A cylindrical projection is a way of transferring the features of a spherical body onto a flat surface by, conceptually, wrapping a cylinder around the sphere and projecting its surface outward. The result is a rectangular map where lines of latitude and longitude form a grid, making it one of the most intuitive and widely used families of map projections in history. But the simplicity comes with trade-offs: every cylindrical projection distorts something, whether that is the size of landmasses, the distances between them, or the shapes of continents near the poles. Which distortion you accept depends entirely on what you need the map to do.
How the Geometry Works
Imagine placing a transparent globe inside a hollow cylinder so the cylinder touches the globe along the equator. If you put a light at the center of the globe, the shadows of the continents and oceans would fall onto the cylinder’s inner wall. Unroll the cylinder and you have a flat map. In practice, cartographers do not literally shine lights through globes. They use mathematical formulas to control exactly how coordinates on the sphere translate to coordinates on the flat sheet. But the cylinder metaphor captures the essence: lines of longitude become evenly spaced vertical lines, and lines of latitude become horizontal lines whose spacing varies depending on the specific projection.
That spacing of the latitude lines is where different cylindrical projections diverge from one another. Spread them farther apart toward the poles and you preserve local shapes but inflate the size of high-latitude regions. Compress them and you can preserve area at the cost of stretching shapes. Keep them evenly spaced and you get a simple grid that is easy to work with digitally but does not preserve much of anything faithfully. These three strategies roughly correspond to the three most common cylindrical projections: the Mercator, the equal-area cylindrical family, and the equirectangular.
The Mercator Projection and Why It Persists
Gerardus Mercator published his projection in 1569 to solve a very specific problem for sailors. On a Mercator map, a straight line between two points gives you a constant compass bearing, known as a rhumb line. If you are navigating the open ocean and want to hold a single heading, you can draw a line on a Mercator chart and follow that bearing from port to port. This made Mercator charts indispensable to maritime navigation for centuries.
The cost of this navigational convenience is dramatic size distortion at high latitudes. Greenland appears roughly the same size as Africa on a Mercator map, even though Africa is about fourteen times larger in reality. Antarctica stretches across the bottom of the map as a massive white band, looking bigger than all other continents combined. Near the equator, shapes and sizes are fairly accurate, but the farther you move toward the poles, the more inflated everything becomes.
Despite being designed for navigation, Mercator’s projection ended up on classroom walls and in atlases for centuries, giving generations of students a warped sense of the relative sizes of countries. Research on how people perceive land area suggests this has real cognitive consequences. In one study, participants who used a Mercator map as a reference tried to mentally compensate for the distortion, but only for certain regions, and only when they already knew Mercator was distorting. When shown other projections, they did not even attempt compensation, even when told the map was distorted. Most people, the researchers found, have limited knowledge of how projections work and struggle to apply whatever knowledge they do have to practical tasks.
Meanwhile, on a Mercator chart, the straight line between two distant cities is not actually the shortest path across the globe. The shortest route is a great circle, which appears as a curve on a Mercator map. Students exploring this on digital maps have confirmed that the shortest distance between two points on a Mercator projection is often a curved line rather than the straight one the map seems to suggest.
The Peters Controversy and Equal-Area Alternatives
In 1973, German historian Arno Peters held a press conference in Bonn to promote what he called a new world map. Peters argued that the Mercator projection distorts the relative sizes of landmasses, inflating countries in higher latitudes at the expense of equatorial nations. He claimed this “overvalues the white man and distorts the picture of the world to the advantage of the colonial masters of the time,” and that his own projection would be completely accurate in terms of relative size.
Peters was right about the Mercator’s area distortion and about its political implications. His framing struck a chord with development organizations, and several UN agencies adopted the Peters projection for their publications. But cartographers were less impressed. The projection Peters presented was essentially the same as one published by James Gall in 1855, now called the Gall-Peters projection. And while it does preserve area, it severely distorts shape: Africa and South America look vertically stretched, as though someone pulled them like taffy. Cartographers pointed out that equal-area does not mean undistorted, and that many other equal-area projections had existed for centuries without the misleading marketing.
The episode illustrates a fundamental tension in cylindrical projections. You cannot flatten a sphere without losing something. An equal-area cylindrical projection keeps the sizes of landmasses in correct proportion to one another, but it warps their shapes. A conformal projection like the Mercator preserves local shapes and angles, but it inflates sizes. No single cylindrical projection does both, and picking one means deciding which kind of distortion your audience can live with.
The Equirectangular Projection and Digital Grids
The equirectangular projection, also called Plate Carrée, is arguably the simplest of all map projections. Latitude and longitude map directly to x and y coordinates with equal spacing. The result looks like a plain grid: no fancy mathematics, no conformal properties, no equal-area properties. It distorts shapes and sizes near the poles, though less dramatically than the Mercator does.
What makes this projection valuable is its computational simplicity. When you need to store planetary data in a raster grid, equirectangular makes indexing trivial, because pixel coordinates correspond directly to latitude-longitude pairs. This is why it shows up across remote sensing, climate modeling, and geographic information systems. A study optimizing global grids for high-resolution remote sensing data recommended the Plate Carrée for global-scale grids, favoring it over equal-area projections that earlier studies had suggested. The reasoning was practical: equirectangular grids are easier to tile, resample, and distribute across different software systems.
The projection is also the default for much of planetary cartography. Maps of Mars, the Moon, and other bodies in the solar system are commonly produced in equirectangular form. In digital, interactive planetary applications, the equirectangular projection serves as a “database” projection: data is stored in equirectangular coordinates and then reprojected on the fly for display. On small-scale global planetary maps, features like impact craters become visibly flattened at higher latitudes, but the computational convenience outweighs the visual distortion for most scientific purposes.
Web Mercator and the Online Mapping Boom
When Google Maps launched in 2005, it used a variant of the Mercator projection that cartographers now call Web Mercator. The choice was driven by engineering convenience, not cartographic philosophy. Mercator’s projection produces a square map at the global scale (if you cut off the extreme polar regions), and square tiles are simple to generate, cache, and serve to millions of users zooming and panning at different scales. The mathematical properties that made Mercator useful for navigation (north is always up, local shapes are preserved) also made it feel natural for street-level and city-level browsing.
Web Mercator quickly became the dominant projection in online mapping, used by virtually every major mapping platform. This is ironic: after decades of cartographers campaigning to retire Mercator from general-purpose mapping, the internet brought it roaring back. At the street or city scale, Web Mercator works fine, because the area distortion only becomes obvious at continental or global scales. But when users zoom out to see the whole world, they see the same inflated Greenland and shrunken Africa that cartographers have been criticizing since the 1940s.
Researchers have noted that Web Mercator’s dominance raises concerns about data display, design, and how people understand spatial patterns. When thematic data like population density, disease rates, or climate variables are displayed on a Web Mercator base map at a global scale, the visual impression can be misleading: high-latitude regions get more visual weight simply because they take up more screen space, not because their values are higher.
Oblique and Transverse Variants
A standard cylindrical projection touches the globe along the equator, but there is no rule saying the cylinder has to be oriented that way. Tilt the cylinder so it touches along a meridian instead and you get a transverse cylindrical projection. The most important example is the Transverse Mercator, which forms the basis of the Universal Transverse Mercator (UTM) coordinate system used by militaries, surveyors, and engineers worldwide. In UTM, the world is divided into sixty narrow north-south strips, each mapped with its own Transverse Mercator projection. Because each strip is only six degrees of longitude wide, the distortion stays small within the strip, giving you a local map that is very close to accurate in both shape and area.
Tilt the cylinder at an arbitrary angle and you get an oblique cylindrical projection, which is useful when the area you care about runs diagonally across the globe rather than along a meridian or the equator. Switzerland, for instance, has historically used an oblique Mercator projection because the country’s long axis runs roughly northeast-southwest.
For satellite remote sensing, the concept was pushed further with the Space Oblique Mercator projection, developed specifically for processing and displaying imagery from satellites like Landsat. A satellite’s ground track curves across the Earth’s rotating surface in a path that does not follow any simple line of latitude or longitude. The Space Oblique Mercator projects that ground track onto the map plane with no length distortion and no normal-view curvature distortion along the track itself. Away from the track, small distortions creep in, but they are negligible for the narrow swath of ground that the satellite’s sensors actually image.
Cylindrical Projections in Medicine
One of the more unexpected applications of cylindrical projection mathematics is in medical imaging. CT colonography, sometimes called virtual colonoscopy, uses CT scans to image the inside of the colon and look for polyps. The standard approach involves scrolling through axial slices or navigating a 3D virtual fly-through of the colon. Both methods can miss polyps hidden behind folds or in hard-to-see crevices.
Researchers addressed this by borrowing directly from cartography. The colon is roughly tubular, so its inner surface can be mathematically “unrolled” using the same cylindrical projection techniques that cartographers use to map the Earth. A Mercator projection of the colon’s inner wall produces a flat, rectangular image where every part of the surface is visible at once, eliminating the blind spots that plague virtual fly-through methods. In one study, Mercator projection of the colon detected polyps seven millimeters or larger with about 88% sensitivity, compared with roughly 63% for viewing axial slices and 68% for conventional virtual colonoscopy. The difference was statistically significant. A stereographic projection variant performed similarly well at around 83% sensitivity.
The principle is straightforward: if you have a roughly cylindrical surface and want to see all of it at a glance, you do exactly what Mercator did with the Earth. The distortions that make cartographers cringe at the global scale are irrelevant when the “globe” is a short segment of intestine and you just need to spot a bump on the wall.
How Cylindrical Projections Shape What People Believe About Geography
The cognitive research on map projections is thin but suggestive. The study mentioned earlier, on how projection knowledge affects perceived land area, found something striking: even when people know a map is distorted, they mostly fail to mentally correct for it. And the one projection people do try to compensate for is the Mercator, probably because it is the one projection whose distortions have been publicly discussed for decades. For every other projection, including equal-area ones that distort shape instead of size, viewers tended to take the map at face value.
This creates a paradox. The more a projection’s flaws are publicized, the more viewers try to compensate, but the compensation is uneven and imprecise, sometimes overcorrecting for some regions and ignoring others. And the less-discussed projections, including ones with their own serious distortions, get a free pass because nobody has told viewers to be suspicious of them. The takeaway is not that one projection is safer than another, but that any flat map creates impressions that viewers absorb uncritically unless they have been specifically warned.
This matters more now than it did a generation ago. When maps lived mostly in classrooms and printed atlases, cartographers controlled which projection was used and could choose one appropriate for the purpose. In the age of Web Mercator, billions of people interact daily with a single projection chosen not for its cartographic merits but because it tiles neatly. The old concern that Mercator gives people a distorted mental model of the world is, if anything, more relevant than it was during the Peters controversy of the 1970s and 1980s.
Mapping Other Worlds
Cylindrical projections are not limited to Earth. Planetary scientists use them to map every solid body in the solar system, from Mars and the Moon to the moons of Jupiter and Saturn. The equirectangular projection dominates planetary cartography for the same reason it dominates terrestrial remote sensing: it is computationally simple, easy to tile, and serves as a universal storage format that can be reprojected as needed.
On a planetary equirectangular map, round impact craters near the poles get visibly squashed into wide ellipses, a distortion that is cosmetically annoying but scientifically manageable because researchers know how to correct for it. For thematic maps of entire planets, both Mercator and equirectangular projections are standard choices. When mapping at regional scales, planetary cartographers reach for the same toolbox terrestrial cartographers use: transverse Mercator for polar regions, conic projections for mid-latitudes, azimuthal projections centered on the poles.
The interesting wrinkle in planetary cartography is that many of these bodies are not perfect spheres. Mars is noticeably oblate, and some small moons and asteroids are so irregular that wrapping a cylinder around them is a stretch of the imagination. For these bodies, the concept of a cylindrical projection still applies mathematically, treating the body as a reference ellipsoid or triaxial ellipsoid, but the gap between the mathematical abstraction and the physical reality is wider than it is for Earth. The further a body departs from spherical, the more any projection’s assumptions strain, and the more creative the cartographer needs to be.