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2.2: Modeling Earth’s Surface

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Lesson Objectives

  • Describe what information a map can convey.
  • Identify some major types of map projections. Discuss the advantages and disadvantages of each.
  • Discuss the advantages and disadvantages of globes.

Vocabulary

  • conic map
  • coordinate system
  • gnomonic map
  • latitude
  • longitude
  • map
  • Mercator projection
  • projection

Introduction

Maps can convey a lot of different types of information. They can tell you where you are or they can tell you something about a location. Earth scientists often use maps that have coordinates so that they can locate themselves or the features they are interested in. Different types of maps show different things well. For example, some types of maps show the tropical areas really well but do a terrible job depicting the polar regions.

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. A map is a picture of specific parts of Earth’s surface. There are many types of maps. Each map gives us different information. Let’s look at a road map, which is the probably the most common map that you use (Figure below).

A road map of the state of Florida. What information can you get from this map?

Map Legends

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.
  • It is the convention for north to be at the top of a map. For this reason, a compass rose is not needed on most maps.

You can use this map to find your way around Florida and get from one place to another along roadways.

Types of Maps

There are many other types of maps besides road maps. Some examples include:

  • Political or geographic maps show the outlines and borders of states and/or countries.
  • Satellite view maps show terrains and vegetation — forests, deserts, and mountains.
  • Relief maps show elevations of areas, but usually on a larger scale, such as the whole Earth, rather than a local area.
  • Topographic maps show detailed elevations of features on the map.
  • 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 show storms and rainfall.
  • Geologic maps detail the types and locations of rocks found in an area.

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?

Map Projections

Earth is a round, three-dimensional ball. In a small area, Earth looks flat, so it is not hard to make accurate maps of a small place. When map makers want to map the round Earth on flat paper, they use projections. What happens if you try to flatten out the skin of a peeled orange? Or if you try to gift wrap a soccer ball? To flatten out, the orange peel must rip and its shape must become distorted. To wrap a round object with flat paper requires lots of extra cuts and folds. A projection is a way to represent Earth’s curved surface on flat paper (Figure below).

A map projection translates Earth's curved surface onto two dimensions.

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.

Mercator Projection

In 1569, Gerardus Mercator (1512-1594) (Figure below) figured out a way to make a flat map of our round world, called the Mercator projection (Figure below).

Gerardus Mercator developed a map projection used often today, known as the Mercator projection.

Imagine wrapping the round, ball-shaped Earth with a big, flat piece of paper. First you make a tube or a cylinder. The cylinder will touch Earth at its fattest part, the equator. The equator is the imaginary line running horizontally around the middle of Earth. The poles are the farthest points from the cylinder. If you shine a light from the inside of your model Earth out to the cylinder, the image projected onto the paper is a Mercator projection. Where does the projection represent Earth best? Where is it worst? Your map would be most correct at the equator. The shapes and sizes of continents become more stretched out near the poles. Early sailors and navigators found the Mercator map useful because most explorations were located near the equator. Many world maps still use the Mercator projection.

A Mercator projection translates the curved surface of Earth onto a cylinder.

The Mercator projection is best within 15 degrees north or south of the equator. Landmasses or countries outside that zone get stretched out of shape. The further the feature is from the equator, the more out of shape it is stretched. 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 the United States. Because Greenland is closer to the pole, the continent's shape and size are greatly increased. The United States is closer to its true dimensions.

In a Mercator projection, all compass directions are straight lines. This 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.

Conic Projection

Instead of a cylinder, you could wrap the flat paper into a cone. Conic map projections use a cone shape to better represent regions near the poles (Figure above). Conic projections are best where the cone shape touches the globe. This is along a line of latitude, usually the equator.

A conic map projection wraps Earth with a cone shape rather than a cylinder.

Gnomonic Projection

What if want to wrap a different approach? Let's say you don't want to wrap a flat piece of paper around a round object? 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 below). The paper only touches Earth at one point. The sizes and shapes of countries near that point are good. The poles are often mapped this way to avoid distortion. A gnomic projection is best for use over a small area.

A gnomonic projection places a flat piece of paper on a point somewhere on Earth and projects an image from that point.

Robinson Projection

In 1963, Arthur Robinson made a map with more accurate sizes and shapes of land areas. He did this using mathematical formulas. The formulas could directly translate coordinates onto the map. This type of projection is shaped like an oval rather than a rectangle (Figure below).

A Robinson projection better represents the true shapes and sizes of land areas.

Robinson’s map is more accurate than a Mercator projection. The shapes and sizes of continents are closer to true. Robinson's map is best within 45 degrees of the equator. Distances along the equator and the lines parallel to it are true. However, the scales along each line of latitude are different. In 1988, the National Geographic Society began to use Robinson’s projection for its world maps.

Whatever map projection is used, maps 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?

Map Coordinates

Most maps use a grid of lines to help you to find your location. This grid system is called a geographic coordinate system. Using this system you can define your location by two numbers, latitude and longitude. Both numbers are angles between your location, the center of Earth, and a reference line (Figure below).

Lines of latitude start with the equator. Lines of longitude begin at the prime meridian.

Latitude

Lines of latitude circle around Earth. The equator is a line of latitude right in the middle of the planet. The equator is an equal distance from both the North and South Pole. If you know your latitude, you know how far you are north or south of the equator.

Longitude

Lines of longitude are circles that go around Earth from pole to pole, like the sections of an orange. Lines of longitude start at the Prime Meridian. The Prime Meridian is a circle that runs north to south and passes through Greenwich, England. Longitude tells you how far you are east or west from the Prime Meridian (Figure below).

Lines of latitude and longitude form convenient reference points on a map.

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!). 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.

Using Latitude and Longitude on a Map

If you know the latitude and longitude of a 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, say the location you want to find is at 30oN and 90oW. Place your right finger along 30oN at the right of the map. Place your left finger along the bottom at 90oW. Move your fingers along the lines until they meet. Your location should be near New Orleans, Louisiana, along the Gulf coast of the United States.

What if you want to know the latitude and longitude of your location? If you know where you are on a map, point to the place with your fingers. Take one finger and move it along the latitude line to find your latitude. Then move another finger along the longitude line to find your and longitude.

Polar Coordinate System

You can also use a polar coordinate system. 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 distance is given in meters or kilometers. To find your location or to 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 20oE 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 20oE.
  • Walk 500 meters in that direction to reach your destination.
  • Polar coordinates are used in a sport called orienteering. People who do orienteering use a compass and a map with polar coordinates. Participants find their way along a course across wilderness terrain (Figure below). They move to various checkpoints along the course. The winner is the person who completes the course in the fastest time.

A topographic map like one that you might use for the sport of orienteering.

Globe

Earth is a sphere and so is a globe. A globe is the best way to make a map of the whole Earth. Because both the planet and a globe have curved surfaces, the sizes and shapes of countries are not distorted. Distances are true to scale. (Figure below).

A globe is the most accurate way to represent Earth's curved surface.

Globes usually have a geographic coordinate system and a scale. The shortest distance between two points on a globe is the length of the portion of a circle that connects them. Globes are difficult to make and carry around. They also cannot be enlarged to show the details of any particular area. Globes are best sitting on your desk for reference.

Google Earth is a neat site to download to your computer. This is a link that you can follow to get there: http://www.earth.google.com/download-earth.html. The maps on this site allow you to zoom in or out, look from above, tilt your image and lots more.

Lesson Summary

  • Maps and globes are models of Earth’s surface. There are many ways to project the three-dimensional surface of Earth on to 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 Earth, but they cannot be carried around easily. Globes also cannot show the details of Earth’s surface that maps can.

Lesson Review Questions

Recall

1. Describe each of the following. What is each one good for? What is each one not good for?

  • Mercator projection map
  • Robinson projection map
  • Globe

2. What does it mean to say that your location is 52 degrees south and 143 degrees west?

3. Why were early explorers happy with Mercator projections?

Apply Concepts

World map with geographic coordinate system

4. Use Figure above. In what country are you located, if your coordinates are 60oN and 120oW?

5. Which of the following map projections gives you the least distortion around the poles?

  • Mercator projection map
  • Robinson projection map
  • Conic projection

Think Critically

6. Imagine that you are going out into the field to do geology. What type of Earth model (map projection, globe) would be best to take with you?

7. Would you choose a map that used a Mercator projection if you were going to explore Antarctica? Explain why. Is there another type of map that would be better?

Points to Consider

  • How does a flight between two cities drawn on a globe compare with the same flight drawn on a map?
  • How do people doing orienteering follow directions across wild terrain?
  • Latitude and longitude give your location in two dimensions. How do you give your location in three dimensions? What is that third dimension?

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