Why Maps Need Projection
Representing the Earth, a three-dimensional sphere, on a flat, two-dimensional map presents a fundamental challenge. Any attempt to translate the Earth’s curved surface to a flat map will introduce some form of alteration. This transformation requires a systematic method to convert spherical coordinates into planar ones. It is impossible to perfectly preserve all properties, such as area, shape, distance, and direction, simultaneously on a flat surface.
The Process of Map Projection
Map projection involves a systematic transfer of geographic coordinates from the Earth’s curved surface onto a flat surface. Conceptually, one can imagine a transparent globe casting shadows onto a surrounding flat material like a cylinder, cone, or plane, which is then unrolled or viewed directly to form the map.
While this analogy helps visualize the process, actual map projections rely on complex mathematical formulas. These calculations translate latitude and longitude coordinates from the spherical Earth into x,y coordinates on a two-dimensional plane. Different mathematical algorithms are employed depending on which properties the mapmaker wishes to preserve or distort.
Major Projection Types
Map projections are broadly categorized based on the “developable surface” used in their conceptual formation. These surfaces include cylinders, cones, and flat planes, each offering distinct ways to transfer the Earth’s features.
Cylindrical projections are conceptually formed by wrapping a cylinder around the globe, usually tangent at the equator. When unrolled, this cylinder produces a rectangular map where meridians appear as equally spaced vertical lines and parallels as horizontal lines. The Mercator projection is a common example, often used for navigation due to its preservation of angles.
Conic projections are created by placing a cone over the globe, typically tangent at a specific line of latitude or intersecting two lines of latitude. When the cone is unrolled, parallels appear as concentric circular arcs, and meridians as straight lines radiating from the cone’s apex. These projections are useful for mapping mid-latitude regions, as they tend to preserve shapes and areas well within a limited latitudinal band.
Planar, also known as azimuthal, projections are formed by placing a flat plane tangent to a single point on the globe, such as a pole or a specific continent. From the central point of tangency, lines of constant direction radiate outwards, and parallels appear as concentric circles. These projections are frequently used for mapping polar regions or for showing great circle routes, as they accurately depict directions from the central point.
Understanding Map Distortion
Every map projection inherently introduces some form of distortion because it is impossible to perfectly represent a three-dimensional sphere on a two-dimensional plane. This means a map cannot simultaneously preserve all properties of the Earth’s surface, such as shape, area, distance, and direction. Mapmakers must choose which properties to prioritize, understanding that others will be altered as a result.
Shape distortion, also known as angular distortion, occurs when angles between lines on the map do not correspond to true angles on the Earth’s surface. This results in landmasses appearing stretched or squashed. Area distortion means that the relative sizes of landmasses on the map are inaccurate, making some continents appear larger or smaller than they are in reality.
Distance distortion signifies that scaled distances between points on the map do not accurately reflect true distances on the globe. A map might show a consistent scale along certain lines, but distances away from those lines could be significantly stretched or compressed. Direction distortion, or bearing distortion, implies that true compass bearings between points on the Earth are not accurately represented. For example, a straight line on a map might not represent the shortest path between two points on the globe.