Shallow Water Bathymetry of Singapore’s Highly Turbid Coastal Waters: A Comparative Approach James F. Bramante, Durairaju Kumaran Raju, Sin Tsai Min Tropical Marine Science Institute, National University of Singapore
Jan 10, 2016
Shallow Water Bathymetry of Singapore’s Highly Turbid Coastal Waters: A Comparative Approach
James F. Bramante, Durairaju Kumaran Raju, Sin Tsai Min
Tropical Marine Science Institute, National University of Singapore
Purpose
• Determine effectiveness of multispectral algorithms in Singapore
• Determine how extra 4 bands may help
• Develop high resolution shallow-water bathymetric map
• Coral/Benthic Surveys• Interface into more complicated IOP models• Determine possible new benthic habitats
Study Area
Study Area (cont.)
Marine EnvironmentStudy Area (cont.)
Wild Singapore
Seagrass-watch
Pulau Hantu
Obstacles
• High turbidity
• Sediment plumes
• Few bathymetric data points in shallow waters
Obstacles (cont.)
• High shipping traffic
• Abundant clouds
• Mixed aerosols from city and ocean
Atmospheric Correction• Stock image (no concurrent field measurements)
• Access to atmospheric information limited
• Clear boundaries for cloud, shadowed, and deep ocean pixels
Atmospheric Correction (cont.)
• Cloud-shadow empirical algorithm• Reinersman et al. (1998) and Lee et al. (2005)
Fig. 1 taken from Reinersman et al. (1998)
Atmospheric Correction (cont.)
Dws
pathtw t
LLL
• General Equation:
• Cloud-Shadow Eq:
• Assumptions:
• Lee et al. simplification:
shpathnepath LL
nersdnewsnepathne REtLL
shrsdiffshwsshpathsh REtLL
shwsnews tt shrsners RR
Atmospheric Correction (cont.)
d
diff
shnenepath
E
ELL
LL
1
)(• Path radiance:
• Reflectance:
• Water-air boundary correction:
rsCloudpathcloud
pathtrs R
LL
LLR
)(
rs
rsrs r
rR
562.11
518.0
Atmospheric Correction (cont.)
0.05
0.1
0.15
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0.05
0.1
0.15
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0
0.005
0.01
0.015
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0.045
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0
0.005
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0.015
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0.025
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0.045
0.05
Band 2 (Blue) Average Radiance
Band 2 Atmospherically Corrected Reflectance
Bathymetry Algorithms
• LUT Classification
• Linear Ratio Algorithm (Stumpf et al. 2003)
• Linear Band Algorithm (Lyzenga et al. 2006)
• Compared results using conventional 4 bands and Worldview-2’s 8 bands
Bathymetry Algorithms (cont.)
• LUT Classification• LUT Library n = 53 for 0 < depth ≤ 2 m• Least squares comparison• Attempted ratio classification
8-band 4-band
Bathymetry Algorithms (cont.)
• Linear Ratio• Based off of Beer’s law:
• Stumpf et al. 2003 : )ln(
)ln(ˆ1
rsHI
rsLOo nr
nrhhh
hWBWrs errrr )(
Bathymetry Algorithms (cont.)
• Linear Band• Lyzenga et al. 2006 :
• Non-real results when LWCj > Lj
)ln( WCjjj LLX
N
j jjo Xhhh1
ˆ
Results
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2Lyzenga et al. Algorithm Band Classification
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Results (cont.)
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Lyzenga et al. Algorithm Band Classification
Results – Platform Comparison
Results – Faulty Relationships
Conclusions
• With more validation, Lyzenga et al. model and band classifications may prove useful in turbid waters
• Assumed relationship between band absorption and depth must be re-examined in extremely turbid waters
Further Investigations• Evaluated cloud-shadow atmospheric correction model against RT Model; former was validated and did not affect results much
• Attempting to use water-column index to adjust for water mass variation in Lyzenga algorithm
• Using spectroradiometer to modify semi-analytical models for Singapore