Notes

A globe is a scale model of the Earth that depicts properties such as area, distance, and direction. Globes accurately display all these properties because they are round just like the Earth. A map is a flat representation of all or part of the planet. But, unlike globes, maps cannot show all the properties accurately.

Mapmakers, called cartographers, use mathematical formulas to transfer information from the three-dimensional globe to the two-dimensional map. However, when the curves of a globe become straight or only slightly curved lines on a map, distortion occurs in shape, distance, area, or direction. A straight line of true direction on a map is not always the shortest distance between two points on Earth. The measured distance between any two points on a flat map will not have the same distance when measured on a round globe. To find the actual shortest distance between any two places, stretch a piece of string around a globe from one point to the other. The string will form an arc that is part of a great circle, an imaginary line that follows the Earth’s curvature. Great circle routes therefore mark the shortest distance that an object can travel between two points. They are useful because they indicate actual distances between two locations. Determining a great circle route is important for travel and transportation. Ship captains and airplane pilots use great circle routes to reduce travel time and conserve fuel.

While globes are useful for portraying the entire Earth, their ability to display detailed features of a particular region are limited. Maps, however, are useful for showing more in-depth information. Cartographers convert the three-dimensional globe image onto a flat map by creating a map projection. But because map projections can distort one or more of the properties of size, shape, distance, area, or direction, the cartographer must choose the projection to use based on the purpose of the map. It is important to know which properties are distorted, and how much they are distorted, so you can use and interpret the map accurately.

There are many kinds of map projections, some with general names and some named after the cartographer who developed them. Three major categories of map projections are planar, cylindrical, and conic. A planar projection, also known as an azimuthal projection, shows the Earth centered in such a way that a straight line coming from the center to any other point represents the shortest distance. Because a planar projection is most accurately represented from its center, it is often used for maps of the Poles.

A cylindrical projection is based on how a map would look if the globe was projected onto a cylinder. This type of projection is most accurate at the Equator because shapes and distances are increasingly distorted when moving away from the Equator and toward the Poles. A Mercator projection is a common example of a cylindrical projection. Because it displays true direction, a Mercator projection is useful for sea navigation.

A conic projection is the Earth’s surface projected onto a map formed into a cone. Shape is relatively accurate on such projections, and straight lines drawn on them approximate great circle routes if distances are not great.

World maps used for general reference use the Winkel Tripel projection. This map projection cannot be used to determine precise distances, sizes, or shapes of specific global features. It does, however, provide a good balance between the overall size and shape of land areas shown.

A Robinson projection looks similar to a Winkel Tripel projection, although its east-west projections run in a straight line. The Robinson projection produces minor distortions, particularly in the polar areas that appear flattened on the map. The sizes and shapes near the eastern and western edges of the map are accurate, and outlines of the continents appear much as they do on the three-dimensional globe. Goode’s Interrupted Equal-Area projection resembles a globe that has been cut apart and laid flat. The process of creating this interrupted projection can be compared to slicing an orange peel in order to lay it flat on a page. Although this projection shows the true size and shape of Earth’s landmasses, distances between land features are generally distorted.