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How do we know where we're going?

Travel depends on the ancient skill of navigation--the ability to find a way from one place to another and back.

Columbus was not sure how far he had to go. In his journal he recorded his latitude observations and estimates of distance traveled underreporting this distance to the crew, "lest the trip be long."

Polar explorers depended on navigational data for survival, as well as success. Sir Ernest Henry Shackleton, who attempted to reach the geographic South Pole several times between 1902 and 1922, was once marooned on a moving ice shelf and his ship was crushed; Shackleton's survival depended on the latitude and longitude observations that described the motion of the ice.

Travelers on land need somewhat different information from those at sea. Travelers on solid ground can follow circuitous routes between important landmarks using schematic maps (for example, the A.D. 250 Peutinger map). Seagoing voyagers need more from their maps, as the ocean moves beneath them and the wind and waves push the ship across a featureless surface (for example, the A.D. 1502 Cantino Planisphere, a portolan chart). Sailors on the open sea have kept track of absolute position using the only reference points they have: the Sun and stars.

The globe is the best way to show the relative positions of places, but a globe that can fit in a ship's cabin cannot show the detail needed for navigation. Flat maps distort the placement of features, but can show great detail and are portable.

The transformation of map information from a sphere to a flat sheet can be accomplished in many ways, called projections. Mapmakers have invented projections that show distances, directions, shapes, or areas as they are on a globe, at least partially. Different projections have different advantages and disadvantages.

Orthographic projections, for example, show shapes as they appear when the globe is viewed from space. Equal-area projections do not distort the size of areas but do distort their shapes. Conformal projections are those on which the scale is the same in any direction at any point on the map.

Many projections retain one geometric quality, and a few retain more than one quality, but no single projection can accurately portray area, shape, scale, and direction. (A map projections poster available from the USGS Earth Science Information Center illustrates the features of the most common map projections.)

The Mercator projection was designed by Flemish cartographer Gerardus Mercator in 1569 to show compass directions as straight lines in all directions from all points on the map. This was an important breakthrough in mapping. Using a Mercator projection, navigators could draw a straight line to a destination, sail in that direction, and expect to reach it, allowing for the effects of ocean currents and other factors.

Mercator made longitude lines parallel and increased the distance between latitude lines away from the equator. As a result, extreme northern and southern areas appear enlarged. For example, Greenland looks larger than South America, although South America is eight times as large in reality. This distortion at high latitudes (north and south) also makes the distances appear larger than they are. Even with these disadvantages, this projection remains one of the most commonly used.