TECHNIQUES FOR DEVELOPMENT OF GLOBAL I-KILOMETER DIGITAL ELEVATION MODELS

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 is developing global digital elevation data with a horizontal grid spacing of 30-arc seconds (approximately I 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. Digital Chart of the World (DCW), a vector cartographic data set based on the 1: 1,000,000 Operational Navigation Charts, was the source data for roughly one-half of the Africa DEM. The hypsography, drainage, and coastline data from the DCW were processed with a surface gridding program. The other half of the Africa DEM was generated from information derived from Digital Terrain Elevation Data (I)TED), 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. Because of differences in topographic detail and accuracy along the irregular boundary between the two data sources, the merging procedure included blending techniques to minimize the discrepancies. The resulting DEM has an absolute vertical accuracy of better than 100 meters (root mean square error). INTRODUCTION Digital elevation data are an important source of information in studies of land surface processes. Medium resolution topographic data, with a horizontal grid spacing up to I kilometer, have many 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. Although the requirements for digital elevation data are well known and have been documented (Topographic Science Working Group, 1988; Guindon, 1993; Gesch, 1994), complete global coverage of digital elevation models (DEM’s) with a resolution of I kilometer has been unavailable. In response to the stated needs of the geospatial data user community, staff members 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 I kilometer). Previously, the best available global DEM was the ETOPO5 data set with a horizontal grid spacing of 5-arc minutes (approximately 10 kilometers). Development of 1-kilometer DEM’s has created unique challenges in the area of spatial data processing techniques. The DEM’s are generated from several topographic data sources, each requiring specific processing methodologies and quality assurance procedures. The large volume of input data, and the significant computer resources required, demand that efficient production procedures be implemented. For this paper, the continent of Africa serves as a representative example for a description of the techniques used in development of the 1-kilometer elevation models. * Work performed under U.S. Geological Survey contract 1434-92-C-40004. ** Work performed as an employee of the U.S. Geological Survey. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. DATA SOURCES 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). Additional sources have been used for areas where DCW and DTED are not available. More than 65 percent of the global land surface is covered by DTED, with coverage concentrated in the northern hemisphere. Consequently, the portions derived from DTED for the North America, Europe, and Asia I -kilometer DEM’s are much more than for Africa. Approximately 56 percent of the Africa DEM is based on DTED, while the remaining 44 percent is based on DCW. Digital Chart of the World 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 (Danko, 1992). The DCW and the ONC series are products of the Defense Mapping Agency (DMA). The topographic information of interest for generating DEM’s is contained in several DCW hypsography layers. The primary contour interval on the source ONC’s is 1,000 feet (305 meters), and supplemental contours at an interval of 250 feet (76 meters) are shown in areas below 1,000 feet in elevation. The DCW drainage layers were also used as input to the DEM generation process; this information included stream networks, lake shorelines, lake elevations, and ocean coastlines. Digital Terrain Elevation Data DTED, also a DMA product, is a raster topographic data base with a horizontal grid spacing of 3-arc seconds (approximately 90 meters). Access to full resolution DTED for areas outside the United States is limited to agencies of the U.S. Government, but permission has been granted by DMA to the EROS Data Center to use and distribute a generalized 30-arc second version. DATA PROCESSING Data processing was accomplished using commercially available geographic information system software, vector-to-raster gridding software, and utilities developed specifically for this project. To more efficiently handle the numerous input data sets and to standardize the proper sequence of processing steps, the production procedures were automated to a great extent by employing preset parameter values, scripted command files, and consistent naming schemes for input and output data files.