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 LC80090452015299LGN00
Oct 10th 2015
Work done july 2016
home



DTM seatruth RMSE=0.56 m
  
Z4SM vs ZDTM+1.5m
  
ZDTM+1.5m and Z4SM
  
ZDTM+1.5m-Z4SM
see legend

Z_final=Z4SM-1.5m
 
  • Spectral K is under satisfactory control
  • Tide correction 1.5 m is too much
    • Something is not under control
    • Likely the Soil Line model for the PAN band, i.e. the reference value of the reflectamce of Coral Sands





Data and Deglinting

TOA TCC: raw image
logaritmic enhancement

TOA TCC deglinted image
same logaritmic enhancement

 




Optical calibration
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=0.55 => Jerlov water type OIB+0.5

 

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

 
  • CoefK=0.0
  • excellent fit of the PAN BPL model
KBLUE/KRED   and  KGREEN/KRED

  • This is obtained by enforcing best fit of
    • KBLUE/KRED   and  KGREEN/KRED 
    • with the BPLpixels
  • This yields KBLUE/KGREEN=0.55
  • This in turn is done by way of the adjustement
    • of the  -LsM... and -CP... arguments
KBLUE/KGREEN

 
  • Assessing the ratio  KBLUE/KGREEN  from this image is far from obvious
  • Fortunately, assessing the ratios  KBLUE/KRED   and  KGREEN/KRED   proved to be straightforward
  • This solves the problem for the ratio  KBLUE/KGREEN 
Reflectance (0-1)
of brightest bottom substrate for this scene
Coastal=1  Blue=2  Green=3  PAN=4  Red=5  NIR=6   SWIR1=7
0.234     0.294   0.367     0.394  0.444  0.501   0.541
2K (m-1) for Jerlov water type OIB+0.5
diffuse attenuation coefficient
Coastal=1  Blue=2  Green=3  PAN=4  Red=5  NIR=6   SWIR1=7
0.101      0.100   0.183               0.751  4.58       
La: reflectance (0-1)
of atmospheric path radiance
0.102    0.077    0.046    0.038   0.025  0.011    0.004
Lw: water volume reflectance (0-1)
 
0.018      0.017   0.002    0.001  0.000  0.000  0.000

Vegetation Line

Vegetation line BLUE vs RED
==>La and Lw for the BLUE band

Vegetation Line

Vegetation line COASTAL vs RED
==>La and Lw for the COASTAL band






Now ready for Modeling
Smoothing applied
  
BOA TCC water column corrected
No enhancement

Retrieved depth in centimeters in decimeters
see legend for retrieved depth in cm
 

RED threshold=2.3
RED and PAN solutions were used here
A threshold of 2.3 was specified for the RED band,
in order to avoid local artifacts caused by turbidity

RED threshold=0.3
for a comparison
Lowering the RED threshold reveals
the full extent of RED bottom detection

Using only PAN solution
for a comparison

 

 




Bottom typing
  
SAM classified image
see legend for SAM
  
ImageB: average bottom brightness