A map projection is a method for translating the curved surface of the Earth onto a flat plane. Because the Earth is roughly spherical, there is no way to flatten it without stretching, compressing, or tearing something. Every flat map you have ever seen uses a projection, and every one of those maps distorts reality in some way. The type of projection determines what gets distorted and what stays accurate.
How Projections Work
At its core, a map projection is a set of mathematical rules that converts every point on a three-dimensional globe into a corresponding point on a two-dimensional surface. The simplest way to picture this: imagine placing a light inside a transparent globe and projecting the shadows of continents onto a sheet of paper. You choose a point called the center of the projection and an image plane, typically a flat surface touching the globe at one point. The image of any location on the globe is found by tracing a line from the center through that location to where it hits the plane.
In practice, most projections don’t literally use light and shadow. They use equations that control exactly how latitude and longitude coordinates map to x and y positions on paper or a screen. The Mercator projection, for instance, spaces latitude lines farther apart as you move away from the equator, using a formula where each small step in latitude gets multiplied by a factor that grows larger toward the poles. Other projections use entirely different math to prioritize different properties.
The Four Things Every Map Distorts
On the actual Earth, you can accurately measure size, shape, distance, and direction all at once. On a flat map, you can preserve some of these, but never all of them simultaneously. This is the fundamental trade-off of cartography, and it is why hundreds of different projections exist. Each one chooses what to sacrifice.
- Shape: Conformal projections keep the shapes of small areas accurate. Coastlines and country borders look “right,” but the sizes of landmasses get badly warped. The Mercator projection is the most famous example: Greenland appears roughly the same size as Africa, even though Africa is about 14 times larger.
- Area: Equal-area projections display the true relative sizes of all regions. A country that is twice as large as another on Earth will be twice as large on the map. The trade-off is that shapes and angles get distorted, so landmasses can look stretched or squished.
- Distance: Equidistant projections preserve true distances, but only from one or two specific points or along certain lines. No flat map can show correct distances between every pair of locations.
- Direction: Some projections preserve the angles between locations, which is critical for navigation. Others let direction drift, especially far from the map’s center.
Visualizing Distortion With Tissot’s Indicatrix
There is a classic visual tool for spotting exactly how a projection warps the Earth. Developed by French cartographer Nicolas Auguste Tissot in the nineteenth century, it works like this: imagine drawing identical circles at regular intervals across a globe, each with the same radius. On a perfect globe, every circle has the same size and shape. When you project those circles onto a flat map, they reveal the distortion.
On a conformal projection like the Mercator, every circle remains a circle (shape is preserved at each point), but the circles grow dramatically larger near the poles, showing how area gets inflated. On an equal-area projection, the circles get squashed into ellipses of varying shapes, but each ellipse has the same area, confirming that size relationships are maintained. On projections that are neither conformal nor equal-area, the circles become ellipses that also vary in size, meaning both shape and area are distorted.
Three Main Projection Families
Projections are grouped by the geometric surface used to “receive” the image of the globe. Each family has natural strengths for certain parts of the world.
Cylindrical Projections
Imagine wrapping a cylinder of paper around the globe, touching it along the equator. Cylindrical projections work well for regions near the equator, where the paper contacts the globe most closely and distortion is minimal. The Mercator projection is the best-known cylindrical type. It became indispensable for ocean navigation because a straight line drawn on a Mercator map represents a path of constant compass heading, called a rhumb line. A sailor could draw a line between two ports, read off the bearing, and hold that heading for the entire voyage. The shortest actual path between two points on Earth (a great circle route) appears as a curve on a Mercator map, but the rhumb line’s simplicity made it far more practical in the age of sail.
Conic Projections
Picture placing a cone over the globe so it intersects along one or two lines of latitude. Conic projections are particularly useful for mid-latitude regions that stretch primarily east to west, like the contiguous United States or Europe. Distortion stays low along the latitudes where the cone touches the globe and increases as you move north or south from those lines. Most regional and national atlases of mid-latitude countries use some form of conic projection.
Azimuthal (Planar) Projections
Here, a flat plane touches the globe at a single point. Directions from that central point to every other location on the map are true, which is why these projections are standard for mapping polar regions and for applications involving radio signals or missile paths radiating from a single point. The stereographic azimuthal projection preserves shapes (it is conformal) and is the basis for the Universal Polar Stereographic system used in military maps of the Arctic and Antarctic. The equidistant azimuthal version preserves distance from the center point but is neither conformal nor equal-area.
Compromise Projections for World Maps
When the goal is a general-purpose world map that simply “looks right” rather than preserving one property perfectly, cartographers turn to compromise projections. These don’t guarantee accuracy in shape, area, distance, or direction, but they minimize the overall visual distortion so that no single property is badly mangled.
The Robinson projection, adopted by the National Geographic Society in 1988, was one of the first widely used compromises. It gently curves the meridians and tapers the poles, producing a map that feels balanced even though it is technically imprecise in every measurable category. National Geographic later switched to the Winkel Tripel projection in 1998, which averages together two other projections to further reduce the combined distortion of area and shape. Today the Winkel Tripel is one of the most common projections you will see in atlases, textbooks, and wall maps.
The Role of Datums and Ellipsoids
Before you can project the Earth, you need a mathematical model of its shape. The Earth is not a perfect sphere; it bulges slightly at the equator and is flattened at the poles. Cartographers use an ellipsoid, a slightly squashed sphere defined by precise measurements, as their starting model. The most widely used ellipsoid today is part of the World Geodetic System 1984 (WGS 84), developed by the U.S. Department of Defense and designated by the International Hydrographic Organization as the universal standard. It is the datum your phone’s GPS uses to determine your position.
WGS 84 differs only very slightly from the GRS 80 ellipsoid used in many national survey systems. For most practical purposes, the two are interchangeable. But the distinction matters in professional surveying and high-precision mapping, where even a fraction of a meter counts. Every projection is built on top of a datum like this: first you define the shape of the Earth, then you flatten it.
Why the Projection Matters in Daily Life
Most people encounter projections without thinking about them. The tiled web maps in Google Maps and Apple Maps use a variant of the Mercator projection called Web Mercator. It was chosen because its rectangular grid makes it easy to slice into image tiles at different zoom levels, and because it preserves local shapes well enough for street-level navigation. But at the global level, it inherits Mercator’s size distortions, which is why zooming out to view the whole world makes high-latitude countries look enormous.
Aviation provides another everyday example. Airlines plan long-haul routes along great circle paths because they are the shortest distance between two points on the globe. On a Mercator map, these routes look like strange arcs curving toward the poles. On a polar azimuthal map, the same routes appear as nearly straight lines. Neither map is wrong; they simply make different trade-offs, and the one you choose depends on what you need the map to do.
Choosing a projection always comes down to purpose. A navigator needs true compass bearings. A demographer comparing country populations needs accurate area. A pilot needs shortest-distance routes. No single flat map can serve all these goals at once, which is why cartographers have invented so many ways to unwrap the globe.