SOLAR SYSTEM

Where in the World Are You?

by Diane Boudreau

Maps are important pieces of information. They describe the places we live, the places we've been, and the places we'd like to go. Maps help us get from one place to another -- here on Earth and, in the future, on other planets as well.

Imagine you're sitting in a classroom in Tempe, Arizona, and you need to tell some students in Russia where you are. How would you describe your location?

You could say that you are in the Southwest region of the United States, but that is pretty vague.

You could also say that you are at approximately 33 degrees and 25 minutes north latitude, 111 degrees and 55 minutes west longitude. That is a very specific way to describe your location, but it's hard to picture in your head.

To provide a frame of reference, you might say that you are 2,142 miles southwest of New York City, and 376 miles east of Los Angeles. You could also say that you are in the Sonoran Desert, about 1,000 feet above sea level.

An easy solution would be to send your friends a map, which would contain all this information and more.

A map is a graphic representation of a particular area of Earth or the heavens. Most maps are smaller than what they represent. The difference in size between a map and the area being mapped is called scale. Scale is shown as a ratio. For example, if one inch on the map represents 10,000 inches on the ground, the map has a scale of 1 to 10,000.

The study of maps and mapmaking is called cartography. Cartography is "as old as the hills," says Robert Edsall, a geographer at ASU who specializes in cartography.

Maps tell stories, just like words. Maps can come from personal experiences, like an ancient villager drawing an easy route to a river or a modern teenager showing a friend how to get to a party. Maps might also come from the experiences of others. Mapmakers use other maps and stories to create maps that communicate something of interest to them or their audiences.

"We've always mapped what's important to us," says Edsall. Over time, maps have tended to cover more and more area, just as human interests have expanded from the local area to the infinite scope of the universe.

What is important to you determines what kind of map you make. There are two basic kinds of maps: thematic maps and reference maps.

Thematic maps show how something is distributed over an area.

For example, ASU geologist Phil Christensen has created a map showing where the mineral hematite is located on Mars (see page Invisible Mars). This map will help scientists decide where exactly on the Red Planet to send future space missions.

The other type of map is a reference map. Most road maps and atlas maps are reference maps. They help us to locate places. But what is the best way to describe the location of places on Earth?

There are two ways of describing location: absolute and relative. Relative location tells where you are in relation to something else. For example, if you say, "My school is two miles south of my house," you are giving a relative location. Another person could easily find your school from your house, but they could not pinpoint it on a map of the city.

Absolute location tells where you are on the planet. This is measured using coordinates called latitude and longitude. Latitude coordinates tell you how far north or south you are. Longitude coordinates tell you how far east or west you are. You can describe any spot on Earth with just these two measurements.

Today, you can easily measure latitude and longitude using a GPS (global positioning system) device. This handheld device receives signals from several of 24 satellites orbiting the Earth. It can tell you your exact location within a few meters.

GPS relies on a technique called triangulation. A GPS unit triangulates your location by determining its distance from at least three satellites. If you know how far you are from three different points, you can always tell precisely where you are.

GPS was developed by the U.S. Department of Defense to locate soldiers on the battlefield. Now, anyone can get a GPS unit. Hikers often use them. They are also being put into cars and cell phones.

How did people measure latitude and longitude before GPS was invented?

Latitude is actually determined by measuring the angle from the horizon to the North Star. At any given latitude, the North Star is always in the same place in the sky, no matter what time of day or year. How far it is from the horizon depends on your latitude. If it's directly overhead, you're at the North Pole. If it's right on the horizon, you're at the equator (the imaginary line that circles the Earth at exactly zero degrees latitude).

You can't see the North Star from the southern hemisphere, but the constellation known as the Southern Cross works almost as well for figuring latitude.

Longitude was a lot more difficult to figure out. Believe it or not, longitude was first measured using a clock!

We can measure distance with a timepiece because the Earth is divided into different time zones. If it is noon in London, it is 7 a.m. in New York and 4 a.m. in California. To measure longitude, a sailor would set a clock in Greenwich, England, then sail out into the open sea. At exactly noon (when the sun appeared directly to the south), he would look at his clock and compare it to Greenwich time to see how far he'd traveled.

This method wasn't very precise back in the 1700s, because clocks of that era weren't very accurate. It wasn't until a watchmaker named John Harrison invented an extremely accurate watch that sailors could measure longitude precisely.

Even with today's advanced mapping technologies, many maps still contain errors. One reason is that most maps are still created by copying and combining older maps. Unfortunately, an error in one map gets passed along through all the ones that are copied from it. Sometimes, people will go out and test the maps to be sure that they are accurate. This process is called "ground truthing."

Other maps contain errors that were put there on purpose. For example, important military locations may be missing or shown in the wrong places so that enemies cannot find them during a war. Camp David is a place in Maryland where the president of the United States and his family can go to rest and relax. You cannot find Camp David on a map. This is an intentional error that helps keep our president safe.

Sometimes, mapmakers do not care about accuracy as long as their maps are useful.

South Pacific Islanders used to make maps out of sticks and shells tied together with rope. The shells represented islands, while the sticks showed trade wind directions and ocean currents. The maps did not show the exact shape or scale of the islands. But they were extremely useful in helping the islanders sail from one island to another.

Even today, some maps are more useful than accurate.

"Did you ever see a AAA TripTik? The maps that give you driving directions from one city to another?" asks Edsall. "They generally show highways as straight lines when they are not. But they still help you get where you need to go."

Even when accuracy is important to a mapmaker, maps are all inherently distorted. This happens because a map is a projection - it shows three-dimensional data on a flat, two-dimensional surface.

Imagine trying to flatten out the peel of an orange you've just eaten. You can't do it without tearing it, or stretching it, or squashing it. You are trying to make a three-dimensional surface two-dimensional, and to do this, you have to distort it somehow.

Maps distort information, too. Mapmakers choose what type of projection to use based on how it stretches or squashes the Earth's surface. The most common projection is called the Mercator. It used to appear in most atlases. The Mercator projection shows the correct shapes of countries, but their sizes are distorted. "Greenland looks huge - as big as Africa - but in fact it is much smaller," says Edsall.

The Peters projection, on the other hand, shows correct sizes, but distorts the shapes of countries. "On this map Greenland looks much smaller. But Africa looks more long and skinny than it really is. And the polar regions look squashed," Edsall says.

Globes can show shape and size correctly because they are three-dimensional. These days, geographers are working to find other ways to display 3D data with the help of computer modeling and graphics programs. "Some of the most cutting-edge stuff in cartography right now is representing lots of variables measured in three dimensions," says Edsall. Even more difficult, some geographers are working to show 3D data over both space and time!

We've come a long way from sticks and shells, but the basic purpose of maps remains the same - to help us find what we need in our town, our country, or even on Mars.