Terrain cartography orrelief mapping is the depiction of the shape of the surface of the Earth on a map, using one or more of several techniques that have been developed.Terrain or relief is an essential aspect ofphysical geography, and as such its portrayal presents a central problem incartographic design, and more recentlygeographic information systems andgeovisualization.
The most ancient form of relief depiction in cartography,hill profiles are simply illustrations of mountains and hills in profile, placed as appropriate on generally small-scale (broad area of coverage) maps. They are seldom used today except as part of an "antique" styling.
In 1921, A.K. Lobeck publishedA Physiographic Diagram of the United States, using an advanced version of the hill profile technique to illustrate the distribution of landforms on a small-scale map.[1]Erwin Raisz further developed, standardized, and taught this technique, which uses generalized texture to imitatelandform shapes over a large area.[2] A combination of hill profile and shaded relief, this style of terrain representation is simultaneously idiosyncratic to its creator—often hand-painted—and found insightful in illustratinggeomorphological patterns.
More recently,Tom Patterson developed a computer-generated technique for mapping terrain inspired by Raisz's work, calledplan oblique relief.[3] This tool starts with a shaded relief image, then shifts pixels northward proportional to their elevation. The effect is to make mountains "stand up" and "lay over" features to the north, in the same fashion as hill profiles. Some viewers are able to see the effect more easily than others.
Hachures, first standardized by the Austrian topographer Johann Georg Lehmann in 1799, are a form of shading using lines. They show the orientation of slope, and by their thickness and overall density they provide a general sense of steepness. Being non-numeric, they are less useful to a scientific survey than contours, but can successfully communicate quite specific shapes of terrain.[2] They are especially effective at showing relatively low relief, such as rolling hills. It was a standard on topographic maps of Germany well into the 20th Century.
There have been multiple attempts to recreate this technique using digital GIS data, with mixed results.
First developed in France in the 18th Century,contour lines (or isohypses) are isolines of equal elevation. This is the most common way of visualizing elevation quantitatively, and is familiar fromtopographic maps.
Most 18th- and early 19th-century nationalsurveys did not record relief across the entire area of coverage, calculating only spot elevations at survey points. TheUnited States Geological Survey (USGS) topographical survey maps included contour representation of relief, and so maps that show relief, especially with exact representation of elevation, came to be called topographic maps (or "topo" maps) in theUnited States, and the usage has spread internationally.
On maps produced bySwisstopo, the color of the contour lines is used to indicate the type of ground: black for bare rock andscree, blue for ice and underwater contours, and brown for earth-covered ground.[4]
TheTanaka (relief) contours technique is a method used to illuminate contour lines in order to help visualize terrain. Lines are highlighted or shaded depending on their relationship to a light source in the Northwest. If the object being illustrated would shadow a section of contour line, that contour would be represented with a black band. Otherwise, slopes facing the light source would be represented by white bands.
This method was developed by Professor Tanaka Kitiro in 1950, but had been experimented with as early as 1870, with little success due to technological limitations in printing. The resulting terrain at this point was agrayscale image.[5] CartographerBerthold Horn later created software to digitally produce Tanaka Contours, and Patrick Kennelly, another cartographer, later found a way to add color to these maps, making them more realistic.[6]
There are a number of issues with this method. Historically, printing technology did not reproduce Tanaka contours well, especially the white lines on a gray background. This method is also very time-consuming. In addition, the terraced appearance does not look appealing or accurate in some kinds of terrain.[7]
Hypsometric tints (also called layer tinting, elevation tinting, elevation coloring, or hysometric coloring) are colors placed betweencontour lines to indicateelevation. These tints are shown as bands of color in a graduated scheme or as acolor scheme applied to contour lines themselves; either method is considered a type ofIsarithmic map. Hypsometric tinting of maps andglobes is often accompanied by a similar method ofbathymetric tinting to convey differences in water depth.
Shaded relief, or hill-shading, shows the shape of the terrain in a realistic fashion by showing how the three-dimensional surface would be illuminated from a point light source. Theshadows normally follow the convention oftop-left lighting in which the light source is placed near the upper-left corner of the map. If the map isoriented with north at the top, the result is that the light appears to come from the north-west. Although this is unrealistic lighting in the northern hemisphere, using a southern light source can causemultistable perception illusions, in which the topography appears inverted.[8]
Shaded relief was traditionally drawn withcharcoal,airbrush and other artist's media. The Swiss cartographerEduard Imhof is widely regarded as a master of manual hill-shading technique and theory. Shaded relief is today almost exclusively computer-generated fromdigital elevation models (DEM). The mathematical basis ofanalytical hillshading is to calculate thesurface normal at each location, then calculate the angle between that vector and the vector pointing to the illumination using theDot product; the smaller that angle, the more illumination that location is receiving. However, most software implementations use algorithms that shorten those calculations. This tool is available in a variety of GIS and graphics software, includingPhotoshop,QGIS,GRASS GIS orArcMap's Spatial Analyst extension.
While these relatively simple tools have made shaded relief almost ubiquitous in maps, many cartographers[weasel words] have been unhappy with the product,[which?] and have developed techniques to improve its appearance, including the following:
Imhof's contributions included a multi-color approach to shading, with purples in valleys and yellows on peaks, which is known as "illuminated shading." Illuminating the sides of the terrain facing the light source with yellow colors provides greater realism (since direct sunlight is more yellow, and ambient light is more blue), enhances the sense of the three-dimensional nature of the terrain, and make the map more aesthetically pleasing and artistic-looking.[9] Much work has been done in digitally recreating the work ofEduard Imhof, which has been fairly successful in some cases.[10]
A common criticism of computer-generated analytical hillshading is its stark, artificial look, in which slopes facing the light are solid white, and slopes facing away are solid black. Raisz called it "plastic shading," and others have said it looks like a moonscape.[2] One solution is to incorporate multiple lighting directions to imitate the effect of ambient lighting, creating a much more realistic looking product. Multiple techniques have been proposed for doing this, including usingGeographic information systems software for generating multiple shaded relief images and averaging them together, using 3-d modeling software torender terrain,[11] and custom software tools to imitate natural lighting using up to hundreds of individual sources.[12] This technique has been found to be most effective for very rugged terrain at medium scales of 1:30,000 to 1:1,000,000.
It is possible to make the terrain look more realistic by imitating the three-dimensional look of not only the bare land surface, but also the features covering that land surface, such as buildings and plants. Texture mapping or bump mapping is a technique adapted fromComputer graphics that adds a layer of shaded texture to the shaded surface relief that imitates the look of the local land cover.[13] This texture can be generated in several ways:
This technique is most useful at producing realistic maps at relatively large scales, 1:5,000 to 1:50,000.
One challenge with shaded relief, especially at small scales (1:500,000 or less), is that the technique is very good at visualizing local (high-frequency) relief, but may not effectively show larger features. For example, a rugged area of hills and valleys will show as much or more variation than a large, smooth mountain. Resolution bumping is a hybrid technique developed byNPS cartographer Tom Patterson to mitigate this problem.[16] A fine-resolution DEM is averaged with a heavily smoothed version (i.e., significantly coarser resolution). When the hillshading algorithm is applied to this, it has the effect of blending the fine details of the original terrain model with the broader features brought out by the smoothed model. This technique works best at small scales and in regions that are consistently rugged.
A three-dimensional view (projected onto a two-dimensional medium) of the surface of the Earth, along with the geographic features resting on it. Imagined aerial views of cities were first produced during the lateMiddle Ages, but these "bird's eye views" became very popular in theUnited States during the 1800s. The advent ofGIS (especially recent advances in 3-D and global visualization) and3-D graphics modeling software has made the production of realistic aerial views relatively easy, although the execution of qualityCartographic design on these models remains a challenge.[17]
This is a map in which relief is shown as a three-dimensional object. The most intuitive way to depict relief is to imitate it at scale. Hand-crafted dioramas may date back to 200 BCE in China, but mass production did not become available untilWorld War II with the invention ofvacuum-formed plastic maps, andcomputerized machining to create molds efficiently. Machining is also used to create large custom models from substrates such as high-density foam, and can even color them based on aerial photography by placing aninkjet printhead on the machining device. The advent of3D printing has introduced a much more economical means to produce raised-relief maps, although most 3D printers are too small to efficiently produce large dioramas.[18]
Terrain rendering covers a variety of methods of depicting real-world orimaginary worldsurfaces. Most commonterrainrendering is the depiction ofEarth's surface.It is used in various applications to give an observer aframe of reference. It is also often used in combination with rendering of non-terrain objects, such astrees,buildings,rivers, etc.
There are two major modes of terrain rendering:top-down andperspective rendering. Top-down terrain rendering has been known for centuries in the way ofcartographic maps. Perspective terrain rendering has also been known for quite some time. However, only with the advent of computers andcomputer graphics perspective rendering has become mainstream.
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A typical terrain rendering application consists of a terraindatabase, acentral processing unit (CPU), a dedicatedgraphics processing unit (GPU), and a display. Asoftware application is configured to start at initial location in theworld space. The output of the application is screen space representation of the real world on a display. The software application uses the CPU to identify and load terrain data corresponding to initial location from the terrain database, then applies the requiredtransformations to build amesh of points that can be rendered by the GPU, which completes geometrical transformations, creating screen space objects (such aspolygons) that create a picture closely resembling the location of the real world.
There are a number of ways totexture the terrain surface. Some applications benefit from using artificial textures, such as elevation coloring,checkerboard, or other generic textures. Some applications attempt to recreate the real-world surface to the best possible representation usingaerial photography andsatellite imagery.
Invideo games,texture splatting is used to texture the terrain surface.
There are a great variety of methods to generate terrain surfaces. The main problem solved by all these methods is managing number of processed and rendered polygons. It is possible to create a very detailed picture of the world using billions of data points. However such applications are limited to static pictures. Most uses of terrain rendering are moving images, which require the software application to make decisions on how to simplify (by discarding or approximating) source terrain data. Virtually all terrain rendering applications uselevel of detail to manage number of data points processed by CPU and GPU. There are several modern algorithms for terrain surfaces generating.[19][20][21][22]
Terrain rendering is widely used incomputer games to represent both Earth's surface and imaginary worlds. Some games also haveterrain deformation (or deformable terrain).
One important application of terrain rendering is insynthetic vision systems. Pilots flying aircraft benefit greatly from the ability to see terrain surface at all times regardless of conditions outside the aircraft.
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Emphasizeshydrologicaldrainage divide and watershed streams.
Portrayal of relief is especially important inmountainous regions. TheCommission on Mountain Cartography of theInternational Cartographic Association is the best-known forum for discussion of theory and techniques for mapping these regions.
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