Assessment of digital elevation models using RTK GPS

Transcription

1 Assessment of digital elevation models using RTK GPS Hsing-Chung Chang 1, Linlin Ge 2, Chris Rizos 3 School of Surveying and Spatial Information Systems University of New South Wales, Sydney, Australia 1 hsing-chung.chang@student.unsw.edu.au; 2 l.ge@unsw.edu.au; 3 c.rizos@unsw.edu.au Abstract A Digital Elevation Model (DEM) represents the height of a patch of terrain. Accuracies of DEMs vary depending upon the requirements of different applications. DEMs have been applied to a wide range of civil engineering and military planning tasks. Besides the conventional methods of creating DEMs, such as photogrammetry and manual field survey, airborne laser scanning and radar interferometry are new technologies capable of creating high quality DEMs in a cost-effective manner. This paper uses Real-Time Kinematic (RTK) GPS (with centimetre-level accuracy) to examine the quality of some DEMs generated by such means as photogrammetry, airborne laser scanning and radar interferometry. The results show that a DEM generated from airborne laser scanning has the highest accuracy with a RMS error of 0.09 ~ 0.3m. The RMS errors of DEMS derived by photogrammetric and radar interferometric means are 1.35 ~ 2.43m and 4.26 ~ 19.39m respectively. 1. Introduction The term digital elevation model (DEM) is used generically to mean the digital cartographic representation of the elevation of the earth surface in any form. It is sometimes also referred as a digital terrain model (DTM). The (horizontal) spacing is specified in arc-seconds, with a smaller horizontal spacing usually implying a better resolution in height - though the height accuracy is actually a function of the production methods (Greve and American Society for Photogrammetry & Remote Sensing., 1996; Mikhail et al., 2001). Currently the majority of DEMs are generated by photogrammetric methods. DEMs can be represented in various formats such as grid, lattices and triangulated irregular network (TIN) model. The formats of grid and lattices have uniform point spacing so that the same density of elevation points is applied to the entire area; while TIN uses data points more closely spaced in complex terrain and more sparsely distributed over other flatter areas. Besides the conventional photogrammetric and 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. The traditional applications of DEMs include a wide range of civil urban planing and military uses. Nowadays, flood estimation (Blomgren, 1999; Sugumaran et al., 2000), landslide detection and surface morphology mapping (McKean & Roering, 2004), and underground mining subsidence monitoring (Fischer & Spreckels, 1999; Spreckels, 2000) can also be done with the aid of high resolution

2 DEMs. The main objective of this paper is to compare the height accuracy of several DEMs derived using the technologies of photogrammetry, laser scanning and radar interferometry against field survey data. Real-Time Kinematic Global Positioning System (RTK GPS) is used to collect the field survey data. The basic methodologies of each technology are discussed separately in the following sections of this paper. A comparison of the elevation profiles derived from the DEMs against RTK GPS data is then given with the aid of Geographic Information System (GIS) software. 2. DEM Generation Technologies The quality of DEMs generated using three methodologies, namely photogrammetry, airborne laser scanning (ALS) and radar interferometry (InSAR), are assessed in this paper. Photogrammetry is a passive system which detects the reflected solar radiation from ground surface and records the returns 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. 2.1 Photogrammetric DEM Production Photogrammetry has the longest history amongst the three DEM generation technologies. It has already proven its efficiency for na range of mapping applications sincluding 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 to 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. The most common DEM format is the raster grid, with elevations given at regularly spaced points, or posts (Mikhail et al., 2001). DEMs are often classified by their post spacing, for example, 1 arc-second (approximately 30m spacing on the surface of the earth) or 3 arc-second DEM (approximately 90m), and so on. Because DEMs are discrete representations of the earth s continuous surface, sudden elevation changes, such as cliffs or deep valleys, may not be represented correctly by a regularly-spaced grid. A 1 arc-second photogrammetric DEM is used in this paper for assessment purposes. 2.2 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

3 usually an inertial measurement unit, to precisely position, attitude and acceleration 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 objects such as trees or buildings can also be measured. In general ALS derives height accuracies of grid points ranging from 0.1 ~ 0.5m, and horizontal accuracies ranging from 0.3 ~ 1.5m, with typical point spacing ranging from 0.2 to 4m (Turton & Jonas, 2003). These accuracies are dependent upon the properties of the terrain. In the cases of hilly or flat terrain densely covered by vegetation, accuracies tend to decrease (Huising & Gomes Pereira, 1998). 2.3 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 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. airborne or spaceborne), or acquired separately in time by re-visiting the scene with a single antenna (e.g. in 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 by 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 and ground surface displacement on DEM generation. In addition to the satellite systems, an 11 day space shuttle radar topography mission (SRTM) was successfully flown in February This mission used InSAR with signals in C (5.6cm) and X (3cm) bands of the microwave spectrum to create the first global DEM of the earth, in the latitude band 60 o N to 57 o S. SRTM used two antennas to scan the earth s surface instantaneously. When this paper was written the C-band 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. 3. Field Survey RTK GPS RTK GPS can deliver almost instantaneous point coordinates with centimetre-level accuracy. There are many applications that can take advantage of RTK technology, including topographic surveying, engineering construction, geodetic control, vehicle guidance and automation, etc. (Riley et al., 2000) RTK positioning uses a static GPS receiver as a reference station located at a known

4 point. Another receiver is used as the rover which can move and survey any points 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 reference to rover, where the calculation of coordinates is carried out. For our field survey, 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. The RTK GPS survey setup is shown in Figure 1. At the end of the field survey the RTK GPS data are imported into Geographic Information System (GIS) software for DEM quality assessment. Figure 1. The setup of the RTK GPS survey. The reference station is at the centre of the photo with a radio transmitter attached to the tripod. The rover antenna and its radio receiver are mounted on the car roof. 4. Comparison of DEMs against RTK GPS The data are interpreted using GIS software, which provides an ideal environment for datum conversion, geo-referencing, profile extraction, interpretation and visualisation. All the data are transformed into the local ISG-56 coordinate system before comparison. ALS data covers only a small portion of the test site, and consists just of points with 3D coordinates. A triangulated irregular network (TIN) DEM model with pixel size 5m is derived based on the raw ALS measurements. The DEMs of the 1 arc-second photogrammetric DEM and the tandem InSAR DEM are in the form of rasters with pixel sizes of 30m and 20m respectively. These three DEMs are displayed in Figure 2.

5 Figure 2. The 3 Digital Elevation Models: ALS DEM with aerial photo as background (top), 1 arc-second photogrammetric DEM (bottom left), and InSAR tandem DEM (bottom right). The elevation of the DEMs is colour-coded as indicated by the scalebar at the top right corner. The elevation profiles are extracted from the DEMs at the same locations where the RTK GPS samples were taken. Due to the very limited coverage of the ALS DEM, only 4 parts of the field survey data have overlapped with all three DEMs. They are denoted as route1, 2, 3 and 4 in this paper. The map of the RTK GPS survey overlaid with an aerial photo, and the ALS DEM are shown in Figure 3. Some roads during the survey have been measured twice, in both directions. In order to demonstrate the accuracy of the RTK GPS measurements, the root-mean-square (RMS) error has been calculated between the forward and backward survey runs. For example, the results of route 1 and 3 are shown in Figure 4. The RMS error for route 1 and 3 are 15cm and 26cm respectively. The profiles extracted from the three DEMs along the four routes of the RTK GPS surveys are shown in Figure 5, and the RMS errors are summarised in Table 1. The profiles show that the ALS DEM is highly correlated with the RTK GPS survey data, while the profiles derived from the photogrammetric DEM have some variations (or shifts), while the InSAR DEM can only provide information on the trends of the terrain in general. RMS error analysis indicates that the ALS DEM has the highest accuracy (as measured against RTK GPS 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 mean RMS errors of 2.08m and

6 14.79m respectively. Figure 3. The maps of the RTK GPS survey (in orange) are overlaid with the ALS DEM (in grey) and an aerial photo. The overlaps between the survey data and ALS DEM are coloured in red, and indicated as route 1, 2, 3 and 4. Figure 4 The RTK GPS survey data along the two sides of the road along route 1 (left) and route 3 (right). The RMS errors for the data collected in opposite directions along the same roads for route 1 and route 3 are 15cm and 26cm respectively. (a) (b)

7 (c) (d) Figure 5. The elevation profiles of the DEMs and RTK GPS survey along (a) route1, (b) route 2, (c) route 3, and (d) route 4. ALS Photogrammetry InSAR Tandem Route1_forward 0.30 m 2.43 m m Route1_backward 0.30 m 2.43 m m Route m 1.35 m 4.26 m Route3_forward 0.10 m 2.26 m m Route3_backward 0.11 m 2.30 m m Route4_forward 0.14 m 1.90 m m Route4_backward 0.15 m 1.85 m m Mean 0.17 m 2.08 m m Table 1. RMS errors of the profiles extracted from the three DEMs compared with the RTK GPS ground truth data. The assessment of the precision of the RTK GPS 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 RTK GPS measurement error at a static point, the height difference caused by the slope of the road, and the effects due to surrounding objects such as trees or buildings. While the absolute accuracy is always of interest, another useful statistic often used is the relative accuracy, which describes the internal consistency of the dataset. For a DEM the relative accuracy specifies the accuracy of the differences in elevation between posts. A DEM might be affected by an overall vertical shift, making its absolute accuracy poor, but still has good relative accuracy (Mikhail et al., 2001). The results described in this paper are preliminary results of the assessment of DEM quality using RTK GPS. More field survey will be conducted in the future. Additional analyses of the accuracy of RTK GPS, in both static and dynamic modes, will also be carried out. 5. Concluding Remarks The accuracy of three DEMs derived using airborne laser scanning, photogrammetry and spaceborne radar interferometry have been examined by comparison with RTK

8 GPS 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 1.35 ~ 2.43m and 4.26 ~ 19.39m respectively. Acknowledgements The authors wish to thank Mr. Andrew Nesbitt of BHPBilliton for his support in supplying laser scanning data and in assisting with the RTK GPS field survey. The assistance of ACRES (the Australian Centre for Remote Sensing) in providing the radar images is also acknowledged. References Blomgren, S. (1999). "A digital elevation model for estimating flooding scenarios at the Falsterbo Peninsula", Environmental Modelling & Software, 14(6), Fischer, C. & V. Spreckels (1999). "Environmental monitoring of coal mining subsidences by airborne high resolution scanner", IEEE Geoscience & Remote Sensing Symposium IGARSS '99. Greve, C. & American Society for Photogrammetry & Remote Sensing. (1996). Digital Photogrammetry : An Addendum to the Manual of Photogrammetry, Bethesda, Md., American Society for Photogrammetry & Remote Sensing. Huising, E.J. & L.M. Gomes Pereira (1998). "Errors and accuracy estimates of laser data acquired by various laser scanning systems for topographic applications", ISPRS Journal of Photogrammetry & Remote Sensing, 53(5), McKean, J. & J. Roering (2004). "Objective landslide detection and surface morphology mapping using high-resolution airborne laser altimetry", Geomorphology, 57(3-4), Mikhail, E.M., J.C. McGlone & J.S. Bethel (2001). Introduction to Modern Photogrammetry, New York, Chichester, Wiley. Riley, S., N. Talbot & G. Kirk (2000). "A new system for RTK performance evaluation", IEEE Position Location & Navigation Symposium. Spreckels, V. (2000). "Monitoring of coal mining subsidence by HRSC-A data", XIXth ISPRS Congress, Amsterdam, The Netherlands. Sugumaran, R., C.H. Davis, J. Meyer & T. Prato (2000). "High resolution digital elevation model and a web-based client-server application for improved flood plain management", IEEE Geoscience & Remote Sensing Symposium IGARSS Turton, D. & D. Jonas (2003). "Airborne Laser Scanning - Cost effective spatial data", Map Asia Conference, Kuala Lumpur, Malaysia.