As a way of representing the curve of the Earth’s surface on a flat two dimensional and easily portable map, map projections were developed. Over time, hundreds of different types of projections were developed, and each of them has its benefits and drawbacks. Each distorts the real world, the earth, in at least one way: area, distance, direction or shape. The scale of the map and its purpose determine the appropriate projection. For example, a projection may be excellent for a large-scale map of a given country, but that same projection would engender too much distortion when used for a map of an entire country.

Since there is no such thing as a perfect projection, cartographers choose different projections for their several qualities. They categorize projections by their distortion characteristics. Each projection maintains some of the Earth’s properties while distorting others. For example, the Mercator projection, because of its straight lines, is chosen for navigation because of the ease of determining compass courses.

Classification of projections based on their distortion characteristics

Equivalent projection. The projection that maintains the relative size of features is called an equivalent projection, and are used for maps that show distributions. This projection shows accurate directions going out from the map’s center point. This is a good projection for land masses such as North America which extends equally out from the center.

Mercator projection. Known as a conformal projection, this maintains accurate angular relationships and are used for meteorological charts or navigational purposes. The U.S. Geological Survey has chosen this projection for most of the topographic maps.

An equidistant projection, used for seismic and radio mapping, maintains proportional distances from the map’s center. Interestingly, this is the projection chosen for the United Nations emblem.

Sometimes cartographers choose the combine the properties of several projections to reach a compromise which suffers distortion of area, distance, shape and direction, but with tolerable limits. These compromise projections are often chosen for world maps.