Geography For Dummies. Jerry T. Mitchell
areas in little detail, while large-scale maps show small areas in big detail. Figure 5-2a has a comparatively small scale. In contrast, Figure 5-2b has a somewhat larger scale, while Figure 5-2c has the largest scale among the three maps. And indeed, as the scales of these maps get larger, the degree of detail increases. Calling the first map a small-scale map makes sense as the representative fraction, if actually divided, would result in a much smaller number than the other two maps.
Showing the Ups and Downs: Topography
All points on Earth have an elevation with respect to sea level. Altogether, they constitute “the lay of the land.” (Keep in mind that elevation also pertains to points on the ocean’s bottom.) Topography is the art and science of depicting heights and depths on a map. Like scale, topographic information is a basic feature of many maps and is commonly represented in three ways as indicated in Figure 5-3. The following sections discuss the three ways of showing topographic information.
(© John Wiley & Sons Inc.)
FIGURE 5-3: The terrain of a make-believe place, as depicted three ways.
Spot heights
A spot height is a symbol (typically a tiny dot, plus sign, or triangle) accompanied by a number that indicates the elevation of a given point in feet or meters (see Figure 5-3a). Sometimes a cartographer wishes to emphasize something other than topography on a map, yet provide elevation information for a few selected points in order to convey the lay of the land. Spot heights serve this purpose.
Contour lines
Contour lines connect points of equal elevation. In so doing, they convey the shape (hence, “contour”) of the land they depict. Near the top of Figure 5-3b is a thin line labeled “50,” which connects points that are 50 feet above sea level. Farther inland is a line labeled 100, which connects points that are 100 feet above sea level. Thus, a walk from the water’s edge to a point on the second line involves a 100-foot gain in elevation. These elevation line features are a major component of topographic maps published by the United States Geologic Survey.
Other maps have similar lines that connect equal data points using an isoline (iso means equal). Want to connect areas of equal temperature on a map instead of elevation? Using the same process, now you have an isotherm instead of a contour line. All maps that use isolines to connect features of the same value are called isopleth maps.
Shading or Color
Colors and gray tones may also be used to indicate elevation above sea level. On color maps, deep green is usually used to depict low-lying coastal land. Light green and yellow are used for progressively higher lands, followed by light brown and dark brown. The peaks of really high mountains are often shown in white (just like snow). Map readers need to understand these color gradations and what they signify; none of us is born knowing that brown means “higher elevation.” Our brain does help, however. Generally, we think of darker colors as representing greater values, so dark brown to show higher elevations makes sense in many of our heads.
While spot heights and contour lines identify the precise elevations of precise locations, shadings refer to a range of elevations over an area. Thus, the lightest gray tone on Figure 5-3c signifies land that is anywhere between sea level and 100 feet above sea level.
DISTORTION FOR A PURPOSE
A cartogram is a map in which different areas are distorted in proportion to numerical data. Following are two maps of Australia. The one on the left shows the true shape of the continent. The one on the right is a cartogram in which the sizes of Australia’s states and territories are distorted in proportion to their populations. As a result, the cartogram looks much different than the “real thing.” New South Wales, which is home to Sydney (the nation’s largest city), contains about 10 percent of the country’s territory, but about 32 percent of its population. On the cartogram, therefore, New South Wales appears bloated. In contrast, Northern Territories accounts for 17 percent of the country’s territory but only 1 percent of its population. On the cartogram, therefore, Northern Territories is quite small. These extremes visually highlight Australia’s uneven population geography. Usually, cartographers seek to minimize map distortion. In the case of cartograms, however, distortion is the purpose of the exercise.
(© John Wiley & Sons Inc.)
Using Symbols to Tell the Story
As highlighted by the discussion of topography, maps commonly show things by means of point, line, and/or area symbols. Each category, in turn, may display either qualitative or quantitative information. That is, each can simply show where something is located, or how much of something exists at a particular location or over a particular area.
Point symbols
Point symbols are used to locate discreet phenomena on Earth’s surface. Most fall within one or more of the following categories:
Nominal icons are tiny likenesses or symbols they name (hence, nominal) and indicate the locations of particular landscape features. Thus a tiny black dot (•) may be used to symbolize a residence while a cross (†) may be used to locate a cemetery or a church. Whatever the symbol, the cartographer must explain its meaning in the map’s legend.
Ordinal icons are very much like nominal icons except that they come in different sizes that suggest comparable size or order (hence, ordinal). On some maps, for example, a tiny airplane might be used to symbolize a small airport, while a larger airplane is used to indicate a major airport. Similarly, a lower case u might be used to pinpoint a minor uranium deposit while a capital U locates a major one.
Dots are often used to show how the distribution of something varies numerically from place to place. Thus, for example, a map showing the geography of dairy cattle might use a series of dots, each one representing, say, 100 head of cattle. Similarly, a map of tobacco farming might use a series of dots, each representing, say, 100 acres of land in cultivation. As each dot represents some measurable quantity in a place, we call these dot density maps. Be careful here. Using dots could be a bad representational