U.S. Geological Survey --EROS Data Center
Techniques for Development of Global 1-Kilometer Digital Elevation Models
Dean B. Gesch, USGS EROS Data Center, Sioux Falls, SD 57198
Kevin S. Larson, Berger & Co., Dallas, TX
In response to well documented requirements for global coverage of regional and continental scale digital elevation models (DEMs), the staff at the U.S. Geological Survey's EROS Data Center (EDC) are developing digital elevation data with a horizontal grid spacing of 30 arc seconds, or approximately 1 kilometer. Such data have many uses including applications such as continental scale land characterization, climate modeling, large area hydrologic studies, and geometric and atmospheric correction of medium and coarse resolution satellite image data. Development of 1-kilometer DEMs has created unique challenges in the area of spatial data processing techniques. The DEMs are generated from several topographic data sources, each requiring specific processing methodologies and quality assurance procedures. The great volume of input data, and the significant computer resources required, demanded that efficient production procedures be implemented. All of the continents have either been completed or are in progress. The continent of Africa serves as a representative example for a description of the techniques used in development of the 1-kilometer elevation models. Digital Chart of the World (DCW), a vector cartographic data set based on 1:1,000,000 Operational Navigation Charts, was the source data for about 50% of the Africa DEM. The hypsography, drainage, and coast line information from the DCW was input to the ANUDEM surface gridding program developed at Australia National University. ANUDEM, developed specifically for creating DEMs 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 surface interpolation routines. However, in order to prepare and format the DCW data for ANUDEM a significant amount of preprocessing was required. This processing included editing and updating the vector stream lines so that the direction of each was oriented downstream, a requirement of ANUDEM. Further preprocessing involved detection and correction of erroneous contour and point elevations. The remaining 50% of the Africa DEM was generated from information derived from Digital Terrain Elevation Data (DTED), a raster topographic data base with a horizontal grid spacing of 3 arc seconds (approximately 90 meters) produced by the Defense Mapping Agency. The DTED data were generalized to 30 arc seconds with a scheme that retains the topographic breaklines (streams and ridges) from the full resolution data in the reduced resolution representation. A merge of the area covered by DCW data with the area covered by DTED data resulted in the final Africa DEM. Because of differences in topographic detail and accuracy along the irregular boundary between the two data sources, the merging procedure included blending to minimize the discrepancies. The discrepancies were reduced by incorporating control from the DTED border into the gridding of the DCW hypsography with ANUDEM. Finally, accuracy characterization was completed for the entire 1-kilometer Africa DEM.