Geography For Dummies. Jerry T. MitchellЧитать онлайн книгу.

to those two roads. Thus, streets are numbered consecutively north and south of Equator Boulevard. Avenues are numbered consecutively east and west of Prime Meridian Way.

A big dot and a letter mark two intersections. I’ll refer to these shortly.

Telling Someone Where to Go

Because geography involves locations and directions, it affords ample opportunity to tell someone where to go. Suppose you live in Gridville and are standing on the sidewalk at Point A, the corner of North 4th Street and East 3rd Avenue. A stranger from out of town comes up to you and asks for directions to Gridville Hospital — can you help her?

Of course, you can. You know the hospital is located at Point B on the map. And you can convey that information to the stranger by stating either the hospital’s relative location or absolute location.

Relative location

In the first instance, you can tell the stranger how to get to the hospital from Point A. For example (pointing west along North 4th Street), “Go that way four blocks, turn left, and walk five more blocks.” This is called relative location because the information you gave is relative to Point A. Give those directions verbatim to the stranger at any other intersection in Gridville, and the result is a lost stranger.

Absolute location

As an alternative, you can convey the location of the hospital with respect to its grid coordinates — that is, its location within the grid system. For example, “Go to the corner of South 1st Street and West 1st Avenue.” This is called absolute location because theoretically, those directions work anywhere in Gridville, not just at Point A.

The best location to use

Both relative location and absolute location have the potential of getting the stranger to the desired destination. And chances are you have used both types of location to direct someone to a destination in your town, neighborhood, or environs.

But in a global context, absolute location is far superior to relative location. When you think about it, the task of directing somebody to a location halfway around the world by means of relative location (for example, “Go that way 11,238 miles and turn right”) is rather mind-boggling. And even if you could do it, that information would only work at the one location where that information was given. It would be far better if every place on Earth had an absolute location such as that hospital in Gridville. Of course, that would be contingent on the existence of a global grid that basically mimics what we’ve seen in Gridville. Fortunately, such a grid exists.

The Global Grid: Hip, Hip, Hipparchus!

Like Gridville, the world as a whole possesses a grid whose coordinates may be used to identify the absolute location of things. Indeed, that is why a Greek named Hipparchus invented the global grid more than 2,000 years ago. Though not necessarily the first with this idea — many credit the earlier ideas of Eratosthenes — he did leave us with what most people use today. (You can read more about Eratosthenes in Chapter 1).

As chief librarian at the great library in Alexandria, Egypt, Hipparchus compiled information about lands and cities all over the expanding Greek world. He saw the value of accurately locating objects on a map, but in those days that was easier said than done. Maps were notoriously inaccurate, due in good measure to lack of a systematic means of stating the location of things. So, Hipparchus set out to rectify the situation, and came up with the global grid that is still in use today (see Figure 3-2).

Schematic illustration of the basics of the global grid.

FIGURE 3-2: The basics of the global grid.

Avoiding gridlock

Proper use of a grid coordinate system to state the absolute locations of things depends on a handful of prerequisites. Think of these as ways of avoiding gridlock:

Familiarity breeds success. Knowledge of the naming and numbering of grid components is essential. If, for example, that stranger were not familiar with Gridville’s grid, then telling her the hospital is at “the intersection of South 1st Street and West 1st Avenue” would have made no sense whatsoever. The same is true with respect to the global grid. That is, knowing how the lines are named and numbered is essential if you are to use the grid successfully.

Unique components. Each road in Gridville and each line on the global grid must have a unique name. In Gridville, for example, there must be only one road named South 1st Street, and only one named East 1st Avenue. If multiples exist, then more than one site could satisfy “the intersection of South 1st Street and East 1st Avenue.” And that would rather defeat the concept of absolute location, whether in Gridville or around the globe.

No double-crossing allowed. Don’t take that as a threat or accusation. What I mean is two roads in Gridville may cross each other only once. The same goes for two lines on the global grid. If they have multiple junctions then, such as the last point, there would be two or more intersections of, say, South 1st Street and East 1st Avenue. And again, that would defeat the concept of absolute location.

Full names, please. You must use the full name of each road in Gridville and each line on the global grid. Again, the absolute location of the hospital is the intersection of South 1st Street and West 1st Avenue. Now suppose you had told that stranger, “The hospital’s at the corner of 1st Street and 1st Avenue.” Well, if you look carefully at the map of Gridville, you find four locations where a 1st Street crosses a 1st Avenue. Obviously, the potential for location confusion here defeats the purpose of absolute location. The remedy is to use the full name of each grid component.

The naming game

While the Gridville grid consists of real roads, the global grid consists of imaginary lines of latitude and longitude (see Figure 3-2). Latitude lines go across the map — latitude comes from the Latin latitudo, meaning breadth, or the measure of the side-to-side dimension of a solid. Longitude lines run from top to bottom — longitude comes from the Latin longitudo, meaning length. This makes sense because when viewed on a globe, lines of longitude are generally lengthier than lines of latitude.

Similar to the roads in Gridville, the global grid contains a principal line of latitude (the equator) and a principal line of longitude (the prime meridian). All other lines of latitude and longitude are named and numbered respectively from these starting lines. It makes sense, therefore, that if you want to make like Hipparchus and draw a grid on a globe, then these are the first two lines you would draw. But where would you put them, and why?

The equator

Because Earth is sphere-like, no compelling locale cries out and says, “Use me to locate the equator!” So where to put it? Old Hipparchus might simply have said, “It’s Greek to me!” and placed it anywhere. Instead, he wrestled with the challenge and came up with an ingenious solution.

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