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 LC80120442014349LGN00, Dec 15th 2014
Work done october 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
Dec 15th 2014    vs   Jan 29th 2014
This is a test of the robustness of 4SM

ZR - ZC
Jan 29th 2014 - Dec 15th 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.
  • Spatial 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.

Low tide -0.70 m  on 
Dec 15th 2014 

Tide reference  on  Jan 20th 2014





Data

TOA TCC: raw image

TOA TCC: deglinted image


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 applied

BOA TCC: water column corrected
 

Retrieved depth
see legend

 







Bottom typing in december
vegetation is dormant, substrates are much brighter
just one spot of discoloured waters

SAM classified image
see legend for SAM

Average bottom brightness