Realistic portrayal and mapping of topographic form is a centuries-old problem: to trick the eye into perceiving a two-dimensional graphic as a three-dimensional landscape. All traditional solutions, including those partly implemented by machine, have been artistic (Imhof, 1965; Horn, 1981). Among the cartographic devices invented by illustrators to supply the necessary visual depth cues are hachuring, hypsographic (elevation) tinting, contour density, parallel-profile density, pictorial relief, and shaded relief (Harrison, 1969; Alpha and Winter, 1971; Gilman, 1981). The latter two manual techniques have been particularly successful. Pictorial relief, which symbolizes topography by stylized morphologic types, was most fully developed in the 50 landform classes of Raisz (1931, 1939). Shaded relief, or hill shading, shows topography by the intensity of shadows cast by a light source (Imhof, 1965; Yoeli, 1965). First drafted by pencil, pen, or brush, shaded relief also has been executed by airbrush, dark-plate, and photography of raised relief models. However, topographic detail is much too complex to be mapped both accurately and economically over large areas by any of these means.

Fast computers, analytical software, digital data, and graphic input/output devices have converged over the last three decades to largely mechanize the craft of mapmaking (for example, Burrough, 1987, p. 4-6). This digital revolution has, among its many accomplishments, also solved the problem of mapping topographic form. Machine visualization now frees terrain portrayal from longstanding limitations (Kennie and McLaren, 1988). Topography need no longer be mapped symbolically, by discrete hand-drawn morphologic types (Raisz, 1931), or subjectively, by manual shading (Imhof, 1965). Where the necessary information is available in digital format, the computer can represent landforms as they actually are—within limitations of the source data—and portray terrain in the infinite variety of form that constitutes the true landscape. No longer is it entirely correct that maps of landforms are “drawn by men and not turned out automatically by machines” (Wright, 1942).

Yet despite the digital revolution in cartography, much truth remains in Wright’s (1942) admonition that map makers are humans, not machines. Design and production of landform maps will be increasingly automated and sophisticated (Kennie and McLaren, 1988; Weibel, 1989), but the conception of a map (Yoeli, 1965, 1967) is fundamentally an intellectual rather than a mechanical process. Moreover, the portrayal of topography using digital data only now is passing from an experimental to a production technique. The many steps to a machine-made map of landforms—from data formatting, editing and processing, through image generation and correction, to preparation of a reproducible master—may be streamlined, if not wholly automated, but the sequence can involve much nonroutine trial-and-error. Such maps of topography, however technical in execution, will continue to remain the constructs of human vision and judgment.

MACHINE IMAGES OF TOPOGRAPHY

Digital image-processing and computer graphics have mechanized much of the art of landform representation by combining the two most effective traditional techniques, pictorial relief and hill shading (Yoeli, 1967; Batson and others, 1975). The resulting image is a shaded pictorial-relief (physiographic) map in vertical perspective. Although automated shaded-relief maps can look deceptively like satellite pictures, they are not acquired directly by Earth-orbiting spacecraft, nor are the data from which they are made. The images are computed from a large array of closely spaced terrain heights, usually in grid cell (raster) format, called a digital elevation (or terrain) model (DEM/DTM) (Miller and LaFlamme, 1958). Most DEMs still are made from conventional topographic maps (for details of a recent example see Hall and others, 1990).

• View the U.S. map, a brief description, and directions for ordering it

• Download all 12 Mb of grey-scale pixels for the US shaded-relief map

• Download the text of the 16-page pamphlet that goes with it

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