Читать книгу Geography For Dummies - Jerry T. Mitchell - Страница 111

A number of important features on Earth’s surface are linear in nature, meaning they look like lines, such as roads or railways. Likewise, migration, travel, trade, and other movements of interest to geography are basically linear phenomena that connect points. Accordingly, line symbols are common features on maps and take one of the following forms:

(© John Wiley & Sons Inc.)

FIGURE 5-4: This map uses proportional circles to indicate the size of cities.

 Nominal lines note the locations of particular linear features, such as roads, railways, rivers, and borders. They may appear as solid, dashed, or embellished lines, the standard symbol for railroads being an example of the latter. Colors may also be employed. Blue lines, for example, are commonly used to indicate rivers.

 Ordinal lines vary in thickness or color to indicate relative importance. On many maps, for example, city, state, and country boundaries are progressively thicker so as to indicate the relative importance of the political units they mark. In Figure 5-4, the line that separates the United States and Canada is thicker than the lines that separate the states and provinces. Similarly, lines that symbolize roads often vary in thickness in proportion to the width of the highway or number of lanes.

 Flow lines indicate movement, travel or trade along a given route or between two points. On some maps, the thickness of the lines varies in direct proportion to the quantity or volume of the flow. Thus, on a map of immigration, arrows of varying widths may be used to indicate the volume of movement between sender and receiver regions (as shown in Figure 5-5).

 Isolines connect points of equal value with respect to a certain phenomenon. Didn’t you just hear this recently? Oh yes, the contour lines shown in Figure 5-3b are an example (flick back if you missed that part of this chapter). Similarly, daily weather maps often contain isolines that connect points with identical atmospheric pressure or the day’s projected high temperature or precipitation.

(© John Wiley & Sons Inc.)

FIGURE 5-5: This map uses flow lines of different widths to indicate hypothetical migrant flow.