TanDEM-X Pol-InSAR Inversion for Mangroves of East Africa

Transcription

1 TanDEM-X Pol-InSAR Inversion for Mangroves of East Africa Seung-Kuk Lee, Temilola Fatoyinbo, David Lagomasino, Batuhan Osmanoglu, Carl Trettin, Marc Simard NASA/Goddard Space Flight Center Biospheric Sciences Laboratory

2 Mangroves are Survivors! Mangroves are salt tolerant plants that can grow in difficult environments regularly inundated by salt water. Other trees can t survive. Live in the intertidal zone along tropical and subtropical coasts between 25 N and 25 S. Inland <Fresh water> Nutrient Intertidal Zone <Brackish water> Salinity Ocean <Salty water> USGS/NASA Global Mangrove Distribution Map Giri et al, 211 Mangrove forests occupy about 15.2 million hectares of tropical coast worldwide: across Africa, Australia, Asia and America (only 1% of the Earth s terrestrial surface).

3 Mangrove Importance Fisheries: home to a large variety of fish, carb, shrimp, and mollusk species Timber and plant products: Construction material and fuel Coastal protection: roots trap sediments / it prevents erosion from waves. Tourism Carbon-rich ecosystem Coastal Wetland Total Carbon stores in tons of carbon dioxide equivalents per hectare (Murray et al, 211). Mangroves and other coastal wetlands have total carbon storage capacities well above that of tropical forests.

4 Mangrove Importance To better understand C emissions an ecosystem structure from mangroves and other ecosystems we need to accurately quantify ecosystem biomass, extent and change by measuring horizontal and vertical heterogeneity Horizontal Structure: in terms of land cover and land cover change Vertical Structure : in terms of mangrove height and biomass In previous work for Vertical Structure SRTM (InSAR) : 9 m 9 m IceSAT/GLA14 (Lidar): about diameter of 6 m and interval of 172 m. Fusion To estimate 3-D mangrove forest parameters using higher resolution TanDEM-X data sets by means of Pol-InSAR

5 TanDEM-X is a Great System for Mangrove Study! The first single-pass polarimetric and interferometric satellite system (bistatic) No temporal decorrelation Higher spatial resolution ( SRTM, IceSAT/GLAS, Landsat series ) Good kz range closed to equator (25 N ~ 25 S) for mangrove height inversion Mangrove Study Dual-Pol. Single-Pol. Polarization HH / VV HH Bandwidth 1-15 MHz 1 MHz Stripmap mode Half wide of single pol. 3 km wide, 5 km long Test Site 1 sites Global Date 213 Dec. ~ Feb. ~ 214

6 Polarimetric SAR Interferometry InSAR Complex Coherence TSX S TSX TDX S ~γ (S S ) = TSX TDX < S TSX < S S TSX TSX S TDX >< S > TDX S TDX > Volume Coherence Volume Coherence ~ γ = ~ γ γ ~ γ γ Temp ~ I γ V = SNR I Vol I SNR h = v ~ γ ( f ( z)) = V Temp 2σz' iκ z' cosθ z G/V Ratio: Temporal decorrelation Additive h noise 2σz' decorrelation v cosθ Vertical Volume I = edecorrelation dz' Wavenumber: e e f dz' h v iκ z z Random f ( z) evolume dz over iκ z z f (z) Ground (RVoG) Model o o e hv (Two-layer scattering model) f ( z) dz o ~ ~ 2σ z ~ i γ ( ) ( ) V + m w cosθ φ ( z) = mv e + m' G δ ( z z) γ w = e Volume Ground layer f (z) Vertical Reflectivity 1 Function + m( w) m( w) = κ = z m ( w) G m ( w) I V 4π θ λ sin( ) θ Volume Height Extinction Topography G/V Ratio σ φ m(w) 4 unknown parameters!

7 Pol-InSAR Inversion: Quad- / Dual- / Single-pol Case Polarization Independent Complex Coherence Assumption Unknowns Condition Quad-Pol. [ ~ ~ ~ ( w ) γ ( w ) γ ( )] m 3 =, σ, φ, m m Unique solution γ 1 2 w3 h v 1, 2 Dual-Pol. [ ~ ~ γ ( w 1 ) γ ( w 2 )] m 2 = h v, σ, φ, m1 Balanced [ ] Single-Pol. ~ γ ( w 1) m 1 = h v, σ, φ Underdetermined problem h σ v Volume Height: Extinction: Ground Phase: G/V Ratio: φ mi Constraints for forest application: X-band / non-fully polarimetric data The best way for inversion performance is to use an external DTM (e.g. Lidar DTM) to estimate the ground phase φ *. However, high resolution DTM rarely exists for (mangrove) forests

8 Water Level Estimation: Z Underlying topography in mangrove forests is negligible and flat due to the unique environment (i.e. water surface). Mangrove forest consists of vegetated area and open water body (e.g. rivers). Height Brackish Water Water Surface z Underlying Topography Open Water Body Low backscattering Strong SNR decorrelation Mangrove Forest Short wavelength Small ground contributions Lack of a Pol. Mangrove Boundary Double bounce scattering The InSAR phase represents We can assume the estimated Z related to ground phase mangrove forest. is a constant value over

9 Water Level Estimation: Z SNR Decorrelation The actual SNR depends on the strength of the returned radar signal. Extract boundary of mangrove forests Coherent Scatterer (CS) technique* Based on correlation coefficient between two parts of full image spectrum. Select stable targets on the boundary *Rafael Schneider et al (26). SNR decorrelation Boundary / Stable targets Histogram of ground elevation on stable targets Volume-only Phase (kz =.85 rad/m) Z Ground Elevation (m)

10 Zambezi Mozambique <Mozambique> Entire 277 km coast (1 2 S ~ 26 5 S latitude The 3 rd largest mangrove area (5211 km 2 ) in Africa High species diversity: 1 species Mangrove-dependent fisheries: 4% of GNP TDX HH amplitude image Rg Az Zambezi Delta Airborne lidar data <Zambezi Delta> 1.2 millions hectares The waters are estuarine and brackish to around 5 km inland. 6 species (Ceriops tagal, Avicenia-marina, Rhizopara-mucronat, Soneratia-alba, Xilocarpis-granatum, and Avicenia-mucronata) Mangrove height up to 35 m Tree diameter up to 6 cm

11 Pol-InSAR Inversion Results & Airborne Lidar Measurement Field Measurement Stereo-image (WorldView-2)

12 Dual-pol InSAR Inversion Zambezi Delta TDX dual-pol. Inversion Height Airborne Lidar H m 4 Latitude Latitude km Radar (m) Polarization: HH and VV Acquisition data: 214/4/3 Acquisition mode: Descending Vertical wavenumber:.83 rad/m Height of Ambiguity: m System Bandwidth: 1 MHz Longitude Longitude R2 = RMSE Acquisition = 1.27m data: 214/5/5 214/5/16 2 Spatial 3 resolution: 4 1 m X 1 m Lidar (m)

13 Single-pol InSAR Inversion Zambezi Delta TDX single-pol. Inversion Height Airborne Lidar H Latitude Latitude Radar (m) Polarization: HH Acquisition data: 211/1/14 Acquisition mode: Descending Vertical wavenumber: -.86 rad/m Height of Ambiguity: m System Bandwidth: 1 MHz Longitude Longitude R2 =.916 RMSE Acquisition = 1.727m data: 214/5/5 214/5/16 Spatial resolution: 1 m X 1 m Lidar (m)

14 Comparison of Single- and Dual-pol Inversions TDX dual-pol. Inversion Height TDX single-pol. Inversion Height m Latitude Latitude km Single_pol (m) Longitude Longitude R2 =.955 RMSE = 1.57m We are able to produce mangrove height Dual_pol maps (m) using dual- and single-pol. TanDEM-X data without any external DTM.

15 Field Zambezi Delta Field Campaign: September m Radius Subplots.52 ha Plot (72 m X 72 m) Total 4 plots / 927 trees Species / Height / DBH Zambezi Delta Validation plot for single-pol TDX result 4 3 : Plot Single_pol (m) 2 1 R2 =.839 RMSE = 2.612m H1 from field data (m)

16 Mangrove Height Estimate using Stereo-Image First Collection for Stereo Second Collection for Stereo Mangrove Height Estimate Stereo-Image Pair Tie Points NASA AMES Stereo Pipeline DSM Generation DSM Open Surface Elevation (z ) Mangrove water Z Mangrove water <Assumption> Flat topography Mangrove Canopy Height Mangrove canopy height (m) = DSM Z

17 Mangrove Height Estimate using Stereo-Image Mangrove Height from Stereo-Image WorldView-2 Satellite Spatial Resolution.6 m DSM Resolution.8 m Acquisition Date: Jan. 7, Latitude m Single_pol (m) Longitude m R2 =.857 RMSE = 2.226m Hi-Res Stereo-Image (m)

18 Entire Mangrove Height Mozambique TanDEM-X Footprint Costal area 3 km 215 TSX/TDX SLC scenes Descending mode Acquisition time 3h15m (UTC) Mangrove Height Frame About 5 frames Resolution: 12 m 12 m Size:.5 deg..5 deg pixels 8 MB Latitude Latitude.5 deg..5 deg. Longitude Longitude

19 Entire Mangrove Height Mozambique 2 1 Maputo Area 12 m 12 m (TanDEM-X) 1.5 deg. 3 2 m 2 Zambezi Delta.5 deg. 9 m 9 m (SRTM) m 3

20 Summary & Future Work Summary Summary Mangrove height estimation using TanDEM-X data provided good results without an external DTM. The TDX mangrove heights over Zambezi Delta was compared and validated against Lidar H1 height (R 2 :.916 / RMSE: m) Field measurement data (R 2 :.839 / RMSE: m) High resolution stereo-image (R 2 :.857 / RMSE: m) The mangrove height canopy map for entire Mozambique was the first country-wide, wall-to-wall estimate of mangrove structure at 12 m resolution. Future Work TanDEM-X Science Phase: Large spatial baselines for small or scrub mangroves. New sensors at longer wavelength DBSAR-1/2 at L-band EcoSAR at P-band Radar (m) Lidar (m)

21 Summary & Future Work Question?