A map projection is a systematic method for transforming the curved, three-dimensional surface of the Earth onto a flat, two-dimensional plane. This process makes global geographic information accessible and usable in a flat format. Coordinates from the globe are converted to coordinates on a planar surface during this transformation.
Understanding Map Distortions
Representing the Earth’s spherical surface on a flat map inherently introduces distortions. It is impossible to flatten a three-dimensional object like a globe without some form of stretching or compressing its features. Cartographers must choose which properties to preserve and which to distort, as no single projection can maintain all of them simultaneously.
The four primary properties that can undergo distortion are area, shape, distance, and direction. Some projections accurately depict relative sizes (equal-area), while others preserve shapes (conformal). However, a projection that maintains true area will inevitably distort shape, and vice versa. Different map projections exist because various applications prioritize different properties, leading to trade-offs in accuracy.
Cylindrical Projections
Cylindrical projections are conceptualized by wrapping a cylinder around the globe, then projecting the Earth’s surface onto it before unrolling the cylinder into a flat map. Parallels of latitude appear as straight, horizontal lines, and meridians of longitude are straight, vertical lines, intersecting at right angles, forming a rectangular map.
A well-known example is the Mercator projection, which is conformal. This property makes it useful for navigation, as lines of constant compass bearing (rhumb lines) appear as straight lines. However, the Mercator projection significantly distorts area, particularly towards the poles, making landmasses in higher latitudes appear much larger than their actual size. The poles themselves cannot be shown on this projection.
Conical Projections
Conical projections are imagined by placing a cone over the globe, typically tangent to a single line of latitude or secant through two lines of latitude. Features from the globe are projected onto this cone, which is then unrolled into a flat surface. On these maps, parallels appear as concentric arcs, and meridians are straight lines that converge at the cone’s apex.
These projections are often suited for mapping mid-latitude regions, such as the United States or Australia. The Albers Equal-Area Conic projection accurately represents the relative sizes of areas, making it suitable for thematic maps. Another common type is the Lambert Conformal Conic, which preserves local shapes and angles and is widely used for aeronautical charts and regional mapping systems.
Azimuthal Projections
Azimuthal, or planar, projections involve projecting the globe onto a flat plane that touches the Earth at a single point. This point is often a pole, but it can be any central location. From this central point, directions are preserved.
On azimuthal maps, parallels appear as concentric circles around the central point, while meridians radiate outwards as straight lines. Examples include the Gnomonic, Stereographic, and Orthographic projections. The Gnomonic projection shows all great circles as straight lines, useful for plotting the shortest routes for ships and aircraft. Orthographic projections often depict the Earth as it would be seen from space, providing a realistic view of a hemisphere.
Compromise Projections
Compromise projections offer a balance between various types of distortion. They aim to minimize overall distortion across the entire map, resulting in a more visually balanced and globally representative map. These projections do not fully maintain area, shape, distance, or direction.
These projections typically feature curved meridians and parallels that are not simple straight lines or arcs. Common examples include the Robinson projection and the Winkel Tripel projection. The Robinson projection provides a reasonable depiction of shape, distance, direction, and size, making it a popular choice for general-purpose world maps and atlases. The Winkel Tripel projection, adopted by the National Geographic Society for its world maps, is known for having low mean scale and area distortion.