Retrieval of coastal sea water parameters from high ...Landsat 7 ETM + true colour image Acquired:...

Retrieval of coastal sea water parameters from high resolution

satellite images

Alice Heng, Liew Soo Chin, Chang Chew Wai, Narvada Dewkurun

and He Jiangcheng

Supported by Agency for Science, Technology and Research (A*STAR), Singapore

22nd APAN Meeting in Singapore 17 Jul 2006

Introduction

Colour of coastal sea water is dependent on

–the absorption and scattering by water constituents:

• dissolved organic matter• chlorophyll• suspended sediments• water

– reflection from bottom

Landsat 7 ETM + true colour image

Acquired: 2000 Apr 28


SPOT 5, 2004 Jan 9SPOT 4, 2001 Jul 14

© CNES, 2001 © CNES, 2004

Image size is approximately 30 km by 30 km


The problem …

To quantify water properties from satellite image data• from the radiance measured by each spectra band

– compute/deduce the water properties– ‘water properties’ can be quantified by

• concentration of dissolved organic matter, suspended matter ,etc.

–or• inherent optical properties

SPOT 4 Image Backscattering coefficient Absorption coefficient


Inherent optical properties (IOP)

Incident beam of photons with spectral radiant power P0(λ)

P0-∆P

∆l

∆∆

=)()(1)(

0 λλλ

PP

lc

Spectral volume beam attenuation coefficient c(λ)

Spectral volume absorption coefficient a(λ)

Spectral volume scattering coefficient b(λ) = c(λ)-a(λ)

Scattering phase function β(λ,γ) ∫ Ω=π

γλβλ4

),()( db


Field tripsOct 18, 2005 (High spring tide)Mar 10, 2006 (Low tide)May 16, 2006


Study sites: P. Hantu & P.Semakau


Study Sites: P. Hantu & P. Semakua


Data acquisition•GPS location

•Sonar depth

•water sampling

• measurement of visibility

•Deployment of the ac9

•Spectra measurement


Instruments(1) – Wetlabs ac9

Absorption and attenuation meter at 9 wavebands: 412, 440, 488, 510, 532, 555, 650, 676, 715 nmLight source: tungsten lampwater is pumped into two tubes:

– dark to measure beam attenuation– highly reflective to measure absorption by water


Instruments (2) – ECO BB2Fmeasures scattering function at 1170 at 470 and 700 nm plus chlorophyll fluorescence at 695 nm


Ac9 and BB2F mounted for deployment

Ac9 and BB2F mounted on a metal casing for deployment


Instruments(3) – GER 1500Measures radiance between 350 and 1050 nm at contiguous bands of about 1.5 nm intervals

Sea Surface

0

5000

10000

15000

20000

25000

400 500 600 700 800 900

wavelength(nm)

digi

tal n

umbe

r


A set of three readings are needed…Spectroradiometer Readings for White Plague

050000

100000150000

200000250000300000350000

390 490 590 690 790 890 990wavelength (nm)

digi

tal n

umbe

r

Sea Surface

0

5000

10000

15000

20000

25000

400 500 600 700 800 900wavelength(nm)

digi

tal n

umbe

r

Sky Radiance

0

20000

40000

60000

80000

100000

400 500 600 700 800 900wavelength(nm)

digi

tal n

umbe

r

−

=

);(;,,();,,(

);,,(0

000 θλ

θλφθρθλφθ

πθλφθ

g

skysfgrs S

SSRR

Down-welling irradiance Up-welling radiance

Sky radiance

22nd APAN Meeting in Singapore 17 Jul 2006Temporal sampling and averaging…

Time (s)

White plaque reading

Sea surface reading

Sky radiance reading

•Ideally the three sets of readings should be simultaneously acquired

•Measurements are repeated to smooth out temporal variability in surface reflectance due to waves and wind


Remote sensing reflectance at Point B, 2005 Oct 18

00.0020.0040.0060.008

0.010.0120.0140.0160.018

0.02

400 500 600 700 800 900

wavelength(nm)

Rrs

(/sr

)


Measured remote sensing reflectance and IKONOS equivalent reflectance.

∫∫=

λλ

λλρλλ

ds

dsR

j

jjIKONOS

)(

)()()(

0

0.005

0.01

0.015

0.02

0.025

400 500 600 700 800 900

wavelength(nm)

rem

ote

sens

ing

refle

ctan

ce (/

sr)


Subsurface remote sensing image Image classified into land, optically deep and optically shallow regions

Image size: 1.28 km by 1.2 km


0.000

0.005

0.010

0.015

0.020

0.025

400 500 600 700 800wavelength(nm)

rem

ote

sens

ing

refle

ctan

ce

(/sr)

Measured remote sensing reflectance and IKONOS equivalent reflectance.

∫∫=

λλ

λλρλλ

ds

dsR

j

jjIKONOS

)(

)()()(


0.00

0.10

0.20

0.300.40

0.50

0.60

0.70

0.80

400 450 500 550 600 650 700 750

wavelength(nm)

abso

rptio

n co

effic

ient

(/m

)

Measured absorption coefficient at Pt B Pulau Hantu, 10 Mar 2006


0.000

0.010

0.020

0.030

0.040

0.050

0.060

400 450 500 550 600 650 700 750

wavelength(nm)

back

scat

terin

g co

effic

ient

(/m

)

Measured scattering coefficient at Pt B Pulau Hantu, 10 Mar 2006

Converted to backscattering coefficient by multiplying by 0.023

22nd APAN Meeting in Singapore 17 Jul 2006Comparison between extracted and measured values at point B

0.1510.155Total absorption coefficient at green (555) (/m)

0.0470.038 Backscattering coefficient at red (664) (/m)

0.0100.011Red subsurface remote sensing reflectance (/sr)

0.0310.031Green subsurface remote sensing reflectance(/sr)

Image(average deep water value)

Point


Conclusion

• Algorithm looks promising•More data needed for validation• Extend to extraction of bottom reflectance spectra• Main issue – atmospheric correction method

Thank you!

Retrieval of coastal sea water parameters from high ...Landsat 7 ETM + true colour image Acquired:...

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