Background

Overview of Topographic Concepts Topography refers to elevations and topographic maps show elevations, usually by lines that connect points with the same elevation above sea level, sometimes by showing spot heights as dots or triangles with numbers. Topographic maps contain much useful information that can be used to calculate slopes and to draw profiles, vertical slices through features such as hills and mountains. Many experienced map readers can visualize a landscape just by looking at a topographic map. This clickable topographic map of Mount Shasta illustrates these concepts. Go to top of page

The Digital Elevation Model While topographic maps are useful, especially for fieldwork, the digital elevation model provides information in a form that facilitates computer analysis. Go to top of page

Sources of Digital Elevation Models DEMs can be made from paper maps. In fact the U.S. Geological Survey has been producing high quality DEMs this way for many years. This data can be downloaded from the National Elevation Dataset and USGS-seamless server. For areas of the world for which there are no DEMs, DEMs can be produced from paper maps. Making digital elevation models from paper maps. The USGS - NASA Distributed Active Archive Center provides DEMs at a resolution of one kilometer for most of the earth. This DEM is known as the Global 30 Arc-Second Elevation Data Set. DEMs at 30 meters resolution for some regions are produced from the stereoscopic imagery of the ASTER insturment carried aboard the TERRA satellite. Both are available from the USGS-NASA. Recently the Shuttle Radar Topography Mission (SRTM) imaged most of the earth land surface using RADAR. The data will be released in the near future at a resolution of 100 meters. NASA-SRTM. STRM DEM data for the USA at 30 meters resolution is now available from the USGS. USGS-SRTM. DEMs are available from several other private and public sources using optical and RADAR satellite instruments. Some privately sourced DEMs have such a high resolution that buildings can be distinguished from terrain. Go to top of page

Errors in DEMs "All DEMs are in some respect flawed (Coops, 2000). Most of the error in current DEMs originated in the contour maps from which they were derived and thus cannot be reduced through efforts of the user." A Bibliography of Terrain Modeling (Geomorphometry), the Quantitative Representation of Topography, Supplement 4.0, USGS OPEN-FILE REPORT 02-465 2002. This judgement is shared by others. Suzanne Perlitsh Wechsler has modelled the effects upon specific topographic parameters (such as slope) of the inherent uncertainty in DEMs. However, Tom Farr has pointed out several features of the Shuttle Radar Topography Mission that indicate SRTM DEMs will have fewer errors than other DEMs. See also: NASA-SRTM Go to top of page

Correcting Errors in DEMs Recognizing that any method of measuring nature has errors, others have focused upon ways and means of reducing the effects of those errors that can be identified and localized. (Michael J. Oimoen, An Effective Filter for Removal of Production Artifacts in USGS 7.5-minute DEMs, USGS EROS Data Center, Sioux Falls SD 57198, Dec 9, 1997). The seamless DEM for the US National Elevation Dataset (NED, USGS-seamless server)) appears to be a major advance over previous DEMs made from maps, not least because there is no longer the need to stitch together several sheets and to remove gaps between them.

DEMs made from maps may have "artifacts", errors not in the original maps but added during the production of the DEMs. To reduce such errors a user might take 10 meter DEMs and resample to 30 meters. Alternatively, a user might obtain SRTM DEMs of the same area and enhance them to remove artifacts peculiar to RADAR technology. In particular, waterplanes and areas in radar shadow will have data errors as shown in this diagram. Note that there is no radar return from water planes or from surfaces in the radar shadow created by hills. (Larry Teng has described some of these problems in detail.) Finally, to combine the advantages of different technologies a user might combine DEMs produced by different methods in a process called "data fusion". For example, 30 meter DEMs for the USA are available from the USGS (from scanned maps) and from the SRTM (RADAR). By combining these two sources a user might aviod the errors of both.

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