A map projection is a mathematical method used to translate the curved, three-dimensional surface of the Earth onto a flat, two-dimensional plane. Since the Earth is a sphere, this transfer is impossible without introducing some form of error or distortion. The idea of a single “best” map projection is a misconception, as every projection must sacrifice the accuracy of one geographic property to preserve another. Cartography is fundamentally a science of trade-offs, where the choice of projection depends entirely on the map’s intended purpose.
The Four Fundamental Properties of Map Accuracy
Mapmakers have identified four fundamental properties that a projection can attempt to preserve, though only one can be perfectly maintained across a large area. The first is the conformal property, which preserves the local shape and angles of small areas. A conformal map maintains the correct angular relationships, meaning that lines of latitude and longitude intersect at right angles. This preservation of shape comes at the expense of area, which becomes increasingly stretched or compressed away from the projection’s center.
The second property is equal-area, which ensures that the relative sizes of all landmasses are accurately maintained. On an equal-area projection, if a country is twice the size of another in reality, it will be twice the size on the map. This property is useful for geographical comparisons, such as analyzing population density or resource distribution. Achieving correct area requires significant distortion of shapes, often making continents appear stretched or compressed near the edges.
The third property, equidistant, preserves accurate distances, but only in specific directions or from one or two designated points. For instance, a projection might correctly show the distance from the map’s center point to every other location. Distances measured between any two other random points, however, would likely be incorrect. This makes equidistant maps unsuitable for general distance measurement but useful for applications centered on a single location.
The final property is azimuthal, which preserves the true direction or bearing from a single central point to all other points. An azimuthal projection is useful for plotting airline or shipping routes that radiate from a hub, as a straight line drawn from the center represents the correct compass bearing. Azimuthal projections are specialized, and they distort area and shape significantly away from the central point.
The Three Geometric Classes of Projection
Map projections are categorized by the geometric surface used to conceptualize the transfer from the globe to the flat map. These three developable surfaces—the cylinder, the cone, and the plane—can be flattened without stretching. Although modern cartography uses complex mathematical formulas, these geometric models provide the framework for classifying projections.
Cylindrical Projections
Cylindrical projections are based on wrapping a cylinder around the globe, typically tangent to the equator. When the cylinder is unrolled, the meridians (lines of longitude) appear as straight, equally spaced vertical lines. The parallels (lines of latitude) are parallel, horizontal lines. This class is best suited for mapping the entire world, but it causes significant distortion of size toward the poles.
Conic Projections
Conic projections are derived by placing a cone over the globe, often positioned to touch the globe along one or two parallels of latitude. When the cone is flattened, the meridians radiate outward from the apex, while the parallels form concentric circular arcs. This geometry makes conic projections effective for mapping areas with a dominant east-west orientation, such as mid-latitude regions or North America.
Planar Projections
The third class is the planar, also known as azimuthal, which projects the Earth onto a flat plane tangent to a single point. If the plane touches a pole, the parallels are concentric circles, and the meridians radiate outward as straight lines. These projections minimize distortion immediately around the point of tangency. They are often used for maps that focus on polar regions or for showing great-circle routes from a central city.
Selecting the Right Projection for the Task
The selection of a specific projection is a practical decision that links the map’s purpose to the property it prioritizes. The Mercator projection is a conformal cylindrical map developed in 1569 that keeps angles and shapes accurate on a local level. Its defining feature is that any line of constant compass bearing (rhumb line) is represented as a straight line, making it the standard projection for marine navigation charts. However, this projection severely exaggerates the size of landmasses farthest from the equator. On a Mercator map, Greenland appears comparable in size to Africa, which is fourteen times larger in reality.
In contrast, the Gall-Peters projection is a cylindrical, equal-area map designed to correct the area distortion seen in the Mercator map. This projection accurately depicts the relative sizes of all countries and continents. It is useful for thematic mapping of global statistics like resource distribution or population density. Its drawback is that it sacrifices shape accuracy, distorting the familiar outlines of continents near the equator and poles.
For general reference, cartographers frequently turn to compromise projections. These projections do not perfectly preserve any single property but attempt to minimize the total amount of distortion across the map. The Winkel Tripel projection, adopted by the National Geographic Society in 1998, is a well-regarded compromise. The name “Tripel” refers to its goal of minimizing distortion in area, direction, and distance, resulting in a visually balanced world map ideal for educational use and atlases.