Assessment of TerraSAR-X for mapping salt marsh

Assessment of TerraSAR-X for mapping salt marsh

Yoon-Kyung Lee1), Wook Park1), Jong-kuk Choi2),Joo-Hyung Ryu2), Joong-Sun Won1)

1) Remote Sensing Lab., Yonsei University2) Korea Ocean Satellite Center, KORDI

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Study area & Data22

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55

Introduction

Processing

Results

Summary

Contents

IntroductionIntroduction

Importance of salt marshBuffer zone from storms and contaminations (Kirwan and Murray, 2007; Li and Yang 2009)

Exchanging materials between tidal flats and open water (Mitch and Gosselink, 2000)

Removing large amount of carbon from the atmosphere (Belyea and Warner, 1996; Choi and Wang, 2004)

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Average global value of salt marsh is 8,535 $/ha/yr (Costanza et al., 1997)

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Accurate mapping of salt marsh is useful for understanding salt marsh functions and monitoring their response to natural and anthropogenic actions (Barker et al., 2006)

Importance of halophyte

Salt marsh vegetation communities (halophyte) play a fundamental role in the topography and stability of coastal wetlands by mean of a soil accretion, resulting from incoming flux of organic matter and sediment trapping.

Physical factor

Biological factor

Elevation of marsh platform in response to rising sea level cause a landward migration of the marsh (Gardner and Porter, 2001). Equilibrium between mean sea level and interactions of physical factors adjust production of halophytes and location of communities (Morris et al., 2002). Type of salt marsh (high marsh/low marsh) is altered in response to the environmental change (Bertness et al., 2002)

Limitation of optical data

Aerial photo

; High spatial resolution Low spectral resolution

Landsat ETM+

; Low spatial resolution Medium spectral resolution

It is difficult to acquire data over tidal flats at the optimum water condition with cloud free.

X-band SAR is suited for the detection and monitoring of herbaceous wetlands because of its short penetration paths to the ground

Objects

1. To To differentiate differentiate halophyte specieshalophyte species based upon radar based upon radar backscattering backscattering characteristicscharacteristics

3. To generate salt To generate salt marsh map marsh map

2. To determine the optimum season of the year and tidal condition for salt marsh mapping

Study area & Data

Study area & Data

Study area

Located in the mid-west of the Korean Peninsula Habitat for migratory birds (especially for the endangered species such as spoonbill, crane etc.) Planned to be nominated as the 1st national tidal flats preservation area. Halophyte densely development: the western of the Donggum-do bridge, northern part of the Yeongjong-do tidal flatTerraSAR-X image acquire on 03, July 2009 (HH pol.)

Ganghwa-do

Inchon internatinal airport

Yeongjong-do

Donggum-do

Main halophyte speciesPhragmites australis Suaeda japonica

• perennial • annual shoots emerge from perennial underground of rhizomes• grows in or near fresh water, brackish water• grows up to 3-4m in height

• annual• stems grows up to 50cm in height• rapid growth at the beginning of the growth stage• grows in saline soil, but cannot grows in the shadow• the color of short succulent leaves change green to red with an accumulation of red pigment (betacyanin)

Sudden dieback of S. japonica

18, May 2006

4, May 2009

: vertical accretion rate, low dissolved oxygen levels, high sulfides, high concentration of nutrients, fungus and sea level rise etc.

Possible reasons of sudden dieback

Data

TerraSAR-X

Field survey

Landsat ETM+

2008: 6/13 (ebb, 225), 7/27 (flood, 272), 10/12 (ebb, 425), 11/25 (ebb, 362), 12/06 (flood, 378)

2009: 4/17 (flood, 466), 7/3 (ebb, 273), 12/15 (ebb, 618)

2010: 3/24 (flood, 347), 7/1 (flood, 464), 9/5 (flood, 421)

2008: 1/30 (ebb, 611) , 4/19 (ebb, 108) , 5/5 (ebb, 839), 8/9 (ebb, 608), 8/25 (ebb, 630), 9/10 (flood, 540), 10/12 (flood, 291), 12/15 (flood, 106)

2009: 2/1 (ebb, 501), 3/21 (flood, 510), 4/6 (flood, 392), 5/8 (ebb, 134), 5/24 (flood, 210), 8/28 (ebb, 574), 9/13 (ebb, 603), 10/15 (flood, 493), 12/18 (ebb, 78)

2010: 1/3 (ebb, 114), 2/4 (ebb, 513), 4/9 (flood, 507), 9/16 (ebb, 504), 10/18 (flood, 460)

2010.05.122010.07.122010.08.202010.09.16

HH

2008: 7/11 (flood, 315), 9/24 (flood, 431), 10/5 (flood, 685)

VV/VH

Processing

Statistical analysis

TerraSAR-X

GPS

Decision tree

ResultsResults

SAVI (Soil adjusted vegetation index)Phragmites australis Suaeda japonica Tidal flats

The seasonal variation pattern of SAVI matched well with phenelogical cycle of halophyte P. australis and S. japonica is hard to be distinguished from SAVI P. australis, S. japonica and tidal flats have similar SAVI values during winter

Characteristics of sigma naught (HH) - 1

Almost 350 pixels for each classes The quality of sigma naught were identified using average of industrial area

P. austalis: - slightly stable and higher than S. japonica

S. japonica:- lower value in winter and increased from spring to peak in summer

Tidal flats: - significant variation according to surface condition - remnant water within the ripple control the backscattering

Industrial area

Tidal flats

P. austalis

S. japonica

Water

Sig

ma

Nau

ght

(dB

)

Sig

ma

Nau

ght

(dB

)

P. australis S. japonica

Characteristics of sigma naught of halophyte

Temporal variation of sigma naught during 3 years- P. australis: not agree with its plant cycle- S. japonica : peak in summer to autumn, presence of S.japonica structure roles as scatter although it is dead in Nov.

Difference between S. japonica and P. austalis- Increased in winter, decreased from summer to autumn

on-season

Characteristics of sigma naught (VV/VH)

The average VV/VH difference of S. japonica:

6.2 dB (from image), 10.2 dB (from field survey)

P. australis S. japonica Similar value in VV also between halophytes during Nov.

(Park et al., 2010)

Consideration tidal condition

Sig

ma

nau

ght

(dB

)

P. a

ust

rali

s

S. j

apon

ica

Tid

al f

lats

Wat

er

Ebb condition during the off-season is the best condition to distinguish between P. australis and S. japonica Ebb condition is good to distinguish halophyte

Decision rule based on statistical analysis

Class PolarizationTidal

condition

Seasons S. japonica Tidal flatOcean water

P.australis

HHebb

On0.10

( p = 0.301)7.32 19.26

Off 6.61 19.29 18.88

flood

On 1.01 5.77 18.08Off 4.33 11.25 18.47

VV On -2.92 3.36 17.39VH On -2.73 8.57 11.06

S. japonica

HHebb

On 7.22 19.16Off 12.67 12.27

flood

On 4.75 17.06Off 6.92 14.14

VV On 6.28 20.31VH On 11.31 12.80

Tidal flat

HHebb

On 11.94Off 0.40

flood

On 12.31Off 7.22

VV On 14.02VH On 2.49

Mean difference of sigma naught (T-test)

HH > -19.28 dBin ebb (on-

season)

HH <-23.57 dBIn flood (on-

season)

HH > -9.43 dB in ebb (off-

season)

HH > -16.16 dB in flood (rains)

Salt marsh map

Reference Data

Tidal flats Runnels WaterS. japonica

P. austalis

Land Row total

Tidal 2062 622 356 0 75 0 3115Runnels 1744 653 53 174 29 0 2653Water 0 0 0 0 0 0 0S. japonica 16 14 0 136 30 0 196P. austalis 0 0 0 0 0 0 0Land 0 0 0 0 0 3329 3329Colum total 3822 1289 409 310 134 3329 9293

Total accuracy: 66.5 %

Summary

Optimum data acquisition plan by high resolution spaceborne X-band SAR should focus on on-season on the ebb tide when the halophyte return the strongest signal and off-season on the ebb tide to distinguish annual and perennial.Generated salt marsh map has 66.5 % total accuracy.

For long-term monitoring the distribution of S. japonica in association with sea level fluctuation, it is important to set up date for annual data acquisition.

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Thank you for your attention.

Thank you for your attention.