Cartographic relief depiction
The most obvious way to depict relief is to imitate it at scale, as in molded or sculpted solid terrain models and molded-plastic raised-relief maps. Because of the disparity between the horizontal and vertical scales of maps, raised relief is typically exaggerated.
On flat paper maps, terrain can be depicted in a variety of ways, outlined below:
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.
Hachures are also an older mode of representing relief. They are a form of shading, although different from the one used in shaded maps. Hachures consist of parallel lines drawn to show the orientation of slope, and by their thickness and overall density they provide a general sense of steepness. The direction of the lines, or the aspect direction, can be clarified using unidirectional arrows to clarify differentiation between confusing hachures . Being non-numeric, they are less useful to a scientific survey than contours, but can successfully communicate quite specific shapes of terrain.
Hachure representation of relief was standardized by the Austrian topographer Johann Georg Lehmann in 1799. There were two types of hachures typically used in manual cartography: slope hachures, wherein hachure width is proportional to slope steepness; and shadow hachures, which incorporate the effects of a hypothetical oblique light source, so that hachure width is determined by whether a hachure is illuminated or in shadow .
Several notable cartographers have used the hachure method in their map making including Eduard Imhof, Patrick Kennelly and Jon Kimerling, and many others . Although many people view hachures as being outdated, some people such as Imhof suggested the contrary. "Hachures alone, without contours, are more capable of depicting the terrain than is [relief] shading alone. Hachures also possess their own special, attractive graphic style. They have a more abstract effect than [relief] shading, and perhaps for this reason are more expressive" .
Contour lines (or isohypses) are isolines showing equal elevation. This is the most common way of numerically showing elevation, and is familiar from topographic maps. They are valuable because they help map readers measure how steep a slope is.
Most 18th and early 19th century national surveys did not record relief across the entire area of coverage, calculating only spot elevations at survey points. The United 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 the United States, and the usage has spread internationally.
On maps produced by Swisstopo, the color of the contour lines is used to indicate the type of ground: black for bare rock and scree, blue for ice and underwater contours, brown for earth-covered ground .
There are several rules to note when viewing topographic maps:
- The rule of V's: sharp-pointed vees usually are in stream valleys, with the drainage channel passing through the point of the vee, with the vee pointing upstream. This is a consequence of erosion
- The rule of O's: closed loops are normally uphill on the inside and downhill on the outside, and the innermost loop is the highest area. If a loop instead represents a depression, some maps note this by short lines radiating from the inside of the loop, called "hachures"
- Spacing of contours: close contours indicate a steep slope; distant contours a shallow slope. Two or more contour lines merging indicates a cliff
Of course, to determine differences in elevation between two points, the contour interval, or distance in altitude between two adjacent contour lines, must be known, and this is given at the bottom of the map. Usually contour intervals are consistent throughout a map, but there are exceptions. Sometimes intermediate contours are present in flatter areas; these can be dashed or dotted lines at half the noted contour interval. When contours are used with hypsometric tints on a small-scale map that includes mountains and flatter low-lying areas, it is common to have smaller intervals at lower elevations so that detail is shown in all areas. Conversely, for an island which consists of a plateau surrounded by steep cliffs, it is possible to use smaller intervals as the height increases .
Illuminated relief is using light to create shadows to form shaded relief. Like a painter painting a picture, they always create a light source. The light source helps the painter add more detail to the painting. In a map, the light sources help the higher elevations stand out. There are many different types of illumination. Most have to do with the angles at which the light source is theoretically hitting the map.
- Upper Left Illumination: When making a map it is important to put the light source at the proper angle. For example, it is by rule of thumb that the light source is in the upper left, or northwest corner of a map. The ArcGIS Hillshade tool default is to set the light source angle at 315 degrees . If it is switched to the lower right corner, the map will be a complete inverse - the mountains will look like canyons and the valleys will look like plateaus. Even a slight change in the angle can affect the look of the elevations and detail of a map . When shadows are angled away from the observer, they produce what is called a pseudoscopic illusion. High points will appear as low points and low points will appear as high points .
- Rotating illumination: Occasionally, some areas of the shaded relief map do not get enough light to show all of the details in that particular spot. If this is occurring, then the angle of the light source can be rotated just a few degrees. At lower angles, there is more shadow cast and relief is easier to distinguish. By rotating the light, more of those hidden details will pop up. In most programs, the light sources are built in at 45 degrees. This works well for things such as high mountains, but it is not perfect. The light source can be changed in almost any direction. Avoid using high angles such as 90 degrees. At high angles, the details of a feature will be washed out and there will be little, if any, shaded relief .
A lot of large maps contain area or topography that is poorly defined. This problem can be fixed by including varying illuminations. When applied skillfully localized adjustments to the illumination go unnoticed by map readers; because variation in shadow pattern difficult to detect. Illumination Discussion
Hypsometric tints are a variant on contour lines in order to make a map easier to read. The lines on a map are called contour lines and the tints are called hypsometric tints. Hypsometric tints show relief or depth depicted by a gradation of colors, usually between contour lines. Each color represents a different range of elevation . Hypsometric tinting is typically associated with small-scale maps and usually gets lighter in color as the elevation increases, hence the rule "higher is brighter" .
The Scottish map firm John Bartholomew and Son is credited with popularizing the technique, and its color scheme has become conventional in depicting gradual value changes in data. When using hypsometric tinting the most popular scheme to use is dark greens at low values, progressing through yellows and ochers, to browns and then grays and white at the highest values. Most maps trying to convey elevation use this technique with the low elevations coming from dark green to high elevations with reds and whites. The reasoning behind this is that cool colors appear to recede and look farther away, whereas warmer colors appear to be closer or popping out of a page. However, this popular coloring scheme can lead to confusion for people who are not familiar with how this color scheme works. For example, someone may see the red on the tips of mountains and think it is a hot desert, or they may see green in a low elevation desert and think that it is a lush green forest. Therefore, it is important to remember who will be using the map and what the goals are for the map when choosing a color scheme .
Another popular use for hypsometric tinting is to display weather patterns from cold to hot temperatures using warm colors for warmer temperatures and cooler colors for colder temperatures. Often when using hypsometric tinting with elevation relief shading is added to bring out the elevation. Depending on the audience, maps may contain hypsometric tint simply because the colors are pleasing.
Hypsometric tinting in maps is often accompanied by bathymetric tinting in oceans, which indicates depths using a similar method. Bathymetric tinting refers to water measurement and using hypsometric tinting to depict water depths or the topography of a body of water.
Tanaka (Relief) Contours
The Tanaka (Relief) Contours technique is designed to highlight the illuminated contour lines with an artificial light source coming from the northwest. Contour lines are drawn on a medium gray color plate. The contour lines are represented as white bands when the light source is incident on the surface and black bands when the light source is obscured by the object. The thickness of the band depends on the angle between the light source vector and the slope.
The most common criticism of this method is its terrace appearance. Tanaka takes into account the aspect and illumination but not the slope, thus causing a 2.5D production .
Pioneered by Hungarian-American cartographer Erwin Raisz, this technique uses generalized texture to imitate landform shapes over a large area. The textures or symbols are derived from oblique views of actual landforms and were originally inspired by geologic block diagrams.
Physiographic illustration is best for depicting landforms and relative ruggedness on small-scale maps. It is easily interpreted by map readers with little cartographic knowledge. One shortcoming is that it does not convey altitude or elevation, and so is often combined with hypsometric tint to convey elevation information.
Because the work of illustration relies on the cartographer's drawing ability as well as geographic knowlege, physiographic illustration is often idiosyncratic to its creator.
One classic example of physiographic illustration is J.R.R. Tolkien's original map of Middle Earth. You can view an interactive, scalable version here.
Resolution Bumping is an approach to cartographic relief depiction focused on representing the topography of mountains using DEMs and Adobe Photoshop. It is a hybrid method that helps correct problems encountered when using a GTOPO30 approach. (Problems include exaggerated and de-emphasized slope, making mountain ranges unrecognizable.) The basic premise of resolution bumping is combining a high-resolution and a lower-resolution GTOPO30 DEM, resulting in a DEM that provides a more realistic and recognizable elevation model. According to Tom Patterson, a cartographer for the U.S. National Park Service, "Resolution bumping in effect 'bumps' or etches a suggestion of topographical detail onto generalized topographic surfaces" . The purpose of Resolution bumping, known also as Patterson's Technique, is to give a more natural and legible appearance to rugged mountains. Patterson's view is that people tend to visualize the Earth's surface from an oblique view rather than a two-dimensional map. He professed that oblique, three-dimensional maps were "probably more understandable to users, especially those with limited map reading skills" .
Forums and associations
Portrayal of relief is especially important in mountainous regions. The Commission on Mountain Cartography of the International Cartographic Association is the best-known forum for discussion of theory and techniques for mapping these regions.
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- Interactive shaded relief map (Shaded-Relief.com)
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- Rendering a map using relief shading technique in Photoshop