DIGITAL ELEVATION MODEL (DEM) GENERATION FROM SAR INTERFEROMETRY M.H. Kamaruddin1, J.R. Abdul Hamid1, P.M. Mather1, and H. Balzter2 1 University of Nottingham, University Park, Nottingham NG7 2RD, United Kingdom 2 Centre for Ecology and Hydrology, Monks Wood, Huntingdon, PE28 2LS, United Kingdom ABSTRACT SAR Interferometry (InSAR) provides data that contain information relating to the phase and coherence components of the backscattered radar signals. Phase information is used to derive Digital Elevation Models (DEM). This paper presents the results of an analysis of the accuracy of InSAR DEM derived from ESAR L-band airborne repeat-pass fully polarimetric InSAR data, which were acquired over Thetford Forest, in the east of England. An area with no tree cover of about 300m by 300m in size was chosen as a test site. Then InSAR DEMs for L-band HH, HV and VV polarisations were generated. The accuracy of the InSAR-derived DEMs was deduced by comparison with reference DEMs, which were generated from field data acquired from both Global Positioning System (GPS) & spot height survey and a Lidar DEM. The Lidar DEM was acquired by the UK Environment Agency. The poster reports the results of these comparisons and some concluding remarks about the relationship between the accuracy of the InSAR DEMs, polarization mode, and the nature of the ground surface cover are highlighted. 1. INTRODUCTION SAR interferometry (InSAR) is an established technique in generating high quality Digital Elevation Models (DEMs) from spaceborne and airborne data (Gens and Genderen, 1996). DEMs are useful in many geosciences applications such as in topographic mapping, earths deformation studies and in particular, forest mapping (Balzter, 2001). The availability of large amounts of InSAR data from many agencies worldwide has encouraged more and more users and researchers to explore the potential of the data in their own applications. This paper intends to address an important issue concerning the validation of the quality or accuracy of InSAR data that was obtained from the UKs SHAC2000 campaign, which is used in generating DEMs over Thetford Forest, in the east of England. The data comprise of L-band in full polarization mode (HH, HV and VV) which were collected in repeat-pass interferometry. The accuracy of InSAR derived DEMs can be determined by comparing them with reference DEMs, which can be taken from other sources (Rosen et al., 2000) such as existing topographic maps, aerial photogrammetry, Lidar and direct ground measurements using Global Positioning System (GPS) and the terrestial method. This paper reports on the results of comparisons between the InSAR derived DEMs and the reference DEMs computed from a GPS and spot height survey, and Lidar data. Some concluding remarks about the relationship between the accuracy of InSAR DEM, polarization mode, and the nature of the ground surface cover are highlighted. 2. STUDY AREA AND DATASETS The study area covers 4km by 3km within the British National Grid 284000m North, 580000m East and 287000m North, 584000m East (Figure 1). It is located in Thetford Forest in the Breckland Zone, East Anglia. The forest is mainly planted with Corsican Pine (60%) and Scots Pine (20%) over lowland with minimal ground variation of less than 50m elevation above sea level. Study Area Test Site 1 1 7 6 16 1 7 18 15 15 2 2 2 1 24 1 9 2 1 26 25 3 20 8 Fig. 1: Study area and Test site in Thetford Forest, East of England, United Kingdom (Map source: Philips Motoring Britain 2002) A smaller area of 300m by 300m with no tree cover was chosen as test site for establishing a reference DEM which is based on ground measurements obtained from GPS and spot height survey. The site is located at near range from the sensor and mainly covered with grass and new planted Corsican pine of less than 0.5m height with a uniform topography and a gradual slope (Fig. 1). The L-band fully polarimetric data were collected by the German DLR ESAR sensor onboard a Dornier DO228 aircraft on 31 May 2000 in medium-resolution, wide-swath mode with a temporal baseline of about two minutes. Table 1 gives the description of the E-SAR data. The L-band InSAR processing was performed using the Gamma Interferometric SAR Processor. DEMs for HH, HV and VV polarizations were generated and resampled to 5m range and azimuth pixel spacing. Band L Wavelength 23.0cm. Polarisation HH,HV & VV InSAR Mode Repeat-pass Baseline ~ 10.0m. Antenna depression angle 35o Flight altitude above sea level 3069.57m. Flight direction 2o Range pixel spacing 2.5m. Azimuth pixel spacing 0.875m. Tab. 1: Description of ESAR data The reference DEMs for the test site were generated by using ground points measured by a Total Station based upon GPS control points that were established in July 2002. The GPS survey was carried out in differential mode with dual frequency GPS receivers. This technique allows transfer of coordinates from UKs GPS Permanent Station at Kings Lynn to the study area within a one-part-per million accuracy in both horizontal and vertical position relative to the global geocentric datum. Lidar data (last return) for the same area were also available from the Environment Agency, UK and were used to generate a DEM. The GPS and Lidar-derived DEMs were adopted as references in validating the quality of the InSAR derived DEMs. 3. RESULTS AND DISCUSSIONS Interferograms for L-band were produced from the product of the first-pass image with the complex conjugate of the second-pass image (Fig. 2). The phase images were then unwrapped and used to compute the surface elevation of the image pixel to pixel basis with the incorporation of the baseline between the two sensor positions. Elevation of points at ground level will be called a Digital Terrain Model (DTM) whilst points which refer to the surface of a feature are known as a Digital Surface Model (DSM). The difference between a DSM and DTM gives the height of the feature above the ground, i.e. canopy height. The interferograms were generated based on 2- and 6-looks in range and azimuth respectively (Balzter et al., 2001). It was found that the Root Mean Square Error height at the GPS points was 5.280m (L-HH), 7.297m (L-HV) and 7.768m (L-VV). The baseline solution is shown in Tab. 2. (a) (b) Fig. 2: (a) LHH interferogram. (b) LHH DEMs Baseline Components Band LHH Band LHV Band LVV Cross Tract (m) 10.67993 10.45113 10.39170 Normal (m) -0.78506 -0.54471 -0.50788 Base length (m) 10.70874 10.46531 10.40411 Parallel (m) -9.3578 -9.0347 -8.9648 Perpendicular (m) 5.2065 5.2818 5.2800 Tab. 2: Baseline solution for L-band. A transect was taken across a no-tree and tree area to visualize the spatial behaviour of the band L against the ground features (Figure 3). The figure illustrates that InSAR DEMs as derived from L-HH (blue), L-VV (red) and L-HV (green) over the same area show some variation or bias in the elevation. The variation was minimal across the treeless area, but quite obvious when the signal interacts with tree cover (Corsican pine stand, 25 yrs. old). This shows that the depth of penetration of radar signal through forest canopies has a significant relationship with the polarisation and forest structural properties. L-band as a longer wavelength penetrates into forest canopies further down towards the ground but does not exactly reach the ground level. This can be validated if the elevation of the ground beneath the canopies is available. This will be investigated in future work. InSAR derived DEMs were compared with the reference GPS DEMs and it was found that band L-VV gives the smallest RMSE of 1.051m and 0.950 correlations. The RMSE results are summarized in Table 3 and the DEMs accuracy comparison is shown in Figure 4. It is interesting to note that the RMSE of Lidar DEMs and band L-VV DEMs are equal. Hence the reliability of band L-VV in generating DEMs in open area should be noted. profile p101-p102 50 Treeless area 40 ) m LHH ( 30 on LVV 20 vati LHV e el 10 Corsican pine (age 25 yrs) 0 distance Fig. 3: Spatial profile of band L along a transect. L-HH L-HV L-VV Lidar RMSE (m) 1.714 1.715 1.051 1.051 Correlations 0.881 0.945 0.950 0.998 coefficient Tab. 3: RMSE of InSAR DEMs and Lidar DEM 4. CONCLUSIONS This study has shown that: • L-band with full polarisation is potentially useful in producing high quality InSAR DEMs. An accuracy of less than 2m is achievable in repeat-pass mode over unvegetated areas. • The same level of accuracy is achieved by LHH and LHV DEMs (RMSE=1.7m) but the LVV DEM has shown a better result (RMSE=1.0m). The L-VV DEM is highly correlated with the ground elevation and it is suggested that this polarization should be applied in building accurate DEMs of bare terrain. • Despite 5.0m - 8.0m Mean Square Height Error (at GCPs) in the interferometric processing, it does not affect the quality of the derived DEMs in a certain incidence angle range. • The Lidar DEM (derived from last return data) shows the best fit to the real topography of the test site. The DEM derived from ground measurements is highly correlated with the Lidar DEM (RMSE= 1.0 and correlation coefficient of 0.998). • The spatial profile of L-HH, L-HV and L-VV DEMs indicates some variations of elevation derived from different polarisations.