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Track: Database Design and Automation
Lisa Mai Olsen
U.S. Geological Survey
Science and Applications Branch
Sioux Falls, SD 57198
Telephone: 605-594-6052
Fax: 605-594-6568
E-mail: Olsenl@edcsnw12.cr.usgs.gov
Norman B. Bliss
Development of a 30-Arc-Second Digital Elevation Model of South America
Defining Issue: A 30-arc-second digital elevation model (DEM) of South America was developed using five different data sources. Examples of the input data are Digital Terrain Elevation Data, which were generalized from a 3-arc-second resolution to a 30-arc-second resolution through systematic sampling, Digital Chart of the World (DCW) contours, point elevations, drainage network, and other sources. These other sources include International Map of the World (IMW) contours and points, Army Map Service contours digitized by Geomatics, Inc., and a river network derived from both DCW and IMW. These data were combined by ANUDEM (Australian National University's Digital Elevation Model software) to create a DEM of the area. This software iteratively applies a spline interpolation algorithm to the data, resulting in a gridded surface. The resultant data set has ten times more points along each line of latitude or longitude than the best continental data set previously available to the public (ETOPO5),
representing a 100-fold increase of resolution on an areal basis. The accuracy of the grid, however, is limited by the accuracy of the source materials. GIS Solution: Due to rather large discrepancies between data sources and types, it was necessary to be extremely selective in which data were included as input to ANUDEM. Problems invariably arose in areas of overlapping data sources, areas where different data sources came together, areas of little relief, and in areas with very little data coverage. Tools utilizing ARC Macro Language were developed to aid in the creation of plots, the inclusion or exclusion of data, and the editing of problem areas. Processing in the Amazon basin posed a unique set of problems. The Amazon river system is very large and has very little elevation change over long distances. To further complicate matters, very little elevation data were available for the basin. Special techniques were developed using dynamic segmentation to interpolate elevation values in the Amazon
basin. These interpolated river elevations were then used to constrain the interpolation of the surrounding topography using ANUDEM. Various artifacts in the data may be introduced through processing including the creation of spurious hills and valleys and a stair-step procession in areas of gradually sloping topography. When possible, anomalous features were minimized by selecting or excluding input data and by localized editing of the results. In these cases, it was necessary to run small patches of data through ANUDEM and to develop GIS-based tools to aid in the merging of several gridded elevation patches into the final output grid. Software: ArcInfo and ARCGRID Version 7.0.3 and ANUDEM (Australian National University's Digital Elevation Model software).
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