Understanding Map Projections: Distortions and Uses

Map projections are essential tools for representing the Earth’s surface on flat media, but they come with inherent distortions. This article explores the main types of map projections, their characteristics, and their best applications, providing insights into how geography is visualised.

Introduction to Map Projections

Map projections are mathematical transformations used to represent the curved surface of the Earth on a flat plane. Because the Earth is a near-spherical object, no flat map can perfectly represent it — every projection introduces some form of distortion. Understanding these distortions is crucial for interpreting maps correctly and choosing the right projection for a given task.

The Challenge of Representing the Earth

The Earth is a three-dimensional object, and flattening it onto a two-dimensional surface leads to distortions in shape, area, distance, and direction. This challenge is often illustrated by the analogy of peeling an orange and trying to lay the peel flat without tearing it. Each map projection offers a different solution to this problem, prioritising certain properties over others to produce a usable representation.

Types of Map Projections: Overview

Map projections can generally be grouped into three families based on the developable surface used: cylindrical, conical, and planar (azimuthal). Each family has its own projection geometry and characteristics that make it suited to different applications.

Cylindrical Projections

Cylindrical projections are created by projecting the Earth’s features onto a cylinder wrapped around the globe. The resulting map is then unrolled into a flat surface. Different cylindrical projections preserve different properties: the Mercator projection preserves angles (conformal), while cylindrical equal-area projections preserve area. Cylindrical projections tend to distort size and shape increasingly toward the poles.

Conical Projections

Conical projections are created by projecting the Earth’s surface onto a cone placed over the globe. This family of projections is particularly well-suited to mid-latitude regions that extend primarily east to west. The Lambert Conformal Conic projection is a widely used example, commonly employed in aviation charts because it minimises scale distortion over large east-west extents and preserves angles locally.

Planar Projections

Planar projections, also called azimuthal projections, project the Earth onto a flat plane touching the globe at a single point. They are most commonly used for mapping polar regions. Depending on the variant, they can preserve distance from the centre point (azimuthal equidistant), area (Lambert azimuthal equal-area), or display great circles as straight lines (gnomonic).

Conformal Projections: Preserving Angles and Local Shape

Conformal projections (also called orthomorphic projections) preserve local angles, which means small shapes are represented correctly. However, this comes at the cost of area distortion — regions far from the standard line or point appear significantly enlarged. The Mercator projection is the most well-known conformal projection, and its angle-preserving property makes it invaluable for maritime and aeronautical navigation.

The Mercator Projection: A Closer Look

Developed by Gerardus Mercator in 1569, the Mercator projection was designed for navigators. It represents lines of constant bearing (rhumb lines) as straight lines, making it straightforward to plot compass courses. However, the projection significantly enlarges areas at high latitudes — Greenland, for example, appears comparable in size to Africa on a Mercator map, even though Africa is approximately 14 times larger in reality.

Despite its area distortions, the Mercator projection remains a standard for many mapping applications, particularly for local and regional navigation. The Universal Transverse Mercator (UTM) system is a widely used variant that applies the transverse Mercator projection in narrow north-south strips (60 zones of 6° longitude each), providing highly accurate local coordinates across the globe.

Other Conformal Projections

• Lambert Conformal Conic Projection: Used extensively for aviation charts and regional topographic mapping, particularly for areas spanning large east-west extents at mid-latitudes.
• Adams Hemisphere in a Square: A conformal projection devised by Oscar S. Adams that maps an entire hemisphere into a square. It is primarily of theoretical and artistic interest.

Equal-Area Projections: Preserving Size

Equal-area projections (also called equivalent projections) accurately represent the relative sizes of geographic features, ensuring that any region on the map covers the same proportion of area as on the globe. This comes at the cost of shape distortion. Equal-area projections are essential for thematic maps where comparing the size of regions is important — for example, population density or land-use maps.

Examples of Equal-Area Projections

• Collignon Projection: An equal-area projection with distinctive triangular shapes toward the poles, used in astrophysics applications such as displaying cosmic microwave background maps.
• Sinusoidal Projection: An equal-area projection that gives a more familiar map appearance while preserving area. It distorts shapes at the edges but is simple to construct.
• Gall-Peters Projection: An equal-area cylindrical projection that preserves relative areas accurately. It elongates features near the poles and compresses them near the equator, resulting in noticeable shape distortion, particularly for equatorial regions which appear stretched vertically.

Equidistant Projections: Preserving Distance

Equidistant projections preserve accurate distances along specific lines or from specific points, but cannot preserve distances universally across the entire map. The exact distances preserved depend on the projection variant: azimuthal equidistant projections preserve distances from the centre point to all other points, while equidistant conic projections preserve distances along the standard parallels.

Examples of Equidistant Projections

• Equidistant Conic Projection: Well-suited to mapping mid-latitude regions with a predominantly east-west extent, preserving distances along two standard parallels.
• Equirectangular Projection: Maps meridians and parallels as equally spaced straight lines, producing a simple grid. It is widely used as a default format for raster geographic data and in web mapping pipelines, though it distorts both area and shape, especially toward the poles.

Compromise Projections: Aesthetic Balance

Compromise projections do not perfectly preserve any single property — neither area, shape, distance, nor direction — but instead minimise overall distortion to produce a visually balanced representation of the world. They are commonly used for general-reference world maps and atlas maps.

Examples of Compromise Projections

• Robinson Projection: A well-known compromise projection with curved meridians, designed to present a visually pleasing world map. It was used by the National Geographic Society from 1988 until 1998.
• Winkel Tripel Projection: A popular alternative to the Robinson projection, adopted by the National Geographic Society in 1998 for its world maps. It minimises the overall distortion of area, shape, and distance.
• Buckminster Fuller Dymaxion Map: A unique projection that unfolds the globe onto the faces of an icosahedron, preserving shapes and relative sizes with minimal distortion while presenting the continents as a nearly contiguous landmass.

Composite and Gnomonic Projections

Composite projections combine two or more different projections to produce a more functional map. The best-known example is Goode’s Homolosine projection, which stitches together sinusoidal and Mollweide projections to create an interrupted equal-area map of the world.

Benefits of Composite Projections

• Reduced distortion in shape, size, and distance across the mapped region.
• The interrupted format allows each lobe to be mapped more accurately, though it breaks the visual continuity of the oceans or continents depending on orientation.

Gnomonic Projections

Gnomonic projections are azimuthal projections in which the point of projection is the centre of the Earth. Their defining property is that every great circle — the shortest path between two points on a sphere — appears as a straight line on the map. This makes the gnomonic projection highly useful in navigation and aviation for planning great-circle routes, even though the projection has significant distortion in area and shape away from the centre point.

Conclusion: Choosing the Right Projection

Understanding the different types of map projections is essential for effective map usage. Each projection serves a specific purpose: conformal projections preserve local angles and are ideal for navigation; equal-area projections preserve relative sizes and are preferred for thematic mapping; equidistant projections preserve distances along defined lines or from a defined point; and compromise projections balance multiple distortions for general-purpose world maps.

Recognising these differences allows map users to interpret geographic information accurately and choose the projection best suited to their specific needs and the area being represented.