The process of creating a flat map from the three-dimensional, spherical surface of the Earth is called map projection. This transformation is a mathematical necessity because a curved surface cannot be flattened without stretching or compression. All map projections inherently introduce distortion. Cartographers must choose which geographical properties they wish to preserve accurately, understanding that any projection will compromise others.
Why Counting Projections Is Impossible
The question of how many map projections exist does not have a fixed numerical answer, because the number is theoretically infinite. A map projection is essentially a mathematical equation that transforms spherical coordinates (latitude and longitude) into planar coordinates (x and y). Since the parameters of these equations—such as the central longitude or standard parallels—can be varied endlessly, a unique projection can be created any time a parameter is changed.
While the theoretical number is limitless, the number of projections formally documented and put into practical use is in the hundreds. These named projections are often grouped into major families based on their construction and the properties they maintain. The utility of a projection depends entirely on its intended purpose, which makes classification methods more informative than a simple count.
Categorization by Geometric Surface
One common way to classify projections is by the geometric surface onto which the Earth is conceptually projected. These surfaces—the cylinder, the cone, and the plane—are considered “developable” because they can be unrolled or flattened without being stretched or torn. Though most modern projections are created purely through mathematical formulas, this conceptual framework helps visualize the resulting distortion patterns.
Cylindrical Projections
Cylindrical projections conceptually wrap a cylinder around the globe, typically tangent to the equator. When the cylinder is unrolled, the lines of latitude and longitude form a straight, rectangular grid. This projection type is often used for world maps or regions near the equator, where distortion is minimal closest to the line of tangency. The well-known Mercator projection is a prime example, though many mathematical variations exist.
Conic Projections
Conic projections are generated by placing a cone over the globe, with the cone’s apex usually positioned over one of the poles. This surface touches or intersects the globe along one or two lines of latitude, known as standard parallels. When the cone is flattened, the lines of latitude appear as concentric circular arcs, and the lines of longitude radiate outward as straight lines. Conic projections are especially well-suited for mapping mid-latitude regions, such as the continental United States, because they minimize distortion across wide, east-west expanses.
Planar (Azimuthal) Projections
Planar, or azimuthal, projections involve projecting the globe onto a flat plane that touches the Earth at a single point. This point of contact is usually a pole, the equator, or another point of interest, and it is the only place on the map free of distortion. On these maps, the lines of longitude radiate outward as straight lines from the central point, and the lines of latitude appear as complete circles. They are often used for showing polar regions or for maps requiring accurate representation of direction from a single central location.
Categorization by Property Preserved
The second major method of classification focuses on which of the four primary types of distortion—shape, area, distance, or direction—the projection is designed to minimize. Since it is geometrically impossible to preserve all four properties simultaneously on a flat map, the choice of projection involves a necessary trade-off. Cartographers select a projection that preserves the property most relevant to the map’s intended purpose.
Conformal Projections
Conformal, or orthomorphic, projections are designed to preserve the local shape and angles of small areas. On a conformal map, the scale is the same in every direction around any single point, meaning that the intersection of latitude and longitude lines always forms a 90-degree angle. This characteristic is particularly important for navigation charts and topographic maps, where maintaining local angles and bearing is necessary for accurate course plotting.
Equal-Area Projections
Equal-area, or equivalent, projections preserve the correct proportional size of all geographical areas on the map. If a continent is one-tenth the size of another on the globe, it will be one-tenth the size on an equal-area map. To achieve this preservation of area, these projections must distort the shapes of landmasses, especially toward the edges of the map. They are frequently used for thematic maps that show the spatial distribution of data, such as population density or land use.
Equidistant Projections
Equidistant projections are designed to preserve true scale along specific lines or directions. While no map can maintain accurate distance across the entire surface, equidistant projections ensure that distances measured from a central point to any other point on the map are correct. Some equidistant projections may preserve scale along all lines of longitude, which is useful for specialized maps focusing on distances from a single origin, such as airline route maps.