81-86_07-039.qxd 12/13/08 1:44 PM Page 81 Accuracy Assessment of Canadian Digital Elevation Data using ICESat Alexandre Beaulieu and Daniel Clavet Abstract latter being used in mountainous area. The data set quality The accuracy of the Canadian Digital Elevation Data (CDED) assessment will be recorded in the data set’s metadata. produced over the past was based on the accuracy of the In order to evaluate the accuracy of these DEMs, good sources used for their creation (elevation extracted form control points are required. For Northern Canada, the Aerial contour lines or provincial data exchange). No means to Survey Data Base (ASDB) of Natural Resources Canada was characterize the absolute vertical precision was available, used; it is a data repository of photogrammetric control points particularly in remote areas. A new production of CDED in the that have been established through the process of aerotriangu- North is currently carried out by the Centre for Topographic lation. However, because of the lack of consistency and the Information, Natural Resources Canada with the support of small amount of existing control points in the North, these the Canadian Space Agency. Approximately 1,500 new data control points are not always sufficient for good quality sets are being produced. Altimetric data is partly acquired control estimates. Furthermore, the acquisition of new control with the European Remote Sensing satellite (ERS) by interfer- points of high accuracy for such a large territory (about ometry (70 percent) and partly by stereo-compilation with 800,000 km2) would be too expensive to be considered. In this aerial photography (30 percent). The assessment of the context, the recent Ice, Cloud and land Elevation Satellite absolute altimetric accuracy of the CDEDs themselves as (ICESAT) lidar data provides such an opportunity for obtaining opposed from the sources is required. ICESAT lidar data gives more ground control. The primary goal of ICESAT is to measure us such an opportunity. The results obtained on the first CDED inter-annual and long-term variations in the polar ice-sheet data sets produced with ERS interferometry are very promis- volume of Greenland and Antarctica. Other objectives of the ing. Accuracy for a group of 21 CDED is in the order of 0.34 m mission include: Global measurements of cloud heights, Ϯ6.22 m, i.e., 10 m at 90 percent confidence level. Accuracy atmospheric profile of clouds, height of vegetation canopies is recorded in the metadata of each data set and is freely and, of prime interest for DEM quality assessments, precise available on the GEOBASE portal (http://www.geobase.ca/). height measurements of land surface (Zwally et al., 2002). This paper demonstrates the contribution of ICESAT data for the assessment of DEM accuracies in a production Introduction environment. The ICESAT instrument and data are first intro- Knowledge of Digital Elevation Model (DEM) quality is duced and described. Previous results obtained while using important for their use in change-detection and land ICESAT as a validation tool are shown. The CDEDs used for this process investigations such as hydrologic, geomorphologic, investigation are described and the data processing to assess and biological studies; errors and imprecision of DEMs can the accuracy is discussed. To determine the horizontal accu- greatly impact the resulting models. Among others, one racy of ICESAT data versus the CDEDs, profiles of ICESAT data important aspect of DEM quality is the accuracy, which is are shifted horizontally over the CDEDs and then checked for critical to environmental modeling because terrain attrib- correlation. The local slope effect on the measured elevation utes often provide direct inputs for environmental models difference between ICESAT and the DEMs is also investigated. (Thompson et al., 2001; Deng, 2005, Wechsler and Kroll, The effect of time difference between the source data for DEM 2006). Recently, the Centre for Topographic Information production and ICESAT in a glacial environment (glacier melt- (CTI) of Natural Resources Canada has initiated a new ing) is also taken into account. Then, the results of CDED and mapping project, in cooperation with the Canadian Space ICESAT comparisons are presented and discussed. Finally, the Agency (CSA) under a Government Related Initiatives absolute difference in height between DEMs and ICESAT data Program (GRIP). This new project, called CartoNord, will are compared, and the results obtained are included in the carry out the production of 1,500 new data sets at the scale CDED metadata. of 1:50 000. A data set includes spatial data (i.e., topo- graphic vector data such as rivers and lakes) as well as elevation data which forms the Canadian Digital Elevation Data and Methodology Data (CDED). CDEDs will either be produced with interferom- The instrument aboard ICESAT, the Geoscience Laser Alti- etry of ERS tandem data (70 percent) or with aerial photog- meter System (GLAS), operates at 40 HZ, and illuminates the raphy (30 percent) depending on the terrain steepness, the Earth’s surface with its laser by spots of about 70 m. Each Centre for Topographic Information, Natural Resources Photogrammetric Engineering & Remote Sensing Canada, 2144 King Street West, Sherbrooke, Quebec, Vol. 75, No. 1, January 2009, pp. 81–86. J1J 2E8, Canada ([email protected]). 0099-1112/09/7501–0081/$3.00/0 Author(s): Beaulieu, A.; Clavet, D. © 2009 American Society for Photogrammetry © Her Majesty the Queen in right of Canada 2007 and Remote Sensing PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING January 2009 81 81-86_07-039.qxd 12/13/08 1:44 PM Page 82 footprint is separated along-track by 172 m intervals. Data for their accuracy. The first group of 21 CDEDs situated on is received by laser pulses with frequencies of 512 nm the Brodeur Peninsula, Baffin Island, has a topography and 1,064 nm, the latter being used for altimetry products. corresponding to lowlands and has moderate relief terrain. ICESAT has a polar orbit and a flight altitude of 600 km. The These DEMs were created with interferometry using ERS-1/ERS-2 GLAS instrument measures accurately how long it takes for C-band INSAR tandem data. Only winter period ERS tandems photons from a laser to pass through the atmosphere to the were used for greater stability and to lessen the impact of surface or clouds and return through the atmosphere. ICESAT snow and ice that may introduce errors in the creation of DEMs is able to know exactly where it is in space with the use of in Arctic environments (Beak et al., 2005). The three next GPS receivers and a startracking camera. More details on the groups are located in mountainous areas with high relief ICESAT products and the mission can be found in Zwally and glaciers; they were created with aerial photography by et al. (2002) and Schutz et al. (2005). stereo-compilation. They consist of a CDED in the Hans Island Calibration and validation tests were conducted with area and a group of 12 CDEDs in Quttinirpaaq National Park, ICESAT GLAS data in various environments in order to assess both on Ellesmere Island, and a last group of four CDEDs in the accuracy and consistency. In the best conditions, on flat ter- Clyde River area on Baffin Island (Figure 1). A DEM on Devon rain, ICESAT derived elevations have sub-decimeters accuracies Island created in the initial project phase is also used to (Fricker et al., 2005; Shuman et al., 2006). The range error demonstrate the effect of glacier variation through time. The of ICESAT (the error in the horizontal plane) is estimated for distribution of ICESAT data points in a CDED data set is not laser 2A data acquisition period (24 October to 18 November always sufficient (over 30 points) to get a statistically robust 2003) to be 0.0 m Ϯ 3.7 m (personal communication with assessment of the accuracy, even if at northern latitudes the David Korn, GLAS team). This is in agreement with what was density of ICESAT profiles increases (Plate 1). Therefore, statis- found by Martin et al. (2005), their range error estimate being tics are evaluated by groups of data sets created in the same Ϫ0.33 Ϯ 3.3 m. In relation with the range imprecision, for manner and in the same area. The difference in elevation is surfaces where slope steepness is high, they also demonstrated calculated as the DEM elevation minus the ICESAT elevation. that elevation errors could reach a meter per arcsec for a 20° slope. However, they claimed that range bias estimates from single pass rarely exceed 20 cm. Interesting results were also Results and Discussions observed while using ICESAT elevation data compared to Shut- Effect of Sloping Terrain tle Radar Topography Mission (SRTM) C-band DEMs (Carabajal In order to get a good distribution of ICESAT elevation points and Harding, 2005). In areas of low relief and sparse tree over the CDEDs, no points were filtered for slope steepness. It cover, Carabajal and Harding (2005) observed a mean and is usually best to use checkpoints on flat terrain or uniformly standard deviation of Ϫ0.60 Ϯ 3.46 m between both data sets sloping terrain because horizontal errors in the DEM or ICESAT (ICESAT elevations minus STRM elevations). They demonstrated may create a vertical displacement. Martin et al. (2005) that the centroid elevation values of ICESAT are very good demonstrated that a one arcsec pointing error on a 2° slope where there is no tree cover and slightly off over dense vegeta- produces a 10 cm error in elevation, which can rise to more tion cover (ICESAT penetrating a little more the vegetation than one meter for a 20° slope. In this study, using data from cover than SRTM).