High School Earth Science/Modeling Earth's Surface
- Describe what information a map can convey.
- Identify some major types of map projections and discuss the advantages and disadvantages of each.
- Discuss the advantages and disadvantages of using a globe.
Maps as Models
Imagine you are going on a road trip. Perhaps you are going on vacation. How do you know where to go? Most likely, you will use a map. Maps are pictures of specific parts of the Earth's surface. There are many types of maps. Each map gives us different information. Let's look at a road map, which is probably the most common map that you use (Figure 2.14).
Look for the legend on the top left side of the map. It explains how this map records different features. You can see the following:
- The boundaries of the state show its shape.
- Black dots represent the cities. Each city is named. The size of the dot represents the population of the city.
- Red and brown lines show major roads that connect the cities.
- Blue lines show rivers. Their names are written in blue.
- Blue areas show lakes and other waterways - the Gulf of Mexico, Biscayne Bay, and Lake Okeechobee. Names for bodies of water are also written in blue.
- A line or scale of miles shows the distance represented on the map – an inch or centimeter on the map represents a certain amount of distance (miles or kilometers).
- The legend explains other features and symbols on the map.
- Although this map does not have a compass rose, north is at the top of the map.
You can use this map to find your way around Florida and get from one place to another along roadways.
There are many other types of maps besides road maps. Some examples include:
- Topographic maps show detailed elevations of landscapes on the map.
- Relief maps show elevations of areas, but usually on a larger scale. Relief maps might show landforms on a global scale rather than a local area.
- Satellite view maps show terrains and vegetation – forests, deserts, and mountains.
- Climate maps show average temperatures and rainfall.
- Precipitation maps show the amount of rainfall in different areas.
- Weather maps show storms, air masses, and fronts.
- Radar maps also show storms and rainfall.
- Geologic maps detail the types and locations of rocks found in an area.
- Political or geographic maps show the outlines and borders of states and/or countries.
These are but a few types of maps that various earth scientists might use. You can easily carry a map around in your pocket or bag. Maps are easy to use because they are flat or two-dimensional. However, the world is three-dimensional. So, how do map makers represent a three-dimensional world on flat paper? Let's see.
The Earth is a three-dimensional ball or sphere. In a small area, the Earth looks flat, so it is not hard to make accurate maps of a small place. When map makers want to map the Earth on flat paper, they use projections. Have you ever tried to flatten out the skin of a peeled orange? Or have you ever tried to gift wrap a soccer ball to give to a friend as a present? Wrapping a round object with flat paper is difficult. A projection is a way to represent the Earth's curved surface on flat paper (Figure 2.15).
There are many types of projections. Each uses a different way to change three-dimensions into two-dimensions.
There are two basic methods that the map maker uses in projections:
- The map maker "slices" the sphere in some way and unfolds it to make a flat map, like flattening out an orange peel.
- The map maker can look at the sphere from a certain point and then translate this view onto a flat paper.
Let's look at a few commonly used projections.
In 1569, Gerardus Mercator (1512-1594; Figure 2.16) figured out a way to make a flat map of our round world, called a Mercator projection (Figure 2.17). Imagine wrapping our round, ball shaped Earth with a big, flat piece of paper to make a tube or a cylinder. The cylinder will touch the Earth at the equator, the imaginary line running horizontally around the middle of the Earth, but the poles will be further away from the cylinder. If you could shine a light from the inside of your model Earth out to the cylinder, the image you would project onto the paper would be a Mercator projection. Your map would be just right at the equator, but the shapes and sizes of continents would get more stretched out for areas near the poles. Early sailors and navigators found the Mercator map useful because most explorers at that time traveled to settlements that were located near the equator. Many world maps still use Mercator projection today.
The Mercator projection best describes the shapes and sizes of countries within 15 degrees north or south of the equator. For example, if you look at Greenland on a globe, you see it is a relatively small country near the North Pole. Yet on a Mercator projection, Greenland looks almost as big as the United States. Greenland's shape and size are greatly increased, while the United States is represented closer to its true dimensions. In a Mercator projection, all compass directions are straight lines, which makes it a good type of map for navigation. The top of the map is north, the bottom is south, the left side is west and the right side is east.
However, because it is a flat map of a curved surface, a straight line on the map is not the shortest distance between the two points it connects.
Instead of a cylinder, you might try wrapping the flat paper into a cone. Conic map projections use a cone shape to better represent regions equally (Figure 2.18). This type of map does best at showing the area where the cone shape touches the globe, which would be along a line of latitude, like the equator.
Maybe you don't like trying to wrap a flat piece of paper around a round object at all. In this case, you could put a flat piece of paper right on the area that you want to map. This type of map is called a gnomonic map projection (Figure 2.19). The paper only touches the Earth at one point, but it will do a good job showing sizes and shapes of countries near that point. The poles are often mapped this way, but it works for any area that you chose.
In 1963, Arthur Robinson made a map that looks better in terms of shapes and sizes. He translated coordinates onto the map instead of using mathematical formulas. He did this so that regions on the map would look right. This map is shaped like an ellipse (oval shape) rather than a rectangle (Figure 2.20).
Robinson's map shows less distortion near the poles and keeps shapes and sizes of continents close to their true dimensions, especially within 45 degrees of the equator. The distances along the equator and lines parallel to it are true, but the scales along each line of latitude are different. In 1988, the National Geographic Society adopted Robinson's projection for all of its world maps.
Whatever map projection is used, maps are designed to help us find places and to be able to get from one place to another. So how do you find your location on a map? Let's look.
Most maps use a grid or coordinate system to find your location. This grid system is sometimes called a geographic coordinate system. The system defines your location by two numbers, latitude and longitude. Both numbers are angles that you make between your location, the center of the Earth, and a reference line (Figure 2.21).
|Figure 2.21: Lines of latitude start with the equator. Lines of longitude begin at the prime meridian.|
Lines of latitude circle around the Earth. The equator is a line of latitude right in the middle of the Earth, which is the same distance from both the North and South Pole. In a grid, your latitude tells you how far you are north or south of the equator. Lines of longitude are circles that go around the Earth from pole to pole, like the sections of an orange. Lines of longitude start at the Prime Meridian, which is a circle that runs north to south and passes through Greenwich, England. Longitude tells you how far east or west you are from the Prime Meridian. You can remember latitude and longitude by doing jumping jacks. When your hands are above your head and your feet are together, say longitude (your body is long!), then when you put your arms out to the side horizontally, say latitude (your head and arms make a cross, like the “t” in latitude). While you are jumping, your arms are going the same way as each of these grid lines; horizontal for latitude and vertical for longitude.
If you know the latitude and longitude for a particular place, you can find it on a map. Simply place one finger on the latitude on the vertical axis of the map. Place your other finger on the longitude along the horizontal axis of the map. Move your fingers along the latitude and longitude lines until they meet. For example, if the place you want to find is at 30°N and 90°W, place your right finger along 30°N at the right of the map (Figure 2.22). Place your left finger along the bottom at 90°W. Move them along the lines until they meet. Your location should be near New Orleans, Louisiana along the Gulf coast of the United States. Also, if you know where you are on a map, you can reverse the process to find your latitude and longitude.
One other type of coordinate system that you can use to go from one place to another is a polar coordinate system. Here your location is marked by an angle and distance from some reference point. The angle is usually the angle between your location, the reference point, and a line pointing north. The other number is a distance in meters or kilometers. To find your location or move from place to place, you need a map, a compass, and some way to measure your distance, such as a range finder. Suppose you need to go from your location to a marker that is 20°E and 500 m from your current position. You must do the following:
- Use the compass and compass rose on the map to orient your map with North.
- Use the compass to find which direction is 20°E.
- Walk 500 meters in that direction to reach your destination.
Polar coordinates are used most often in a sport called orienteering. Here, you use a compass and a map to find your way through a course across wilderness terrain (Figure 2.23). You move across the terrain to various checkpoints along the course. You win by completing the course to the finish line in the fastest time.
A globe is the best way to make a map of the whole Earth, because the Earth is a sphere and so is a globe. Because both the Earth and a globe have curved surfaces, sizes and shapes of countries are not distorted and distances are true to scale (Figure 2.24).
Globes usually have a geographic coordinate system and a scale on them. The shortest distance between two points on a globe is the length of the arc (portion of a circle) that connects them. Despite their accuracy, globes are difficult to make and carry around. They also cannot be enlarged to show the details of any particular area. Google Earth is a neat program to download to your computer. This is a link that you can follow to get there: http://earth.google.com/download-earth.html. The maps on this program allow you to zoom in or out, look from above, tilt your image and lots more.
- Maps and globes are models of the Earth’s surface. There are many ways to project the three-dimensional surface of the Earth onto a flat map. Each type of map has some advantages as well as disadvantages.
- Most maps use a geographic coordinate system to help you find your location using latitude and longitude.
- Globes are the most accurate representations, because they are round like the Earth, but they cannot be carried around easily. Globes also cannot show the details of the Earth's surface that maps can.
- Which of the following gives you the most accurate representations of distances and shapes on the Earth's surface?
- Mercator projection map
- Robinson projection map
- Explain the difference between latitude and longitude.
- (Use Figure 2.25.) In what country are you located, if your coordinates are 60°N and 120°W?
- Which map projection is most useful for navigation, especially near the equator? Explain.
- In many cases, maps are more useful than a globe. Why?
- Which of the following map projections gives you the least distortion around the poles?
- Mercator projection map
- Robinson projection map
- Conic projection
- conic map projection
- A map projection made by projecting Earth's three dimensional surface onto a cone wrapped around the Earth.
- coordinate system
- Numbers in a grid that locate a particular point.
- gnomonic map
- A map projection made by projecting onto a flat paper touching just one spot on the Earth.
- Horizontal imaginary lines drawn around the Earth parallel to the equator, which is 0° latitude.
- Vertical imaginary lines drawn around the Earth from pole to pole; the Prime Meridian is 0° longitude.
- A two dimensional representation of Earth's surface.
- Mercator projection
- A two dimensional projection which was invented by Mercator and uses a cylinder wrapped around the Earth.
- A way to distort and/or represent a three dimensional surface in two dimensions.
Points to Consider
- Imagine you are a pilot and must fly from New York to Paris. Use a globe and a world map to do the following:
- Plot your course from New York to Paris on a globe. Make it the shortest distance possible.
- Measure the distance by using the scale, a ruler, and a string.
- Draw the course from the globe on a world map.
- Draw a line on the map connecting New York and Paris.
- How does the course on the globe compare with the line on the map? Which is the shortest distance? Write a brief paragraph describing the differences and explain why they are different.
- Would you choose a map that used a Mercator projection if you were going to explore Antarctica? Explain why this would not be a good choice. What other type of map would be better?
- Maps use a scale, which means a certain distance on the map equals a larger distance on Earth. Why are maps drawn to scale? What would be some problems you would have with a map that did not use a scale?