Optical calibration, bathymetry, water column correction and bottom typing of shallow marine areas, using passive remote sensing imageries
Bathymetry and water column correction
LANDSAT 8 OLIP 
at Caicos Bank, Bahamas
4018*4149, 30 m pixel size, UTM zone 18, downloaded from USGS
 
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

Please refer to Bora Bora and Sanaa
for use of the PANchromatic band 
and for water column correction
 


 
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 LC80090452014200LGN00
Sep 19th  2014
work done august 2014
DTM seatruth
home

This is summer time in the Bahamas




DTM seatruth   RMSE=0.73 m
Some conclusions
  • 0-10 m: retrieved depths are ~correct
  • > 10 m in the SE corner: the DTM is not suitable
    • this causes the branching out of the regression
    • retrieved depths are ~correct
  
ZDTM+0.5m  - Z4SM in centimeters
tide correction applied to ZDTM: 0.50 m

see legend
    
Z4SM vs ZDTM regression

The SE corner  of the DTM
has very few depth sounding:
most of them are derived from Landsat image (purple)

 


The SE corner  of the DTM
is quite coarse
and does not cover the whole site
(backdrop is retrieved depth from 
caicosoli-20140108)
 

Z4SM vs ZDTM along profile_green
tide correction applied

 
  • 0-10 m: the regression is correct
  • deeper: calibration is not under control
Quite clearly, the so-called clearer waters
in the SE corner of this image
is the cause of a bad artifact


see location of this profile

The conclusion is:
  • 0-10 m: retrieved depths are ~correct
  • > 10 m in the SE corner: the DTM is not suitable
    • this causes the branching out of the above regression
    • retrieved depths are ~correct
  • layered waters are confirmed

Heterogeneous waters 0-10-30 m

in the above calibration,
we assumed a layered water body
with apparent success for water column correction,
except that retrieved depths are over-estimated
in the 10-30 m depth range
  • 0-10 m:  KBLUEKGREEN=0.62
  • 10-30 m: progressively decreasing to 0.42
Black line: the 10 m retrieved isobath line is shown

Homogeneous waters 0-30 m

Ignoring the problem is not an option
as it yields extremely and unacceptable
water column corrected bottom brightness
over the SE corner of this image (flat blue tone)
  • 0-30 m:  KBLUEKGREEN=0.62
 
 






Data and Deglinting

TOA TCC: raw image
logarithmic enhancement
quite a complex atmosphere

BOA TCC deglinted image
logarithmic enhancement
glint regressions over genuine sea-surface clutter
are excellent

 





Optical calibration, optimized
16U data are scaled to allow for comfortable screen display

Calibration diagram for the whole scene
for bands Blue, Green, Red and NIR
KBLUE/KGREEN is modeled
decreasing
from 0.62 (0-10 m) to 0.42 at depth


This is done through three -CP... arguments

 

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

Calibration of the PAN band
still not under goodcontrol
This might be caused by
slight mis-registration of the PAN band
 
Reflectance (0-1)
of the path radiance for this scene
Ra    0.105 0.073 0.047 0.043 0.030 0.015 0.006

 
Reflectance (0-1)
of the water volume for this scene
Rw    0.011 0.015 0.003 0.001 0.000 0.000 0.000

 




Now ready for Modeling
Smoothing applied
Red and PAN solutions applied

BOA TCC water column corrected
scaled 0-250
no enhancement

 

Retrieved depth




 





Bottom typing
    
SAM classified image
This is September 2014

see legend for SAM

ImageB: average bottom brightness
This is September 2014