Accuracy Assessment of Canadian Digital Elevation Data Using Icesat

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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., topographic 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

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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). In the present case, since the validation of the different groups of CDEDs together (for a total of 8,703 valid DEM accuracies occurs only for DEM in the far North (over ICESAT points), it was found that the effect of slope is of 69.75° North), trees are not an issue. These results demon- similar magnitude as Martin et al. (2005). When comparing strate that ICESAT is a powerful tool for the vertical accuracy height differences between ICESAT and DEM elevations versus assessment of DEMs. The ICESAT data used in the present study is extracted with a tool provided on the GLAS website (http://nsidc.org/ data/icesat/), the NSIDC GLAS Altimetry elevation extractor Tool (NGAT). It extracts elevation and geoid data from GLAS altimetry products (GLAS/ICESAT L2 Global Land Surface Altimetry Data, product GLA14), and it outputs latitude, longitude, elevation, and geoid in ASCII columns. ICESAT elevation data is referenced to the TOPEX/Poseidon-Jason ellipsoid (Schutz et al., 2005). According to Canadian Digital Elevation Data standards, CDED elevations must be referenced to the Canadian Geodetic Vertical Datum of 1928 (CGVD28) with the North American Datum of 1983 (NAD83). In order to obtain absolute vertical accuracy estimates between CDED elevations with ICESAT GLAS data, and at the same time to be compatible with modern positioning techniques (e.g., GPS heights, altimetry), the Canadian Gravimetric Geoid model of 2005 (CGG05) proposed by Geodetic Survey Division of Canada (Véronneau et al., 2006) was used instead of the CGVD28 model to translate ICESAT GLAS data into orthometric heights. Bilinear interpola- tion using four points is used to get coinciding elevation over the DEM for each ICESAT point location. ICESAT data points have to be filtered to remove potential elevation anomalies resulting from clouds or valley fog. In order to do that, any point location showing a difference between the interpolated DEM elevation and the ICESAT elevation greater than 50 m is removed from the statistical estimations. Figure 1. Map of Baffin and Ellesmere Island, North of Four groups of CDED data sets (a data set being equivalent Canada, showing the areas chosen for DEM accuracy to a 1:50 000 scale map sheet), which are the first CDEDs analysis with ICESAT data. produced in the Northern mapping program, were assessed

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Plate 1. Distribution of the ICESAT laser 2A profiles over a group of 21 CDEDs generated from ERS interferometry. There are a total of 9,869 elevation points over these CDEDs.

the slope, it is hard to say if a real tendency is present, mostly offset in longitude. The CDED data is geo-referenced with because there is not enough information for higher slopes (i.e., Ortho-Landsat-7 imagery available on the GEOBASE website, over 20°). In order to get a more realistic picture of the slope which has an evaluated absolute circular error of 24 meters at variation, the average of the absolute elevation differences is a 90 percent confidence level, or 16 meters at 68 percent in plotted against the slope in Figure 2. In this case, we get this area. Knowing that most of the ICESAT data is situated in a variation of about 12 cm per 2° of slope, which would areas of low to moderate terrain (70 percent under 5° and 83 produce an average error of 1.2 meter on a 20° slope. This percent under 10°), hence the average slope is very low for result is very close to the one obtained by Martin et al. (2005). most CDEDs, and effect of slope on the accuracy estimation is ICESAT range error and DEM horizontal positioning error negligible compared to other potential sources of error. could both affect the final accuracy measurements. In order to estimate this effect, tests were run on ICESAT profiles, which Effect of Glacier Dynamics on DEM Accuracy were translated horizontally over the DEM; DEM elevation is In Northern areas, glaciers and ice caps can cause accuracy recalculated for each ICESAT point. Correlation coefficients assessment problems of major importance when comparing were then calculated for each translated profiles and then DEM created from old air photos with recent ICESAT data. plotted on a grid with the translation parameters in X and Y. The temporal gap between photos and ICESAT can introduce An example of this data manipulation is shown in Figure 3 a vertical shift due to ice ablation or accumulation that may for one ICESAT profile. In this example, an offset of Ϫ0.3 arcsec impair the accuracy estimation. This effect was observed in X and one arcsec in Y in between the original location when tests using ERS interferometry and aerial photo stereo- (shown by a black dot on Figure 3) on the ICESAT profile compilation for DEM production were conducted on Devon and the DEM corresponding elevations and the centroid of Island in the initial phase of the project. The effects of glacier the contours (shown as a grey star) is observed. For the 21 advance on the DEMs (ERS interferometry DEM and aerial photo CDED produced by interferometry, this offset is approximately stereo-compilation DEM) and their compared elevations with Ϫ2 arcsec in X and 0.1 arcsec in Y, corresponding to a 20 m ICESAT elevation points, demonstrated vertical displacements

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Figure 2. Plot of the average of the absolute values for ICESAT-CDED elevations in meters per degree of slope versus the slope in degrees for 8,703 data points. Figure 3. Correlation map of an ICESAT profile translated over a CDED. Square of the Pearson correlation coefficient of the ICESAT elevation versus related to the movement of the glacier. The ERS interferometry the CDED elevation is calculated for each increment DEM is more concordant with ICESAT elevation than the photo in X and Y. Contour interval is 0.001, peaking at DEM, which shows, at the Devon Island ice cap front, eleva- 0.963 around the grey star. tion differences of up to 30 m (Figure 4). The time difference for ERS data (1995 to 1996) with ICESAT (2003) is shorter (ϳ8 years) than the one between ICESAT and the aerial photography (1958 to 1960) which spans up to 43 to 45 years. This would explain the discrepancy between the two DEMs (ERS When evaluating accuracy, inclusion or exclusion and air photo) over the Devon Island ice cap; the accumula- of ICESAT data over glaciers can create large differences in tion of ice over time and the progression of the ice front the final estimation. Using vector data, ICESAT points that created this difference. The example on Devon Island is fall on glaciers or ice-caps can be flagged as such, and shown because of the two available DEMs (old photo DEM and then removed if necessary. Such examples of vertical varia- new ERS interferometry DEM) and their differences and tions were observed in two areas. In the Clyde River area on relations with ICESAT data. In most other instances, receding Baffin Island at latitude 70° North, bias and standard devia- glaciers are observed, such as on Baffin Island in the Clyde tion of 18.0 m and 18.4 m, respectively, were observed for River area and on Ellesmere Island in Quttinirpaaq Park. height differences between ICESAT and DEMs over valley glaciers. The 18 m bias shows that ICESAT is in average 18 m lower than the DEM, hence indicating that melting and/or retreat of glaciers in this area is very important. In the Quttinirpaaq Park area, a bias of seven meters and a standard deviation of 17.6 m were observed with a total of 9,440 ICESAT elevation points on ice caps. It becomes obvious that, in order to obtain the accuracy of the DEM, ICESAT data falling on glacier and ice-cap area need to be removed from the statistics. There are many factors involved in the difference observed in the magnitude of ice melt between the two areas (Quttinirpaaq Park on Ellesmere Island and Clyde River area), where the dynamics of climatic changes probably vary from one area to another. Although interesting, it was not in the scope of the present study to investigate such process. For the Brodeur Peninsula area, over which the DEMs were produced by interferometry, there are no glaciers, thereby not affected by this issue.

Accuracies Obtained with ICESat This investigation of the accuracy of CDEDs was conducted only with ICESAT laser 2A acquisition data period in version 24, and as already explained, the density of points over each CDED is not always sufficient (over 30 points) to get robust statistics. This is why CDEDs were analysed by groups. With Figure 4. Elevation profiles from an ERS DEM, a the new version of ICESAT data 26 and now 28, new data Photo DEM and ICESAT data over an ice cap on sets (acquisition periods) are made available, filling in the Devon Island. gap where there were clouds in the first ICESAT release. This constraint of validating accuracy by groups may well be over

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with the incorporation of the whole ICESAT data acquisition periods (11 in total presently). Last minute investigation with the new available ICESAT datasets is showing promising coverage. The total of ICESAT points over Canada increases from 5,000,000 to 12,000,000 with just the addition of the 2B and 3D data releases. When points or profiles of ICESAT data from different data sets (2A, 2B, and 3D) are too close or overlap, they are filtered, keeping the points according to the following criteria: higher elevation points are removed first if the elevation difference is more than 50 m between the two overlapping ICESAT points. If this difference is lower, then the most imprecise point (accuracy provided by the ICESAT team) is removed. Tests on the coverage of these new data points in northern Canada showed that only 160 of the 1,486 CDEDs are not covered properly, either because there are not enough points caused by the presence of clouds or because the CDEDs contain only a small proportion of land, the latter case occurring most of the time. As for the vertical accuracy estimation of the CDEDs, the ICESAT elevation data are closely correlated with the DEM Figure 5. ICESAT elevations versus DEM elevations for elevations interpolated under the location of each point all data analysed excluding points on the ocean and (Figure 5). The slope and the intercept show that the DEM is on glaciers, points potentially showing cloud eleva- concordant with ICESAT data, the latter being considered tions and outliers due to cliff areas (very high slope absolute in comparison because of its higher vertical under the point). accuracy. Estimation of the accuracy for the Brodeur Penin- sula area data sets, which were produced by interferometry in a single contract (Plate 1), gave promising results (Table 1). After the removal of incoherent or unwanted ICESAT data from the Canadian Aerial Survey Database (ASDB), and points (e.g., points in the ocean, points showing cloud since ICESAT data are similar to these CDED, it demonstrates elevations, outliers due to cliff areas, points on glaciers), that the ASDB is an appropriate source of ground control there is a total of 4,649 points remaining over 9,869 ICESAT points in terms of absolute values. This information could data elevation points for this area, yielding a result of 0.34 help to qualify the ASDB in areas where the absolute accu- m Ϯ 6.22 m. As for the three other groups, results are in the racy is not well known (such as in the North). As stated same order (see Table 1), although a little high, probably before, since trees are not an issue in the North, the only because of the greater variations in elevation in these areas. thing that has to be taken into account when estimating the Elevations on the Brodeur Peninsula area vary from 0 m (sea absolute accuracies of the DEMs, and particularly those level) to 603 m over the 21 CDED cells, with moderate relief created by aerial photography, are glaciers or ice caps that variations. In the other areas, steep elevation variations are should be masked out if they are present in the DEM being 1,126 m, 2,559 m, and 1,859 m for Hans Island, Quttinirpaaq evaluated. It is also demonstrated by this investigation that Park and Clyde River, respectively. the quality of DEMs generated by ERS interferometry meets our expectations. DEMs created through this process are of high quality and up-to-date. ICESAT land elevation data Conclusion introduces a new type of data validation source to existing As demonstrated, ICESAT’s land elevation data is a valuable sources, such as the ASDB composed of photogrammetric source for the assessment of DEM vertical accuracy, particu- control points, for the estimation of the absolute accuracies larly in remote areas such as the Northern latitudes in of CDED product. Canada. The ICESAT data collection for Canada, for the laser 2A period version 24, sums up to over 5,000,000 data points; elevation points were extracted using the NGAT tool. Acknowledgments When the data from all of the laser acquisition periods will We would like to thank the ICESAT Science Project and the be available, this collection of high precision land eleva- NSIDC for their accomplishment and support, more specifically tion will be exceptionally useful for applications such as: M. David Korn for his help, and moreover for the free access accuracy assessments, vertical control, change detection of of ICESAT data (see http://nsidc.org/data/icesat/). We are also ice caps and glaciers, and even correction of DEMs. thankful to the Canadian Space Agency for their contribution, The CDED generated from aerial photo are of particular particularly to M. Paul Briand for his support. Finally, the interest because they are generated using the control points contribution of M. Marc Véronneau from Geodetic Survey

TABLE 1. ACCURACY ASSESSMENT OF CDEDS WITH ICESAT LAND ELEVATION DATA

Area Terrain Data type # of CDED # of points Mean (m) STD (m) LE901 (m)

Brodeur Peninsula area, Baffin Lowlands INSAR 21 4649 0.3 6.2 10.2 Hans Island, Ellesmere Mountains Photo 1 108 5.3 6.4 13.7 Quttinirpaaq Park, Ellesmere Mountains Photo 12 3257 2.0 8.1 13.7 Clyde River area, Baffin Mountains Photo 4 689 3.0 7.6 13.4

1- Absolute Linear Error at 90 percent including the bias; LE90 ϭ (STD2ϩMean2)0.5*1.6449

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Division was greatly appreciated. Natural Resources Canada, Schutz, B.E., H.J. Zwally, C.A. Shuman, D. Hancock, and Earth Science Sector contribution number: 20070364. J.P. DiMarzio, 2005. Overview of the ICESat Mission, Geophysi- cal Research Letters, 32(21), doi:L21S0110.1029/2005GL024009. Shuman, C.A., H.J. Zwally, B.E. Schutz, A.C. Brenner, J.P. DiMarzio, V.P. Suchdeo, and H.A. Fricker, 2006. ICESat Antarctic References elevation data: Preliminary precision and accuracy assessment, Baek, S., O. Kwoun, A. Braun, Z. Lu, and C.K. Shum, 2005. Geophysical Research Letters, 33(7), doi:10.1029/2005GL025227. Digital elevation model of King Edward VII Peninsula, Thompson, J.A., J.C. Bell, and C.A. Butler, 2001. Digital elevation West Antarctica, from SAR interferometry and ICESat laser model resolution: Effects on terrain attribute calculation and altimetry, IEEE Geoscience and Remote Sensing Letters, quantitative soil-landscape modeling, Geoderma, 100:67–89. 2(4):413–417. Véronneau, M., R. Duval, and J. Huang, 2006. A gravimetric geoid Carabajal, C.C., and D.J. Harding, 2005. ICESat validation of SRTM model as a vertical datum in Canada, Geomatica, 60(2):165–172. C-band digital elevation models, Geophysical Research Letters, 32(22), doi:L22S0110.1029/2005GL023957. Wechsler, S.P., and C.N. Kroll, 2006. Quantifying DEM uncertainty and its effect on topographic parameters, Photogrammetric Deng, Y., 2005. Evaluating Input Data for DEM-Based Environmen- Engineering & Remote Sensing, 72(9):1081–1090. tal Analysis and Classification, Thesis dissertation, University of Southern California, California, 209 p. Zwally, H.J., B. Schutz, W. Abdalati, J. Abshire, C. Bentley, A. Brenner, J. Bufton, J. Dezio, D. Hancock, D. Harding, T. Fricker, H.A., A. Borsa, B. Minster, C. Carabajal, K. Quinn, and Herring, B. Minster, K. Quinn, S. Palm, J. Spinhirne, and R. B. Bills, 2005. Assessment of ICESat performance at the Salar Thomas, 2005. ICESat’s laser measurements of polar ice, de Uyuni, Bolivia, Geophysical Research Letters, 32(21), atmosphere, ocean, and land, Journal of Geodynamics, doi:L21S0610.1029/2005GL023423. 34(3–4):405–445. Martin, C.F., R.H. Thomas, W.B. Krabill, and S.S. Manizade, 2005. ICESat range and mounting bias estimation over precisely-surveyed terrain, Geophysical Research Letters, (Received 01 May 2007; accepted 19 September 2007; revised 12 32(21), doi:L21S0710.1029/2005GL023800. November 2007)

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