Modeling Earth's Surface

Lesson Objectives

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


Vocabulary

  • map
  • projection


Introduction

Different representations of Earth’s surface are valuable for different purposes. Accuracy, scale, portability, and features represented are among the many factors that determine which representation is most useful.

Globe

Earth is best represented by a globe like the one seen in Figure below because Earth is a sphere. Sizes and shapes of features are not distorted and distances are true to scale.

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 is the length of the arc (portion of a circle) that connects them.

Math problem: How would you measure the distance between two points on a globe in miles?

  • Here’s an idea: Pull a string taut between the two locations and mark both locations. Lay the string on the equator of the globe. Count the number of degrees between the marks, starting with one end at 0. The number of miles per degree at the equator is 69.17; now multiply the number of degrees by that number to get the distance in miles between the two locations.


A location on a globe must be determined using polar coordinates because a globe is curved (Figure below).

The polar coordinate system is useful for curved surfaces.

Globes are difficult to make and carry around, and they cannot be enlarged to show the details of any particular area. As a result, people need maps.

Maps as Models

A map is a visual representation of a surface with symbols indicating important features. Different types of maps contain different information. Examples of some maps that are important in Earth science are:



Map Projections

Maps are 2-dimensional (2D) representations of a 3-dimensional (3D) Earth. In a small area, Earth is essentially flat, so a flat map is accurate. But to represent a larger portion of Earth, map makers must use some type of projection to collapse the third dimension onto a flat surface. A projection is a way to represent the Earth’s curved surface on flat paper. One example of a projection is shown in the Figure below.

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

There are two basic methods for making 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 looks at the sphere from a certain point and then translates this view onto a flat paper.


Let’s look at a few commonly used projections.

Mercator Projection

In 1569, Gerardus Mercator (1512-1594) developed the Mercator projection (seen in the Figure below). A flat piece of paper curves around the spherical Earth to make a cylinder. The paper touches the sphere at the equator, but the distance between the sphere and the paper increases toward the poles. The features of Earth’s surface are projected out onto the cylinder and then unrolled, creating a Mercator projection map.

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

Where do you think a Mercator map is most accurate? Where is it least accurate? Near the equator the shapes and sizes of features are correct, but features get stretched out near the poles. For example, on a globe, Greenland is fairly small, but in a Mercator map, Greenland is stretched out to look almost as big the United States.

In a Mercator projection, all compass directions are straight lines, but a curved line is the shortest distance between the two points. Many world maps still use Mercator projection today. Early explorers found Mercator maps useful because they visited the equatorial regions more frequently.

A good explanation of the distortion that results from the projection of a sphere onto a flat surface can be seen in Alternative World Maps.

Conic Projection

A conic map projection uses a cone shape to better represent regions and best depicts the area where the cone touches the globe. Looking at Figure below, what is the advantage of a conic projection over a Mercator projection?

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

Gnomonic Projection

A gnomonic map projection is illustrated in Figure below. With a gnomonic map projection, paper is placed on the area that you want to map. The projection is good for features near that point. The poles are often mapped this way.

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 created an attractive map projection in which latitude lines are projected but meridians are curved, resulting in a map that is an ellipse rather than a rectangle (see Figure below for an example). This projection has less distortion near the poles, and features within 45 degrees of the equator are closer to their true dimensions. The distances along latitude lines are true, but the scales along each line of latitude are different. Robinson projections are still commonly used.

A Robinson projection more accurately reflects the size and shape of features near 45 degrees.

Winkel Tripel Projection

The National Geographic Society uses the Winkel Tripel Projection, which uses mathematical formulas to create a map projection that is also distorted at the edges (Figure below).

The Winkel Tripel Projection of Earth.

Locations on a map are determined using rectangular coordinates (see Figure below).

Rectangular coordinates are useful for flat surfaces.

Google Earth is a neat site to download to your computer. The maps on this site allow you to zoom in or out, look from above, tilt your image, and a lot more.

Lesson Summary

  • Maps and globes are models of the Earth’s surface.
  • Globes are the most accurate representations because they are spherical like the Earth is, but using a globe as a map has practical disadvantages.
  • There are many ways to project the three-dimensional surface of the Earth on to a flat map. Each type of map has some advantages as well as disadvantages.
  • Most maps use latitude and longitude to indicate locations.


Review Questions

  1. Which of the following gives you the most accurate representations of distances and shapes on the Earth’s surface?

    1. Mercator projection map
    2. Robinson projection map
    3. globe


  2. Explain the difference between latitude and longitude.
  3. In what country are you located if your coordinates are 60oN and 120oW?
  4. Which map projection is most useful for navigation, especially near the equator? Explain.
  5. In many cases, maps are more useful than a globe. Why?
  6. Which of the following map projections gives you the least distortion around the poles?

    1. Mercator projection map
    2. Robinson projection map
    3. conic projection




Further Reading / Supplemental Links

  • National Geographic has an introduction to maps and one at MyWonderful World.
  • An atlas for the United States with many types of maps.
  • Location and relief on maps.


Points to Consider

  • Imagine you are a pilot and must fly from New York to Paris. Use a globe to determine the distance. Now do the same with a map. How are these activities the same and how are they different?
  • Would you choose a map that used a Mercator projection if you were going to explore Antarctica? What other type of map could you use?
  • 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?


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