A map projection is the mathematical process of transforming the Earth’s three-dimensional, curved surface onto a flat, two-dimensional plane. This transformation is necessary for creating any physical map that is not a globe. The question of the “most accurate” map projection is complex because no single flat map can perfectly represent the Earth in every respect. Every projection involves a systematic trade-off, requiring cartographers to choose which geographic property to preserve and which to sacrifice. Therefore, accuracy is not absolute but is measured by a map’s fitness for a particular purpose.
The Geometry of Projection Distortion
The fundamental reason distortion is inevitable lies in the geometric impossibility of flattening a sphere without altering its properties. The Earth’s surface is considered a curved, non-developable surface, meaning it cannot be unrolled into a plane.
When the coordinates from the globe are transferred to a flat map, the relationship between distances, angles, and areas must change. Cartographers use mathematical equations to control exactly how and where this inherent distortion occurs. The resulting stress on geographic features is a known and calculable consequence of the 3D-to-2D transformation. Different projections are systematic solutions to managing this unavoidable geometric conflict.
The Four Properties Maps Attempt to Preserve
Since total accuracy is unattainable on a flat map, cartographers choose to preserve one or more specific properties. The choice of which property to maintain determines the projection’s characteristics and its best use. These preserved properties are categorized into four primary types.
- #### Equal-Area (Equivalence)
Equal-area projections maintain the correct proportional size of all landmasses relative to one another across the entire map. Projections that preserve area must necessarily distort the shapes, especially toward the edges of the map. This property is useful for thematic maps that analyze resource distribution or population density. - #### Conformal (Shape Preservation)
Conformal projections preserve the local shape of features and the angular relationships between lines of latitude and longitude. They ensure that small shapes on the map look the same as they do on the globe, meaning the scale at any point is the same in all directions. This maintenance of local angles is helpful for navigation. A projection cannot be both equal-area and conformal; these two properties are mutually exclusive. - #### Equidistant (Distance Preservation)
Equidistant projections accurately represent true distances, but only from one or two specific points or along certain lines. Distances between any two random points on the map are generally distorted. This property is particularly valuable for mapping applications centered on a single location, such as air travel routes from a hub airport. - #### Azimuthal (Direction Preservation)
Azimuthal projections correctly show the true compass direction, or azimuth, from one central point to every other point on the map. These maps are often used for navigation or strategic planning, as they represent the shortest path, known as a great circle route, as a straight line passing through the center.
Comparing Major Map Projections
Different well-known map projections illustrate the practical reality of these trade-offs by prioritizing one property over others. The Mercator projection, developed in 1569, is a conformal cylindrical projection that became the standard for nautical charts. Its strength is that it shows lines of constant compass bearing as straight lines. However, it severely exaggerates the size of landmasses as they approach the poles, making Greenland appear visually comparable in size to South America.
The Gall-Peters projection is an example of an equal-area projection promoted for its size accuracy. While it accurately shows the relative area of continents, it achieves this by significantly distorting their shapes, making equatorial landmasses appear elongated vertically. This projection highlights the principle that preserving area requires sacrificing the accurate representation of local shape.
The Robinson projection, created in 1963, is a compromise projection that attempts to achieve a visually pleasing balance of distortion across all four properties. It is neither strictly equal-area nor strictly conformal, but it reduces the extreme distortions seen in maps like the Mercator. The Robinson map is widely used in educational materials and atlases because it offers a more balanced view of the world.
Determining Accuracy Based on Purpose
Since all flat maps contain distortion, the most accurate map projection is the one best suited to the user’s specific task. Accuracy is defined by the intended purpose, not by a single universal standard. The map that preserves the most relevant property for the job is the correct choice.
For example, if the goal is to compare the size of different countries for a study on global resource consumption, an equal-area map is required. For a pilot or navigator planning a route, a conformal projection that preserves angles and directions is the most accurate tool. If the objective is to show true distance from a single point, such as a radio transmission range, an azimuthal equidistant projection would be the most accurate. Evaluating any map requires understanding which property it preserves and whether that property aligns with the information being sought.