To make any map, a cartographer must make two decisions:

• How much of Earth’s surface to depict on the map (map scale).

• How to transfer a spherical Earth to a flat map (projection).

Map Scale

What is the purpose of the map? Is it necessary to show the entire globe on a map, or just one continent, or a country, or a city? To make a scale model of the entire world, many details must be omitted because there simply is not enough space. Conversely, if a map shows only a small portion of Earth’s surface, such as a street map of a city, it can provide a wealth of detail about a particular place.

The level of detail and the amount of area covered on a map depend on its map scale. When specifically applied to a map, map scale refers to the relationship of a feature’s size on a map to its actual size on Earth. Map scale is presented in three ways (Figure 1-16).

• Ratio. A ratio or fraction shows the numerical relationship between distances on the map and Earth’s surface. A scale of 1:1,000,000 means that 1 unit (for example, inch, centimeter, foot, finger length) on the map represents 1 million of the same unit on the ground. The 1 on the left side of the ratio always refers to a unit of distance on the map, and the number on the right always refers to the same unit of distance on Earth’s surface. 1:24,000 is an example of a ratio representation scale

• Written. A written scale describes the relationship between map and Earth distances in words. For example, in the statement “1 centimeter equals 10 kilometers,” the first number refers to map distance and the second to distance on Earth’s surface.

• Graphic. A graphic scale usually consists of a bar line marked to show distance on Earth’s surface. To use a bar line, first determine with a ruler the distance on the map in inches or centimeters. Then hold the ruler against the bar line and read the number on the bar line opposite the map distance on the ruler. The number on the bar line is the equivalent distance on Earth’s surface.

The four images show (from top to bottom) an area centered on Houston, Texas, at progressively larger scales: (top) southeast Texas, (second) the city of Houston, (third) central Houston, and (bottom) Minute Maid Park. Each map includes ratio and graphic scales.

Maps often display scale in more than one of these three ways.

The appropriate scale for a map depends on the information being portrayed. A map of a downtown area, such as Figure 1-16 (bottom), may have a scale of 1:10,000, whereas a map of southeast Texas, such as Figure 1-16 (top), may have a scale of 1:10,000,000. One inch represents about 800 feet on the Minute Maid Park map and about 158 miles on the southeast Texas map.

At the scale of a small portion of Earth’s surface, a map can provide detailed information about an area such as a city’s downtown. At the scale of the entire globe, the amount of detail must be greatly reduced, but the map can still effectively communicate processes and trends that affect everyone.

Projection

Earth is very nearly a sphere and is therefore accurately represented with a globe. However, a globe is an extremely limited tool with which to communicate information about Earth’s surface. A small globe does not have enough space to display detailed information, whereas a large globe is too bulky and cumbersome to use. And a globe is difficult to write on, photocopy, display on a computer screen, or carry in the glove box of a car. Consequently, most maps—including those in this book—are flat. Three-dimensional maps can be made but are expensive and difficult to reproduce.

Earth’s spherical shape poses a challenge for cartographers because drawing Earth on a flat piece of paper unavoidably produces some distortion. Cartographers have invented hundreds of clever methods of producing flat maps, but none has produced perfect results. The scientific method of transferring locations on Earth’s surface to a flat map is called projection.

The problem of distortion is especially severe for maps depicting the entire world. Four types of distortion can result:

1. The shape of an area can be distorted, so that it appears more elongated or squat than in reality.

2. The distance between two points may become increased or decreased.

3. The relative size of different areas may be altered, so that one area may appear larger than another on a map but is in reality smaller.

4. The direction from one place to another can be distorted.

Most of the world maps in this book use the Winkel equal area projection Other common world projections are Mercator, Goode Homolosine, and Gall-Peters.

Winkel equal area projection

The relative sizes of the landmasses on the map are the same as in reality. The projection minimizes distortion in the shapes of most landmasses. Areas toward the North and South poles, such as Greenland and Australia, become more distorted, but they are sparsely inhabited, so distorting their shapes usually is not important. However, by allocating space to the oceans, the land areas are much smaller than on interrupted maps of the same size.

Mercator

Shape is distorted very little, direction is consistent, and the map is rectangular. However, relative size is grossly distorted toward the poles, making high-latitude places such as Antarctica look much larger than they actually are.

Goode Homolosine Projection

Shape is distorted very little, direction is consistent, and the map is rectangular. However, relative size is grossly distorted toward the poles, making high-latitude places such as Antarctica look much larger than they actually are.

Mercator Projection

The Mercator Projection Dramatically distorts the size of areas the farther north or south of the equator. This distortion has skewed many people's view of the world.

It is difficult to flatten out a globe

Here is the true size of the Earth's countries ranked by size.