A planetary body's surface is inherently three-dimensional — and not just because of its peaks and valleys but because it must wrap around the whole world.
That's obvious but easy to forget, since Earth is large enough that we experience it as flat in our everyday lives. But it's a major challenge for cartographers looking to create two-dimensional world maps, like the familiar Mercator projection and the design recently endorsed by an overwhelming majority of the United Nations' General Assembly, called the "Equal Earth" projection. And the same difficulties apply beyond Earth, albeit with much lower stakes, at least for now: any map is a mix of accuracy and distortion.
"It's important to know that sometimes, as with statistics or anything else, you can tell a story with a map," Corey Fortezzo, a geologist and map coordinator for the Planetary Geologic Mapping Program operated jointly by the U.S. Geological Survey (USGS) and NASA, told Space.com. "And it's not necessarily the true story."
That's because flattening a three-dimensional surface into a two-dimensional map is an inherently messy endeavor. "Every projection chooses what to preserve—in some, it's the area, some preserve shape, some preserve distance, some preserve the direction — but not all of them at once," Jue Wang, a cartographic data scientist at Washington University in St. Louis, told Space.com. "It's not [about] which projection is correct, it's [about] what are you trying to do with the map."
In the U.S., people tend to be most familiar with the Mercator projection, which dates to 1569, when a cartographer working in what is now Belgium designed a map on which any straight represents a constant compass direction. "It's built for the purpose of navigation, so the Mercator projection preserves angles," Wang said. "Which is very useful and why sailors have used the Mercator projection for centuries."
But of course, most of the 8.2 billion people currently living on Earth aren't sailors, and today world maps are less about navigation and more about understanding the world we live in. That means many fewer people are getting lost at sea than are operating under the mistaken impression that Greenland is larger than South America — hence the movement backing the Equal Earth projection to take Mercator's place in common settings like classrooms.
That projection prioritizes preserving the relative area of countries and continents, but sacrifices the accuracy of angles of the Mercator projection. As a result, most north-south longitude lines become curved in the Equal Earth projection, while east-west latitude lines are spaced closer together near the poles, rather than farther away.
Beyond Earth
Mapping worlds beyond Earth is only a smidge younger than the Mercator projection: the oldest known map of the moon dates to 1609. In some ways, the activities are inherently different, since mapping Earth has relied on people's observations in place, whereas the foundation of extraterrestrial mapping is remote data gathered first with telescopes, then with spacecraft.
Often, that data has come in the form of images — themselves two-dimensional, warped representations of a three-dimensional surface. "You're stitching together this quilt of imagery and the problem is all the pieces are curved and you have to figure out where they fit together," Fortezzo said.
Another quirk of the process is that because neither Earth nor any other solar system object is perfectly spherical, projection systems can't seamlessly translate between worlds. And each world needs its own unique definition for 0 degrees longitude, an extraterrestrial translation of Greenwich's supremacy as backed by the British empire.
Mapmakers are still chipping away at these challenges, although the International Astronomical Union has operated a working group dedicated to cartographic coordinates since 1979. Right now, this work may induce yawns, but as long-term exploration efforts of the moon and other bodies become both more serious and more varied, the complexity of mapping these worlds may increase.
Consider the needs of NASA's Apollo program — where landing sites were clustered toward the equator on the moon's near side —compared to the current Artemis program, which intends to focus initiatives at the lunar south pole. While something like the Mercator or Equal Earth projections would both make sense for a map related to Apollo, neither excels at depicting the poles, as their treatment of Antarctica attests.
That, of course, is the starkest difference when map-making goes to space: no other world's surface comes with the complications of Earth's patchwork of continents and oceans, which can influence projection techniques. (Take the so-called "orange-peel" or Goode homolosine projection, a bizarre lobed depiction that keeps continents intact and accurately represents their relative sizes, but breaks up the oceans and polar regions.)
In contrast, except for volcanic Io and Titan's water-ice surface sculpted by organic liquids, an extraterrestrial mapper's material is straightforward. "We are able to just take a piece and that piece is going to have nothing but rock and ice," Fortezzo said.
"It makes it much easier," he said. "There's no feelings involved."
