Читать книгу Geography For Dummies - Jerry T. Mitchell - Страница 110

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 choice. If we used one dot for each person in the United States on a map, you’d have trouble seeing the base map and each dot would overlay another. So dots are not always a good way to convey information.

 Proportional symbols vary in size in direct relation to numerical values. Thus, circles whose areas are proportional to population may indicate the locations and sizes of cities (Figure 5-4).