Latitude & Longitude

Thought question: when you are standing at the North Pole, which direction are you looking?

A topic that I think people find a bit confusing is the coordinate systems commonly used in their handheld GPS units. The handheld GPS can tell you your exact location, and this is because cartographers have partitioned the surface of the Earth so that one point can be located with regard to any other point. However, over time they have developed a variety of different coordinate systems to meet various needs.

The most commonly used coordinate system is probably the latitude and longitude grid. This system is based on two 360 degree circles that are envisioned to circle the planet. The first great circle spans the planet from “head to toe,” or along the axis of rotation. The second great circle goes around the “waist” of the planet, at right angles to the axis of rotation, and along the planet’s midline. We call this great circle the equator.

You no doubt learned that a circle can be divided into 360 degrees. The circle of the equator can likewise be divided, but where should we start? The line projected from the North Pole through Greenwich, England, through the equator to the South Pole is the prime meridian. Here, “prime” means “first” or “initial.” The point where it crosses the equator is the starting point for dividing the equatorial circle.

Why Greenwich, England, you ask? This standard was really only recently set, in 1884, when delegates from 25 nations met in Washington, D.C. for the International Meridian Conference. They adopted the meridian passing through the Transit Instrument at the Greenwich Observatory as the “prime” one. You have to start somewhere!

From the prime meridian, we can measure around the circle of the equator east and west up to 180 degrees, covering the full circle. This establishes the lines of longitude (running from the pole to pole) and defines the directions “east” and “west.”

We can divide the prime meridian into its 360 degrees also, and we find it useful to start at the equator and count 180 degrees from pole to pole. So, you can go from zero to 90 degrees in both directions, north and south, and this establishes lines of latitude, and defines “north” and “south.”

Notice that lines of latitude do not converge on a spot—they remain parallel to each other on the globe. However, lines of longitude do converge, at the poles. What this means is that the distance on the ground remains the same between degrees of latitude. But the degrees of longitude get closer together as you approach the poles. In other words, one degree of longitude at the equator is a longer physical distance on the ground than one degree of longitude farther to the north, say in Greenland. This is just an interesting complication of living on a sphere.

And it is because of that complication that I know exactly which direction you are looking when you are standing on the North Pole—no matter how you turn your body, you are by definition looking south. At 90 degrees north there is so other way to go but down (in latitude, that is).

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Handheld GPS basics
Basic features in a handheld GPS unit

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