Thank you chairman. Good morning, Thank you for giving me a great chance to present my paper at IGARSS. I’m yoon kyung lee who studying at remote sensing lab. in Yonsei university. To represent coauthors, Mr. Park, Dr. Choi and Dr. Ryu in KORDI, and Pro. Won in Yonei university, I’m going to talk about “assessment of TerraSAR-X for mapping salt marsh.
This is the contents of presentation. First, I’ll give you the importance of salt mash, background knowledge and objectives in introduction. Then, I’ll talk about study area and data, I’m going to explain how I processed these data. I ’ ll show you the results of this study. Then I ’ d like to conclude with summary.
First, I’m going to talk about the importance of salt marsh. Salt marsh are ecoclines between terrestorial and salt or brackish water ecosystems. This ecosystem acts as a buffer zone from storms, typhoon, and prevents the rapid diffusion of contaminant to the ocean. It also exchanges materials between tidal flats and open water. An important aspect of matter exchange is the trapping effects of the salt marsh. The stability of salt marsh ecosystem is explained by interactions of sea level and trapping effects. It also removes large amount of carbon fro the atmosphere. Salt marsh has known as the largest sinks of carbon among the soil ecosystem. If these ecological services are estimated, the current ecological value of salt marsh is 8535 $/ha/yr. But, these values are oftenly little weight in policy decision. Therefore, Accurate mapping of salt marsh is useful for understanding salt marsh functions and monitoring their response to natural and anthropogenic actions.
Among the many living organisms in salt marsh, salt marsh vegetation communities which are called halophyte play a fundamental role in the topography and stability of coastal wetlands by means of a soil accretion, resulting from incoming flux of organic matter and sediment trapping. Physical and biological factors control the distribution of halophyte. It is known that physical factors such as the duration of inundation, fluctuation of water level are more affect to the distribution of halophyte than biological factors. From the previous researches, it is known that elevation of marsh platform in response to rising sea level should cause a landward migration of the marsh. Because of equilibrium between mean sea level and interactions of physical factors, production of halophytes and location of communities could be altered. Type of salt marsh (high marsh / low marsh) is altered in response to increase/decrease of exposure time due to the environmental change such as change of sea level rise, reduced/increased sediment supply, sediment accretion, etc Therefore, attenuation of species and its boundary between species can be an environmental indicator to understand status of salt marsh.
Optical images have advantages for monitoring coastal area, However, it is oftenly difficult to acquire data over tidal flats at the optimum water condition with cloud free. Even we could get cloud free image, areal photo has a limit spectral resolution to extract the information about halophyte. Optical images with medium or high spectral resolution have a low spatial resolution. Because of this, low spatial resolution, spectral signatures between halophyte species are overlapped. Therefore, they are hard to be distinguished each other. SAR provides images regardless of meteorological conditions. Especially, X-band SAR such as TerraSAR-X and COSMO-SkyMed which is available to public and provide high resolution radar image is well suited for the detection and monitoring of herbaceous wetlands because have shorter penetration paths to the ground.
This is the objective of this study. At first, we’d like to differentiate halophyte species based upon radar backscattering characteristics using high resolution X-band SAR. Then, we also want to determine the optimum season of the year and tidal condition for salt marsh mapping. Finally, we’d like to generate salt marsh map using the decision tree based on the statistical analysis of the backscattering coefficient.
This is the image of study area acquired on 3 July in 2009 using TerraSAR-X HH polarization. Ganghwa tidal flats is located in the mid-west of the Korean Peninsula near the estuaries of the Han-river. This tidal flats is famous habitat for the endangered migratory birds, and one of the largest mud flats in the world. For several reasons, this tidal flats is going to be nominated as the first national tidal flats preservation area. The surface sedimentary facies are mainly mud flats in the eastern part marked as green, and sand flats in the western part marked in red, and mixed flats in between. Doggum-do is another island which is connected to the Ganghwa-do by a bridge. Halophyte densely develops to the western of the bridge, but halophyte rarely distributed to the eastern of bridge. A large northern part of the Yeongjong-do tidal flats is compactly covered with halophyte.
Main halophyte species in this area are P. austalis and S.japonica. P. Australis is a perennial grass which grow and bloom over the spring and summer and then dieback every autumn and winter. Then in the spring annual shoots emerge from perennial underground of rhizomes. It grows in or near fresh water and brackish water. In optimum condition, it grows up to 3-4m height. S.Japonica is annual plant, and its stems grows up to 50 cm in height. It grows in saline soil, but cannot grows in the shadow. Rapid growth of underground part of S. japonica at the beginning stage of the growth (generally, 40 days needs from germination to settlement. Because of this rapid growth, the habitat of S. japonica could be settled down regardless of the fluctuation of tidal currents. Interestingly, the color of short succulent leaves change green to red with an accumulation of red pigment. It is important to identify the area of S.japonica because the area of S. japonica is subject to dynamic change according to the sea level fluctuation while the habitat of P. australis is relatively stable. Therefore, we focus to know the boundary between S. japonica and P. australis and boundary between S. japonica and exposed tidal bottom.
These photo were taken from the southern part of Donggu-do Upper image was acquired on may 2006, and lower image acquired on may 2009. They have 3 years gap. Suddenly, the large dense patch of S.japonica were disappeared since 2009. The reasons of sudden dieback have been known as increase of vertical accretion rate, low dissolved oxygen levels, high sulfide,s high concentration of nutrients, fungus and sea level rise from previous study for other study areas. Therefore, the phenomenon of sudden dieback of halophyte could be an indicator of health of salt marsh. Understanding of the habitat of S.japonica is useful to know the statues of salt marsh.
These are data what we were used for analyzing. We had 4 field surveys. one of them was to get GCP points for accurate rectification of TSX to coincide between field data and image. 3 surveys were for tracking the boundary between P.australis and S.japonica, and the boundary between exposed tidal flats and S.japonica. 14 TerraSAR-X images were acquired between 2008 to 2010. This is the tidal condition (ebb/flood) and tidal height at the point of image acquisition. These HH polarization data were acquired in the standard stripmap mode with a 3m pixel resolution in 39 degree incidence angle. These VV/VH dual polarization data were acquired with 6m resolution and 31 degree incidence angle. 22 Landsat ETM+ images were also acquired between 2008 to 2010.
TerraSAR-X SSC image were provided by the DLR. Intensity image from SLC data in converted to the multi-look image and they were georectified using orbit. Absolute radiometric calibration was carried out to minimize the differences in the image radiometry. Although TerraSAR-X images were geo-rectified, there were position disagreement with field survey data about 3-4 pixels. To refine the geo-location of the image, GCPs obtained from field survey were used. Statistical analysis were carried out, then based on the result of statistical analysis, salt marsh map was constructed using decision tree.
Because halophyte in salt marsh is sparsely populated in comparison with inland vegetation, the effect of soil background needs to be minimize. So, among the vegetation index, we used soil adjusted vegetation. The SAVI was calculated from Landsat ETM+ to examine Phragmites australis and Suaeda japonica. The seasonal variation pattern of SAVI matched well with phenelogical cycle of halophyte. Increasing from march until september and the rapid decreasing since then. Tidal flat showed low SAVI value through all month. The little variation of tidal flat comes from the tidal condition and algae bloom in spring. Although Phragmites australis and Suaeda japonica has a different color, it is difficult to distinguish from vegetation index alone. Even P.australis and S. japonica have similar SAVI values with tidal flats during winter. Using the image when SAVI value of halophyte is high, halophyte can be distinguished from tidal flats. However, as I told at introduction, using a optical image to distinguish halophyte species is very difficult.
Almost 350 pixels were selected for the classes what we already knew from field survey. The quality of sigma naught were identified using average of industrial area, because many artificial structures in industrial area act as a corner reflector and have permanent backscatterer. Value of average sigma naught of industrial area is quite stable through 3 years. Therefore, any compensation doesn’t need before further analysis. The average sigma naught of Phragmite austalis was slightly stable through 3 years, and its value is higher than S. japonica. Standard deviation of Phragmite austalis was 1.95 and Suaeda japonica was 2.09. Unlike P.australis, there is seasonal variation in Suaeda japonica It has low sigma naught value in winter, and its value is increased from spring to peak in summer. Therefore, there is a possibility to distinguish S. japonica and P.australis using this seasonality of S.jaonica. The average sigma naught tidal flats showed a significant variation with a standard deviation 2.48 according to surface condition. Remnant water within the ripple plays an important role to control the radar backscattering
To examine the seasonal variation of radar backscattering patterns, the temporal variation of sigma naught were plotted based on month during 3 years. The seasonal variation of phragmite austalis was slightly higher in winter than in summer. The sigma naught behavior of phragmite does not agree with its plant cycle. Although the leaf dry during winter, structure of Phragmite and exposed presence of remnant water make higher sigma naught value than growing season. As I mentioned before, Suaeda japonica showed higher value from mid July to end of November, and fall sharply during december. The behavior of sigma naught was different with the temporal variation of plant cycle. Although S. japonica is dead in November, presence of S. japonica structure roles as scatters and make high backscattering. After the structure of Suaeda japonica collapse in tidal flats due to the tidal effects, it shows sudden low value in December. Difference between Suaeda japonica and phragmite austalis were calculated. Difference was large in Feb., and gap are getting smaller until September. Then, the difference are getting larger in winter. For convenience to differentiate P.australis and S.japonica, where the difference less than 3dB is called as on-season. The rest of them grouped as off-season.
This is the characteristics of sigma naught in VV/VH dual polarization. Unfortunately, we could get only three dual polarization images. Radar returns from tidal flats is slightly lower than halophyte in VV polarization. In September, the value from phragmites austalis and suaeda japonica were very similar. However, in July, radar returns more signals by Suaeda japonica than Phragmites australis by 5dB. From this we could know that Suaeda japonica has more seasonal variation than Phragmites autralis. The average VV/VH difference of Suaeda japonica calculated from image was 6.2 dB while measured from field survey was 10.2 dB. Because signal were measured from the very dense patch of suaeda japonica at field, difference from field survey is larger than image.
We found the off-season is good for distinguish P.austalis and S.japonica. However, we also consider the the change of tidal condition. Due to the flood/ebb, the surface of tidal flats are affected by the remnant water within the ripples. During ebb tides, specular scattering occurs in remnant water within ripples. Therefore tidal flats has low sigma naught in ebb tide during on-season and off season. Because of the longer exposure time of tidal flats, tidal flats during flood tide has higher sigma naught. Difference between P.australis S.japonica and tidal flats is large during ebb tide than flood tide. Therefore, ebb tide is more effective to differentiate halophyte from tidal flats. The sigma naught of P. austalis is almost similar between on-season and off-season regardless of tidal condition in HH polarization. The sigma naught value between P.australis and S.japonica is very similar to each other during on-season. Even they are almost same in ebb condition during on-season. Therefore, these two halophyte species are hard to be distinguished during on-season. As I told before, the difference of sigma naught between P.austalis and S.japonics is large during off-season. Especially, sigma naught of S.japonica in Ωbb-tide during off-season is very low. And the difference between s.japonica and p.australis is about 7dB. And difference is 4dB in flood tide during off-season. Because of the large present of remnant water under the S.japonica during ebb tide, sigma naught during ebb-tide is low. Therefore, ebb tide during the off-season is the best condition to distinguish these two species. Due to the remnant water within the ripples, some part of exposed tidal flat of ebb tide is considered as water. Therefore, sigma naught of tidal flat is low in ebb. Therefore, to distinguish tidal flats from water, flood tide is useful. However to differentiated halophyte from tidal flats, ebb tide is good because low reflectance of tidal flats makes big difference with halophytes
Based on these results, we could make the rule for decision tree. To determine thresholds for each step, a mean and standard deviation was used for each class and condition on data acquisition. Decision tree were performed via the statistical analysis of backscattering coefficient to generate salt marsh map. At first tidal flats and sea water are divided. Actually, VV has the highest difference between them, However, it covers a limited study area. Instead of VV image, HH polarization in ebb condition was used. Although the difference between tidal flats and halophyte was big in ebb condition during off-season, it was confirmed that sigma naught of tidal flats were very low because of the remnant water. Instead of HH in ebb condition, VH in flood condition during on season was used. Because it has a second big difference between tidal flats and S. japonica which has a boundary with tidal flats. Using the difference in off season, P. australis and S. japonica were distinguished. From the previous research, HH image acquired in rain could be useful to differentiate runnels from tidal fats. Based on that, we could distinguish runnels and exposed tidal flats.
This is the generated map from the decision tree. Orange color means mud flat, yellow means runnels, red means S.japonica, green means P. australis, purple means water and white is the land which is masked out. Right image is reference image which is generated based on field tracking data. From the several field surveys, we tracked the boundary between exposed tidal flat and S. japonica, and boundary between S. japonica and P. australis. Runnels were extracted from the Kompsat images with 1m spatial resolution. Due to the limit of access, we traced from the bridge to here. Although we got a reference data for a very limited area, the total accuracy is 66.5 %
The behavior of sigma naught was different with the temporal variation of plant cycle. Although S. japonica is dead in November, presence of S. japonica structure roles as scatters and make high backscattering. After the structure of Suaeda japonica collapse in tidal flats due to the tidal effects, it shows sudden low value in December.
Assessment of TerraSAR-X for mapping salt marsh Assessment of TerraSAR-X for mapping salt marsh Yoon-Kyung Lee 1) , Wook Park 1) , Jong-kuk Choi 2) , Joo-Hyung Ryu 2) , Joong-Sun Won 1) 1) Remote Sensing Lab., Yonsei University 2) Korea Ocean Satellite Center, KORDI
1 Study area & Data 2 4 3 5 Introduction Processing Results Summary Contents
Buffer 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) 1 2 3 Average global value of salt marsh is 8,535 $/ha/yr (Costanza et al ., 1997) 4 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 salt marsh
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. <ul><li>Elevation of marsh platform in response to rising sea level cause a landward migration of the marsh (Gardner and Porter, 2001). </li></ul><ul><li>Equilibrium between mean sea level and interactions of physical factors adjust production of halophytes and location of communities (Morris et al ., 2002). </li></ul><ul><li>Type of salt marsh (high marsh/low marsh) is altered in response to the environmental change (Bertness et al ., 2002) </li></ul>Importance of halophyte Physical factor Biological factor
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 Limitation of optical data Aerial photo ; High spatial resolution Low spectral resolution Landsat ETM+ ; Low spatial resolution Medium spectral resolution
1. To differentiate halophyte species based upon radar backscattering characteristics 3. To generate salt marsh map 2. To determine the optimum season of the year and tidal condition for salt marsh mapping Objects
<ul><li>Located in the mid-west of the Korean Peninsula </li></ul><ul><li>Habitat for migratory birds (especially for the endangered species such as spoonbill, crane etc.) </li></ul><ul><li>Planned to be nominated as the 1 st national tidal flats preservation area. </li></ul><ul><li>Halophyte densely development: the western of the Donggum-do bridge, northern part of the Yeongjong-do tidal flat </li></ul>TerraSAR-X image acquire on 03, July 2009 (HH pol.) Ganghwa-do Inchon internatinal airport Yeongjong-do Donggum-do Study area
Phragmites australis Suaeda japonica <ul><li>perennial </li></ul><ul><li>annual shoots emerge from perennial underground of rhizomes </li></ul><ul><li>grows in or near fresh water, brackish water </li></ul><ul><li>grows up to 3-4m in height </li></ul><ul><li>annual </li></ul><ul><li>stems grows up to 50cm in height </li></ul><ul><li>rapid growth at the beginning of the growth stage </li></ul><ul><li>grows in saline soil, but cannot grows in the shadow </li></ul><ul><li>the color of short succulent leaves change green to red with an accumulation of red pigment (betacyanin) </li></ul>Main halophyte species
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
Phragmites australis Suaeda japonica Tidal flats <ul><li>The seasonal variation pattern of SAVI matched well with phenelogical cycle of halophyte </li></ul><ul><li>P. australis and S. japonica is hard to be distinguished from SAVI </li></ul><ul><li>P. australis , S. japonica and tidal flats have similar SAVI values during winter </li></ul>SAVI (Soil adjusted vegetation index)
Characteristics of sigma naught (HH) - 1 <ul><li>Almost 350 pixels for each classes </li></ul><ul><li>The quality of sigma naught were identified using average of industrial area </li></ul><ul><li>P. austalis : </li></ul><ul><ul><li>- slightly stable and higher than S. japonica </li></ul></ul><ul><li>S. japonica: </li></ul><ul><ul><li>- lower value in winter and increased from spring to peak in summer </li></ul></ul><ul><li>Tidal flats: </li></ul><ul><ul><li>- significant variation according to surface condition </li></ul></ul><ul><ul><li>- remnant water within the ripple control the backscattering </li></ul></ul>Industrial area Tidal flats P. austalis S. japonica Water
Characteristics of sigma naught of halophyte <ul><li>Temporal variation of sigma naught during 3 years </li></ul><ul><ul><li>- P. australis : not agree with its plant cycle </li></ul></ul><ul><ul><li>- S. japonica : peak in summer to autumn, presence of S. japonica structure roles as scatter although it is dead in Nov. </li></ul></ul><ul><li>Difference between S. japonica and P. austalis </li></ul><ul><ul><li>- Increased in winter, decreased from summer to autumn </li></ul></ul>Sigma Naught (dB) Sigma Naught (dB) P. australis S. japonica on-season
Characteristics of sigma naught (VV/VH) <ul><li>The average VV/VH difference of S. japonica : </li></ul><ul><ul><li>6.2 dB (from image), 10.2 dB (from field survey) </li></ul></ul><ul><li>Similar value in VV also between halophytes during Nov. </li></ul>P. australis S. japonica (Park et al ., 2010)
Consideration tidal condition <ul><li>Ebb condition during the off-season is the best condition to distinguish between P. australis and S. japonica </li></ul><ul><li>Ebb condition is good to distinguish halophyte </li></ul>Sigma naught (dB) P. australis S . japonica Tidal flats Water
Decision rule based on statistical analysis Mean difference of sigma naught (T-test) Class Polarization Tidal condition Seasons S. japonica Tidal flat Ocean water P.australis HH ebb On 0.10 ( p = 0.301 ) 7.32 19.26 Off 6.61 19.29 18.88 flood On 1.01 5.77 18.08 Off 4.33 11.25 18.47 VV On -2.92 3.36 17.39 VH On -2.73 8.57 11.06 S. japonica HH ebb On 7.22 19.16 Off 12.67 12.27 flood On 4.75 17.06 Off 6.92 14.14 VV On 6.28 20.31 VH On 11.31 12.80 Tidal flat HH ebb On 11.94 Off 0.40 flood On 12.31 Off 7.22 VV On 14.02 VH On 2.49 HH > -19.28 dB in ebb (on-season) HH <-23.57 dB In flood (on-season) HH > -9.43 dB in ebb (off-season) HH > -16.16 dB in flood (rains)
Salt marsh map <ul><li>Total accuracy: 66.5 % </li></ul>Reference Data Tidal flats Runnels Water S. japonica P. austalis Land Row total Tidal 2062 622 356 0 75 0 3115 Runnels 1744 653 53 174 29 0 2653 Water 0 0 0 0 0 0 0 S. japonica 16 14 0 136 30 0 196 P. austalis 0 0 0 0 0 0 0 Land 0 0 0 0 0 3329 3329 Colum total 3822 1289 409 310 134 3329 9293
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. 1 2 3
Thank you for your attention. Thank you for your attention.
March July September December 3 Feb., 2010 Seasonal change of S. japonica