Validation of DEMs Derived from Radar Interferometry ... - CiteSeerX

Validation of DEMs Derived from Radar Interferometry, Airborne Laser Scanning and Photogrammetry by Using GPS-RTK Hsing-Chung Chang1, Linlin Ge2, Chris Rizos3, Tony Milne4 1,2,3 4

1

School of Surveying and Spatial Information Systems School of Biological, Earth and Environmental Science University of New South Wales Sydney, Australia

[email protected]; [email protected]; [email protected]; [email protected]

Abstract—A high resolution digital elevation model (DEM) enables easy derivation of subsequent information for various applications. This paper uses Real-Time Kinematic (RTK) GPS to examine the quality of some DEMs generated by such means as radar interferometry (InSAR), airborne laser scanning (ALS) and photogrammetry. The preliminary results show that a DEM generated from ALS has the highest accuracy with a RMS error of 0.09 ~ 0.3m. The RMS errors of DEMs derived by photogrammetric and radar interferometric techniques are 1.03 ~ 3.75m and 4.26 ~ 27.81m respectively. Keywords-DEM; GPS-RTK; Interferometric Synthetic Aperture Radar (InSAR); Airborne Laser Scanning (ALS);Photogrammetry

I.

INTRODUCTION

A high resolution digital elevation model (DEM) enables easy derivation of subsequent information for many applications such as urban planning, flood plain mapping, mobility mapping in military, crop planning and mapping, etc. DEM is also sometimes used in differential interferometric synthetic aperture radar (DInSAR) processing to remove the topographic information, in order to detect any ground surface displacement. Currently the majority of DEMs are generated by photogrammetric methods, and sometimes by field surveying techniques. The new technologies of radar interferometry, or so-called interferometric synthetic aperture radar (InSAR), and airborne laser scanning (ALS) can also be used to generate high quality DEMs. New applications such as flood estimation [1][2], landslide detection and surface morphology mapping [3], and underground mining subsidence monitoring [4][5] have demonstrated the benefit of high quality DEMs. This paper demonstrates the feasibility of real-time kinematic GPS (GPS-RTK) for validating digital elevation models derived by photogrammetry, InSAR and ALS for a selected area in the eastern state of New South Wales, Australia. The basic methodologies of each technology are

outlined in the following sections. A comparison of the elevation profiles derived from the DEMs against GPS-RTK field survey data is then presented with the aid of Geographic Information System (GIS) software. II.

REMOTE SENSING TECHNOLOGIES FOR DEM GENERATEION

Three DEMs generated by photogrammetry, airborne laser scanning (ALS) and radar interferometry (InSAR) have been assessed in this paper. Photogrammetry is a passive system which detects the reflected solar radiation from the ground surface, and records the reflections digitally or on film. Unlike photogrammetry, ALS and InSAR are active systems that provide their own energy source for transmitting signals, the reflected signals then being recorded digitally. ALS and InSAR are all-weather, 24-hour systems, while the photogrammetric method is more restricted by time-of-the-day and weather conditions. A. Photogrammetric DEM Production Photogrammetry has the longest history amongst the three DEM generation technologies. It has already proven its efficiency for a range of mapping applications including the production of orthophotos, cartographic maps, DEMs, etc. With the rapid growth of the usage and maturity of GIS, digital photogrammetry is now very widely used as a versatile spatial information acquisition technology. The generation of a DEM using photogrammetric principles has two operational parts: firstly the measurement phase, and secondly the derivation of the DEM. The main data source is from aerial photography (digital or film-based), and through either an interactive (operator-based) measurement procedure, or via a highly automated procedure, digital image processing methods are applied. The interpolation process identifies DEM points from the stereo-pair of aerial photos based on feature

matching. A 1 arc-second (30m) photogrammetric DEM is used in this paper for assessment purposes, as shown in Fig. 1. B. Airborne Laser Scanning DEM Production ALS is a member of the so-called “Light Detection Ranging” (LiDAR) group of surveying methodologies that include airborne laser profiling and terrestrial laser scanning. Data is collected by the laser scanner mounted on the airplane as a stream of discrete reflected laser points from the ground. The system also exploits GPS, and usually an inertial measurement unit, to give precise position, attitude and acceleration information of the aircraft. At least two recordings, the first and last received signals, of each of the reflected laser points are recorded. By determining the difference between the two received signals, the height of trees or buildings can also be measured. In general ALS derives height accuracies at grid points ranging from 0.1 ~ 0.5m, and horizontal geo-referencing accuracies ranging from 0.3 ~ 1.5m, with typical point spacing ranging from 0.2 to 4m [6]. These accuracies are dependent upon the characteristics of the terrain. In the cases of hilly, or flat terrain densely covered by vegetation, accuracies tend to decrease [7]. Only a partial area of the test site is covered by ALS data. The original samples (points) of ALS data have been converted to a triangulated irregular network (TIN) DEM model with pixel size 5m. The ALS DEM, with the local aerial photograph as background, is shown in Fig. 2.

Figure 2. The ALS DEM covering only a small portion of the test site.

C. InSAR DEM Production Synthetic Aperture Radar (SAR) is a side-looking active radar-ranging system. It uses the microwave portion of the electromagnetic spectrum, encompassing frequencies in the range 0.3GHz to 300Ghz (or in wavelength terms, from 1m to 1mm). Each SAR image contains information of both the amplitude and phase of the reflected signals. InSAR requires two SAR images acquired over the same scene. These two images can be acquired either at the same time by using two separate antennas mounted on the platform (e.g. typical airborne, and some spaceborne systems), or acquired separately in time by re-visiting the scene with a single antenna (e.g. as in typical satellite radar systems). The two images are then co-registered precisely to each other so that the phase difference between the pixels in the two images can be calculated. This phase difference, or so-called interferogram, can be used to derive the DEM of the imaged area. The InSAR DEM used for the assessment in this paper was derived using the images collected during the ‘tandem’ mission of the ERS-1 and ERS-2 satellites, where there was only one day difference between the two radar image acquisitions. This short temporal separation reduces the impact of noise, vegetation growth and ground surface displacement on DEM generation. The details of the two radar acquisitions of the tandem pair and the derived DEM are shown in TABLE I and Fig. 3.

Figure 1. 1 arc-second photogrammetric DEM of the test site.

TABLE I.

THE TANDEM PAIR USED TO GENERATE THE INSAR DEM.

Mission

Date (yyyy/mm/dd)

Orbit

Track

Frame

Baseline (metre)

ERS-1

1995/10/29

22434

402

4293

0

ERS-2

1995/10/30

2761

402

4293

49

In addition to the satellite systems, an 11 day space shuttle radar topography mission (SRTM) was successfully flown in February 2000. This mission used InSAR with signals in the C (5.6cm) and X (3cm) bands of the microwave spectrum to create the first DEM of the entire earth, in the latitude band 60°N to 57°S. SRTM used two antennas to image the earth’s surface instantaneously. When this paper was written the Cband SRTM DEM for the Australia region have not yet been released by NASA, and only very limited regions (but not our test site) had been covered by the X-band data. Therefore, only the ERS-1/2 tandem DEM is used as an example of an InSAR DEM for assessment purposes.

at a static point, the height difference caused by the slope of the road, and the influence of surrounding objects such as trees or buildings. An example of the difference in height measured along the two sides of the road is given in Fig. 4. The effect of tilted road surface is strongly suggested by the offset in height shown in Fig. 4, with a RMS of 26cm. B. Assessment of DEMs against GPS-RTK The data collected by GPS-RTK over the test site are displayed with aerial photo and ALS DEM in Fig. 5. The comparisons between the DEMs and GPS-RTK are made in two parts. The first part is to compare the elevation profiles drawn from the 3 DEMs based on the very limited overlap between the ALS DEM and the survey data. The second part is to compare the elevation profiles drawn from the photogrammetric and InSAR DEMs along other survey paths.

Figure 3. The InSAR tandem DEM over the test site.

III.

GPS-RTK FIELD SURVEY

GPS-RTK can deliver instantaneous point coordinates with centimetre-level accuracy. There are many applications that take advantage of RTK technology, including topographic surveying, engineering construction, geodetic control, vehicle guidance and automation, etc. [8]. RTK positioning uses a static GPS receiver as a reference station located at a known point. Another receiver is used as the rover which can move and survey any point of interest. Both receivers make observations of the GPS signals at the same time and a radio data link between the two receivers permits data to be sent from the reference to rover, where the calculation of coordinates is carried out.

The RMS errors of the profiles extracted from the three DEMs along the overlapped GPS-RTK surveys are summarised in TABLE II. RMS error analysis indicates that the ALS DEM has the highest accuracy (as measured against GPS-RTK ground truth data) among the three DEMs considered. The ALS DEM has the lowest mean RMS error of 0.15m and a maximum of 0.3m. Photogrammetric and InSAR DEM have RMS errors ranging from 1.03 ~ 3.75m and 4.26 ~ 27.81m respectively. One example shown in Fig. 6 indicates that the ALS DEM is highly correlated with the GPS-RTK survey data, while the profiles derived from the photogrammetric DEM have some variations (or shifts), and the InSAR DEM can only provide information on the general trends of the terrain.

For the field survey at the test site, the reference receiver station is set up on top of a hill at a pillar whose precise coordinates are known. The rover is mounted on a car roof so that the survey can be easily performed by driving along roads within radio link coverage, which is approximately 9 ~ 10km in radius from the reference station. Only the samples of the survey data having 3-dimensional accuracy better than 5 centimetres are imported into Geographic Information System (GIS) software for DEM quality assessment. IV.

RESULTS AND DISCUSSIONS

The data are interpreted using GIS software, which provides an ideal environment for datum conversion, georeferencing, profile extraction, interpretation and visualisation. All the data are transformed into the local ISG-56 coordinate system before comparison. A. Precision Assessment of GPS-RTK Survey The assessment of the precision of the GPS-RTK survey is made by comparing the data collected along the same paths, either driving both sides of the road or from another repeated field survey. The assessment indicates that the variation of between the two trajectories is in the range of 15 ~ 26cm. This variation is the composite of the GPS-RTK measurement error

Figure 4. GPS-RTK survey data colloected along the two sides of the road.

V.

CONCLUDING REMARKS

The accuracy of three DEMs derived using airborne laser scanning, photogrammetry and spaceborne radar interferometry have been examined by comparison with GPS-RTK field survey results. Our results show that ALS has the best accuracy, ranging from 0.09 ~ 0.3m, while the photogrammetric DEM and InSAR DEM have accuracies of 1.03 ~ 3.75m and 4.26 ~ 27.81m respectively. ACKNOWLEDGMENT The authors wish to thank Mr. Andrew Nesbitt of BHPBilliton for his support in supplying laser scanning data and in assisting with the GPS-RTK field survey. The assistance of ACRES (the Australian Centre for Remote Sensing) in providing the radar images is also acknowledged. Figure 5. The maps of the GPS-RTK survey (in red) are overlaid with the ALS DEM (in grey) and an aerial photo.

REFERENCES [1]

[2]

[3]

[4]

[5] Figure 6. The elevatino profiles of the DEMs and GPS-RTK survey along the path 2.

[6]

TABLE II. RMS ERRORS OF THE PROFILES EXTRACTED FROM THE THREE DEMS COMPARED WITH THE GPS-RTK GROUND TRUTH DATA

[7]

ALS

Part 1

Part 2

Path 1 forward Path 1 backward Path 2 Path 3 forward Path 3 backward Path 4 forward Path 4 backward Mean Path 5 Path 6 Path 7 Path 8 Mean

Photogrammetry

InSAR

0.30 m

2.43 m

11.74 m

0.30 m

2.43 m

11.70 m

0.09 m

1.35 m

4.26 m

0.10 m

2.26 m

18.57 m

0.11 m

2.30 m

18.45 m

0.14 m

1.90 m

19.38 m

0.15 m

1.85 m

19.39 m

0.17 m

2.08 m 1.03 m 3.10 m 1.22 m 3.75 m 2.28 m

14.79 m 7.89 m 14.87 m 27.81 m 26.39 m 19.24 m

[8]

not available

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