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 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 LC80090452014088LGN00, March 29th  2014
Work done august 2016
home

This is early spring time in the Bahamas

PAN solution  safer than  RED solution





DTM seatruth  RMSE=0.63 m
  
ZDTM - Z4SM
1.20 m tide correction applied to ZDTM

see legend
     
Seatruth regression
ZDTM vs Z4SM
1.20 m tide correction applied to ZDTM

COARSE

DTM is very coarse!
There is nothing like a
MBES or LIDAR DTM for seatruthing

Retrieved depth
see legend



Data and Deglinting

TOA TCC: raw image
linear enhancement

BOA TCC deglinted image
linear enhancement

 




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=0.51 => Jerlov water type OIB+0.37


 

Calibration diagram for the whole scene
for bands Coastal, Blue, PAN and RED



 
Reflectance (0-1)
of the path radiance Ra for this scene 
0.113 0.087 0.048 0.044 0.033 0.018 0.007


 
Reflectance (0-1)
of the water volume Rw for this scene
0.012 0.008 0.002 0.001 0.0 0.0 0.0



Now ready for Modeling
No smoothing applied
PAN solution applied

BOA TCC water column corrected
scaled 0-250, Linear enhancement
Rosy areas denote excess radiance in the RED band
  • is this caused by some "whiting"?
  • or, more likely, is this caused by local turbulence?

BOA TCC deglinted image
scaled 0-250
Same linear enhancement







 




PAN solution  safer than  RED solution

BOA TCC water column corrected
shows location of Profile_White
from West to East:
  • section A: fake bottom detection in the RED band accross a ~9 m deep trough: see waypoint A1
  • section B: local depth underestimation (~1 m) accross the 6-7 m deep channel
  • section C: correct depth retrieval by all three GREEN, PAN and RED solutions over a low lying dark mound over green bottoms. Some more fine-tuning might be in order
  • section D: PAN solution is just as good as RED solution at very shallow depth
This gives an insight into the advantages of the PAN solution against the RED solution
  • The RED solution is quite sensitive to any hint of local increase of content in suspended particles
    • much more than the PAN solution
  • Such local increase of water volume reflectance causes underestimated depth retrievals in 4SM
    • this can easily result in fake "shallow" bottom detection, where the bottom is actually optically deep:
    • see for example  Arcachon tutorial
  • I suppose an analytical approach would correct for that?
  • But is clear that PAN and GREEN bands are much less affected
    • this is demonstrated in the plots below

Profile_white:
GREEN and PAN solutions
yield approx the same depth

Profile_white:
PAN and RED solutions
RED solution yields an underestimated depth,
caused by the excess radiance in the RED band
  • see at waypoint A1 on section A
  • see from 0 to 2.5 km on section B




Bottom typing
      
Classified image
This is end March 2014

see legend for SAM

ImageB: average bottom brightness
This is end March 2014
note exceptional brightness (blue)