The Nature of Geographic Information

10. Geographic Coordinate System

Picture showing how longitude and latitude fall on a globe.
Figure 2.11.1 The geographic coordinate system.
Credit: David Dibase

Longitude specifies positions east and west as the angle between the prime meridian and a second meridian that intersects the point of interest. Longitude ranges from +180 (or 180° E) to -180° (or 180° W). 180° East and West longitude together form the International Date Line.

Latitude specifies positions north and south in terms of the angle subtended at the center of the Earth between two imaginary lines, one that intersects the equator and another that intersects the point of interest. Latitude ranges from +90° (or 90° N) at the North pole to -90° (or 90° S) at the South pole. A line of latitude is also known as a parallel.

At higher latitudes, the length of parallels decreases to zero at 90° North and South. Lines of longitude are not parallel but converge toward the poles. Thus, while a degree of longitude at the equator is equal to a distance of about 111 kilometers, that distance decreases to zero at the poles.

Try This!

Geographic Coordinate System Practice Application

Nearly everyone learned latitude and longitude as a kid. But how well do you understand the geographic coordinate system, really? My experience is that while most everyone has heard of latitude and longitude, only about half can point to the location on a map that is specified by a pair of geographic coordinates. The Flash application linked below lets you test your knowledge. The application asks you to click locations on a globe as specified by randomly generated geographic coordinates.

You will notice that the application lets you choose between "easy problems" and "hard problems." Easy problems are those in which latitude and longitude coordinates are specified in 30° increments. Since the resolution of the graticule (the geographic coordinate system grid) used in the application is also 30°, the solution to every "easy" problem occurs at the intersection of a parallel and a meridian. The "easy" problems are good warm-ups.

"Hard" problems specify coordinates in 1° increments. You have to interpolate positions between grid lines. You can consider yourself to have a good working knowledge of the geographic coordinate system if you can solve at least six "hard" problems consecutively and on the first click.

Click here to download and launch the Geographic Coordinate System practice application (5.7 Mb). (If the globe doesn't appear after the Flash application has loaded, right-click and select "Play" from the pop-up menu.)

Screenshot of the Geographic Coordinate System practice application, download above
Figure 2.11.2
Note: You will need to have the Adobe Flash player installed in order to complete this exercise. If you do not already have it, you can download the Flash Player for free at the Adobe website.
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