A map is a two-dimensional representation of a geographical area. This act of translating the spherical shape of the Earth onto a plane is a process known as map projection. Map distortion is the unavoidable consequence of this geometric transformation. These distortions affect how we perceive geographical relationships, such as the size of continents or the distance between two points.
The Inevitable Tradeoff of Flattening the Earth
Distortion is a mathematical certainty because a sphere is not a developable surface. This means the surface cannot be flattened into a plane without being stretched, torn, or compressed.
The Earth’s curved geometry dictates that the mathematical relationships between points on its surface cannot be perfectly maintained when those points are moved to a flat sheet. For example, on a sphere, meridians of longitude converge at the poles, but on a rectangular map, they must be drawn as parallel lines. This necessary alteration stretches the area and distances near the poles dramatically.
This fundamental geometric constraint means that cartography is a science of compromise. Every map projection requires a systematic, mathematical transformation to convert three-dimensional coordinates (latitude and longitude) into two-dimensional coordinates (x and y). This process is essentially a deliberate choice about which geographical properties to preserve and which to sacrifice through distortion.
The Four Key Properties Affected by Distortion
The process of projecting the globe onto a flat map affects four spatial properties: area, shape, distance, and direction. No single projection can preserve all four attributes. Cartographers must choose which properties are maintained, knowing the others will be systematically distorted.
Area
Area refers to the relative proportions of landmasses on the map compared to their actual size on the Earth. Projections with area distortion, such as the common Mercator projection, famously exaggerate the size of landmasses near the poles. For instance, the Mercator makes Greenland appear larger than Africa, when Africa is approximately fourteen times larger. Projections that preserve true relative size are called equal-area or equivalent projections.
Shape
Shape distortion occurs when the outlines of geographic features are altered from their true form. This often happens in equal-area maps where continents appear stretched or compressed to maintain their correct size proportion. Projections that preserve local angles and the shape of small areas are known as conformal projections. However, preserving shape locally requires significant distortion of area globally.
Distance
Distance is distorted on most maps, meaning the measured length between two points on the map does not accurately reflect the true distance on the ground. Only equidistant projections preserve true distances, and they can only do so accurately from one or two specific central points to all other points. Distances between any other pair of points on an equidistant map will likely be incorrect.
Direction
Direction, or azimuth, relates to the true compass bearing between two locations. While some projections, such as the Mercator, preserve direction accurately along straight lines, others can significantly warp the true bearing between points. A map designed to preserve direction from a central point is known as an azimuthal projection, which is particularly useful for plotting flight paths or radio communications.
How Projections Prioritize Different Features
Since distortion is inescapable, cartographers select a specific map projection based on the map’s intended purpose. The choice of projection is a deliberate tool used to manage the four types of distortion. This prioritization ensures the most relevant spatial property for the map’s use remains accurate.
For instance, navigation charts historically used for seafaring rely on conformal projections like the Mercator. This projection is chosen because it preserves local angles and directions, allowing navigators to plot a constant compass course as a straight line. This accuracy comes at the expense of severely distorting the size of polar regions.
Conversely, thematic maps that display statistical data, such as population density, must use equal-area projections. These maps prioritize the preservation of area so that a feature’s size accurately represents its proportion on Earth. The correct comparison of geographic scale is more important than the visual accuracy of a continent’s shape.
Other projections are designed as compromises, minimizing overall distortion across all four properties without perfectly preserving any one of them. The Robinson projection is a well-known example of this type, created to provide a visually balanced and aesthetically pleasing view of the entire world for general reference or educational purposes.