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| Earth Resources Observation and Science (EROS) |
Dean B. Gesch, Hughes STX Corporation, EROS Data Center, Sioux Falls, SD
57198
e-mail:gesch@dg1.cr.usgs.gov
Kevin S. Larson, Berger & Co., 2828 Routh Street, Suite 350, Dallas, TX
75201
e-mail: klarson@berger.com
Work your way through the poster using the imagemap, or use the links below.
Read the Abstract | Introduction | Data Sources | Data Processing | Accuracy Assessment | Summary | Read the Full Text of the Paper
In response to well documented requirements for regional and continental scale digital elevation models (DEM's), the staff at the U.S. Geological Survey's EROS Data Center are developing global digital elevation data with a horizontal grid spacing of 30 arc seconds (approximately 1 kilometer). The DEM's are generated from several topographic data sources, each requiring specific processing methodologies and quality assurance procedures. The continent of Africa serves as a representative example for a description of the techniques used in development of the 1-kilometer elevation models.
The global 1-kilometer DEM's are based primarily on data derived from two sources: Digital Chart of the World (DCW) and Digital Terrain Elevation Data (DTED). DCW is a vector cartographic data set based on the 1:1,000,000-scale Operational Navigation Chart (ONC) series, which is the largest scale base map source with global coverage. DTED is a raster topographic data base with a horizontal grid spacing of 3 arc seconds (approximately 90 meters). Approximately 56% of the Africa DEM is based on DTED, while the remaining 44% is based on DCW.
The full version of this image is 33K.
The topographic information from the DCW was converted into an elevation grid through a vector-to-raster gridding approach. The hypsography, drainage, and coastline data were input to the ANUDEM surface gridding program developed at the Australian National University. Preprocessing of the DCW data included editing and updating the vector stream lines so that the direction of each was oriented downstream (a requirement of ANUDEM). This figure shows the data for one of the 197 DCW 5-degree tiles processed for the Africa DEM.
The full version of this image is 83K.
ANUDEM, specifically designed for creating DEM's from digital contour, spot height, and stream line data, employs an approach known as drainage enforcement to produce raster elevation models that represent more closely the actual terrain surface and contain fewer artifacts than those produced with more general purpose surface interpolation routines. This figure illustrates the advantages of incorporating stream line data and drainage enforcement in ANUDEM, leading to better topographic definition, especially in areas of sparse hypsography data.
The full version of this image is 83K.
The 3 arc second DTED were generalized to 30 arc seconds by selecting one elevation value to represent the area covered by 100 full resolution grid cells (a 10 by 10 matrix). The generalization approach emphasized the topographic breaklines (ridges and stream channels) as found in the full resolution data. The breaklines, extracted directly from the full resolution data, guided the selection of elevation values for the generalized cells, thus preserving significant topographic features in the reduced resolution representation.
The full version of this image is 182K.
Merging of the generalized DTED and the DCW grid to produce the Africa DEM was accomplished by mosaicking the two data sets. The generalized DTED had the highest priority so that coverage of the source with the greater topographic detail and accuracy was maximized. The DCW grid filled in the areas of the African continent not covered by DTED. The merging procedure included blending of the two data sources along the irregular boundary to minimize the differences and smooth the transition. This figure shows the coverage and percent of the total area of the final merged DEM derived from each data source.
The full version of this image is 50K.
The horizontal coordinate system of the final Africa DEM is decimal
degrees of latitude and longitude referenced to WGS84, and the vertical
units represent elevation in meters above mean sea level.
The full version of this image is 264K.
The absolute vertical accuracy of the Africa DEM varies by location according to the source data. Generally, the areas derived from DTED have a higher accuracy than those derived from the DCW. The full resolution 3 arc second DTED have a vertical accuracy of ± 30 meters linear error at the 90% confidence level. If the error distribution is assumed to be Gaussian with a mean of zero, the statistical standard deviation of the errors is equivalent to the root mean square error (RMSE). Under those assumptions, vertical accuracy expressed as ± 30 meters linear error at 90% can also be described as an RMSE of 18 meters. The areas of the Africa DEM derived from DTED retain that same level of accuracy because through generalization an actual elevation value from one full resolution DTED cell is chosen to represent the area of the reduced resolution cell.
To characterize the accuracy of the areas of the Africa DEM derived from the DCW, the DCW grid was compared to 30 arc second DTED, which had been aggregated by averaging. If the averaged DTED are thought of as the reference data set, the RMSE of the DCW grid is 95 meters. To get an idea of the overall absolute accuracy of the DCW grid, the relative error between the DCW and DTED can be combined with the known error of the DTED itself in a sum of squares. The root of that sum of squares is 97 meters. Using the assumptions about the error distribution cited above, an RMSE of 97 meters can be expressed as ± 162 meters linear error at 90% confidence. The following table summarizes the accuracy of the Africa DEM:
Absolute vertical accuracy (in meters):
Data source RMSE LE at 90%
Generalized DTED 18 ± 30
DCW Grid 97 ± 162
A digital elevation model of Africa at a 30 arc second horizontal grid spacing was produced by incorporating the elevation information derived from the Digital Chart of the World and Digital Terrain Elevation Data. The goal of the processing techniques that were implemented was to maximize the amount of topographic information that could be derived from each source. The resulting digital elevation grid, with an absolute vertical accuracy of better than 100 meters RMSE, provides new topographic data with a level of detail and accuracy suitable for many regional and continental applications.
This figure illustrates the increase in topographic information
content provided by gridded DCW data and generalized DTED over that of the
ETOPO5 data set, which previously was the highest resolution DEM covering
all of Africa.
The full version of this image is 297K.
The techniques developed and implemented for production of the Africa DEM have been extended to expedite completion of 1-kilometer DEM's for the other continents.
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For further assistance, please contact: Customer Services U.S. Geological Survey Center for Earth Resources Observation and Science (EROS) 47914 252nd Street Sioux Falls, SD 57198-0001 Tel: 800-252-4547 Tel: 605-594-6151 Fax: 605-594-6589 Email: custserv@usgs.gov Business Hours: Monday thru Friday, 8:00 a.m. to 4:00 p.m., central time |
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