on a map and the corresponding actual distance on Earth’s surface. Calculating distance between locations and comparing the size of areas are two of the more important functions of maps.
Going the distance
The scale of a map may be stated in three rather different ways, described in the following sections. Figure 5-1 shows you what the three ways look like. Some maps include just one of them. Others include two, and still others all three. Perhaps the most important thing to remember is that every map has a single scale, but a cartographer has three ways to tell you what it is. If, therefore, a single map contains two or three of the scale-types, then each is saying the same thing, albeit in a different way.
(© John Wiley & Sons Inc.)
FIGURE 5-1: Three ways to indicate the scale of a map.
Scale bar
A scale bar looks like a miniature ruler. But whereas the ruler you use may show inches and millimeters, the one on the map shows miles or kilometers, (as shown in Figure 5-1). The principal virtue of the bar graph is that it provides a clear visual reference to the size of the area portrayed on the map. For actual measurement, however, it may be a bit unwieldy because you can’t pick it up like you can a real ruler.
Verbal scale
A verbal scale (also called statement of scale) communicates the relationship between map distance and real-world distance in a sentence or sentence-like format. In Figure 5-1, “One inch equals one mile” is the example. (By the way, if you want to know why a mile is a mile, see the nearby sidebar “Whence comes the mile?”)
As far as most people are concerned, the verbal scale is particularly convenient for measuring distances on a map, provided a ruler is available. In the case of “one inch equals one mile,” one need only measure the number of inches between two points to arrive at the number of miles that separate them on Earth. If, on the other hand, the verbal scale on another map reads “one inch equals 20 miles,” then the number of inches between the two points on the map needs to be multiplied by 20 to render the actual distance.
Representative fraction (RF)
Check out Figure 5-1 again. The RF shown is 1:63,360. That means the map is 1/63,360th the size of the area it shows. Stated differently, a distance of one inch on the map equals 63,360 inches on the Earth’s surface.
Once more, a given map has a given scale, but you can express it in different ways. In the example, therefore, “One inch equals one mile” and “1:63,360” must mean the same thing. And, indeed, they do. Proof is obtained by calculating the number of inches in a mile. To do that, multiply the number of inches per foot times the number of feet per mile (12 × 5,280). The answer is 63,360, so the statement of scale and the RF are, in fact, the same.
Comparing Earth at different scales
A mile is a unit of linear measurement that equals 5,280 feet. While most of the world has adopted metric units (kilometers), Americans continue to express distance in miles, which, therefore, commonly appear as units of measurement on maps made in the U.S. But exactly what is a mile? And why does it consist of 5,280 feet instead of a more convenient figure, like 5,000?
“Mile” comes from the Latin milia, meaning thousand. In Roman times, a unit of linear measure called the milia passum, or thousand paces, was common. Somehow, somebody’s thousand strides became a standard Roman mile, equal to about 1,650 yards. This measurement became widely used in Britain following the Roman’s invasion. After the Empire’s demise, however, the milia passum fell into disuse, although “mile” endured in the British vocabulary as a word applicable to a substantial distance.
The mile’s present length has its origins in medieval English agriculture. Back then, a team of oxen was used to pull a heavy wooden plow. The farmer walked behind, making liberal use of an ox goad — a big stick — to influence the animals’ behavior. The stick was known as a rod, and at some point its length was standardized to 16.5 feet. The length of a parcel of farmland was “a furrow long,” or furlong. That was the distance the oxen could pull the plow before the farmer had to stop and rest them. Naturally, that length varied. In time, however, the furlong was standardized to a distance of 40 rods (660 feet or 220 yards). Sometime later, a distance of 8 furlongs (5,280 feet or 1,760 yards) became the standard mile, and remains so to this day.
In Figure 5-2a, 1 inch represents 630 miles. As a result, this map shows a comparatively large area that includes most of the Great Lakes, Upper Midwest, a handful of major cities, and a portion of Canada.
In Figure 5-2b, 1 inch represents 190 miles. What is shown now is a much smaller area that includes parts of Lake Michigan and Midwest states, a few medium-size towns, and a few major regional highways.
In Figure 5-2c, 1 inch represents 64 miles. Now we have “zoomed in” to the extent that the map shows Greater Chicago, southern-most Lake Michigan, more municipalities, local highways, and several streets.
Notice that as the area shown on these maps decreases, the amount of detail increases. And if you think about it, that makes a great deal of sense. When 1 inch represents 630 miles — a large area — only very large surface features (such as the Great Lakes) can be shown. But when 1 inch represents 64 miles — a much smaller area — then comparatively small surface features (such as roads) can be effectively shown.
(© John Wiley & Sons Inc.)
