Coastal/PAN is caused by slight misregistration of the resampled PAN band




Ready for modeling
No Smoothing

BOA TCC: water column corrected
 

Retrieved depth
see legend
I had to slightly increase LswPAN
in order to ballpark retrieved depths
in the 9-12 m range as indicated
in the figure speaker_harris_fig_01.png

 



Bottom typing

SAM classified image
see legend for SAM

Average bottom brightness