Shallow Water Bathymetry of Singapore’s Highly Turbid Coastal Waters: A Comparative Approach

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

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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