The relationship between globes and map projections centers on representing the Earth’s three-dimensional curved surface on a flat, two-dimensional plane. A globe serves as the initial, scaled model of the Earth, maintaining the true proportions and geometric relationships of the spherical body. A map projection is the systematic, mathematical method used to translate geographic coordinates from that sphere onto a flat surface, such as a map or digital screen. This transformation is necessary for practical applications, making the globe the source material and the projection the derived representation.
The Globe as Earth’s True Model
The globe is the most accurate model of the Earth because it is the only representation that simultaneously preserves all four spatial properties: area, shape, distance, and direction. Since the Earth is nearly spherical, a miniature sphere reproduces its features without geometric deformation. The scale remains constant everywhere on the globe, ensuring all regions are shown in their correct relative size and form.
Despite this geometric perfection, a globe has significant limitations that restrict its utility. It is difficult to transport, impractical for showing fine detail due to its small scale, and cannot display the entire world at once. Furthermore, accurately measuring distances and angles on a curved surface is cumbersome. These constraints necessitate the use of flat maps derived from the globe.
Transforming Three Dimensions to Two
The process connecting the globe to a map involves a mathematical transformation known as projection, which systematically translates geographic coordinates (latitude and longitude) from the sphere to a plane. The Earth’s curved surface is an unrollable, or undevelopable, surface, meaning it cannot be flattened without tearing or stretching its material. This geometric constraint is often compared to the impossibility of flattening an orange peel without distortion.
Because of this constraint, cartographers use precise mathematical algorithms to convert spherical coordinates into planar coordinates. This is not a physical peeling process but a systematic translation of every point on the globe’s grid system onto a flat grid. The mathematical function determines where each point on the three-dimensional surface will land on the two-dimensional map, forming the basis of the map’s appearance. This systematic translation allows for the creation of maps that are easily stored, displayed, and measured.
The Inevitable Trade-offs of Mapping
The consequence of transferring data from a curved surface to a flat one is the introduction of distortion. Four primary characteristics are subject to this deformation: shape, area, distance, and direction. Every map projection must compromise one or more of these properties, as no single flat map can perfectly preserve all four simultaneously.
A projection designed to maintain the correct area of landmasses is called an equal-area or equivalent projection. This preservation of area, however, comes at the cost of distorting the shapes of geographic features. Conversely, a map that preserves the correct shape and angles of small areas is known as a conformal projection. Preserving shape means that the area of regions will be incorrect, often appearing exaggerated toward the poles.
Other projections may prioritize maintaining accurate distances from a single point or line (equidistant) or preserving true compass bearings (azimuthal). The choice of which property to preserve depends entirely on the map’s purpose. For instance, maps used for measuring land resources must be equal-area, while navigation maps must be conformal to maintain correct angles.
Categorizing Projection Methods
Map projections are classified based on the geometric surface used to conceptually receive the globe’s coordinates before being flattened. These surfaces—the cylinder, the cone, and the plane—are considered developable because they can be unrolled without further distortion. The three main categories are cylindrical, conic, and planar (also known as azimuthal) projections.
In a cylindrical projection, the globe is imagined inside a cylinder, and coordinates are transferred to this surface. The Mercator projection is a famous example, resulting in a rectangular map highly useful for sea navigation because it maintains true compass bearings. Conic projections are created by placing a cone over the globe, which is used for mapping mid-latitude regions.
Planar, or azimuthal, projections involve projecting the globe onto a flat disk, typically touching the globe at a single point, such as a pole. These projections are valuable for showing true direction from the central point. Many complex modern maps, like the Robinson or Winkel Tripel projections, are mathematical compromise projections that attempt to minimize overall distortion across all four properties.