This component of the tapestry is the digital shaded-relief image, created by Thelin and Pike (1991), that shows the shape of the land surface by variations in brightness. The degree of light and dark artificially mimics the intensity of the Sun's light on different types of topography. This technique, called chiaroscuro, dates back some 400 years to sketches of the Tuscan landscape by Leonardo da Vinci. Because manual (artistic) portrayals of terrain can practicably show only small areas both accurately and in detail, the technique has been automated by the computer to cover large regions. The resulting digital shaded-relief map of the lower 48 states succeeded the unique, if less accurate, hand-drawn landform map by master cartographer Erwin Raisz.
Landscape features contain many of the clues needed to understand the Earth and the agents that have shaped it. Finding and decoding these clues enables scientists to learn more about natural hazards, the events of Earth history, and resources of land, water, energy, and minerals. Many methods of landscape portrayal are used to view surface features, but shaded-relief imaging by computer is unique in the following respects:
- It provides fine-scale detail over a wide area, a combination not possible in other techniques of illustration;
- It shows terrain accurately and in its true complexity, two properties commonly lost in sketches and diagrams of large areas;
- Unlike mosaics of aerial photos and radar images, the view
-- limited only by extent of the digital data set -- is continuous
across the country;
- Relief shading is free of distortion and the vegetation and cultural features that conceal topographic form on satellite images.
Thelin, G.P., and Pike, R.J., 1991,
Landforms of the conterminous United States; A digital shaded-relief portrayal:
U.S. Geological Survey Miscellaneous Investigations Map I-2206 scale 1:3,500,000.