Before answering that question, we need to take a step back and look at the fundamental challenge of cartography (mapmaking), and that is the impossibility of representing a globe — a three-dimensional object — accurately in two dimensions by making a map on a sheet of paper or a computer screen.
You can easily understand the problem by trying to cover an orange with a sheet of paper without wrinkling or folding the paper. Clearly, it cannot be done. So what cartographers have done for centuries is invent ways to project the image of a curved surface onto a flat surface, and the results of their efforts are called map projections.
One of the real treasures in my personal library is a book published in 1989 by the United States Geological Survey entitled “An Album of Map Projections: U.S. Geological Survey Professional Paper 1453.” Its authors are John P. Snyder of the Geological Survey and Philip M. Voxland of the University of Minnesota. Amazingly, after 30 years this marvelous publication is still available, in hard copy or as a pdf.
For each projection, an illustration of the earth using that particular projection, and then the name of the projection’s author, date of publication, and various technical and practical aspects of the projection are provided. The first projection in the book is the Mercator projection.
Gerardus Mercator (1512 — 1594) was a Flemish polymath; fluent in at least six languages; a maker of maps, globes and scientific instruments; and author on many topics. He is most famous today for his large map of the world, published in 1569. Many generations of schoolchildren have grown up seeing a large, colorful, canvas wall map of the world in their classrooms, always using the Mercator projection.
You will hear many criticisms of the Mercator map, but its genius is that straight lines drawn on the map can be used for navigation, because the angles are accurate. You can draw a straight line from Miami to Anchorage on a Mercator world map, and if you follow the direction of that line from one of those cities in an airplane (after adjusting for wind), you’ll arrive at the other one. And indeed the Mercator projection is still used for navigation charts.
But, as USGS publication notes, the Mercator projection is “[o]ften and inappropriately used as a world map in atlases and wall charts. It represents a misleading view of the world because of the excessive distortion of area.” And that distortion increases with latitude; at the highest latitudes (near the poles) the distortion is so great that it cannot be shown on the map.
What do you find at high latitudes, especially in the Northern Hemisphere on Mercator plots? You find people who ruled the world or large parts of it for centuries: Scandinavians, Germans, Dutch, Spanish, French, British, Russians and Japanese. In a Mercator world map, these countries appear much larger than they really are, as do some others. Tropical countries are distorted only slightly, so for people using Mercator maps, they seem smaller, and inevitably, less important than they really are.
In addition, global warming has been, and will continue to be, greater at high than at middle and low latitudes. But plotting climate change on a Mercator map makes things seem even worse than they really are, because high latitude areas seem larger than they really are. So northern countries seem more important than they really are and the threat they face from global warming seems even worse than it really is — and it’s quite serious enough without distorting it. To summarize all the above, a Mercator projection, while brilliant for navigation, is a bad projection for a thematic map.
Why does this matter, and what’s the solution to this problem? It matters because scientists’ most valuable currency is honesty. Our job in society, as I and many others see it, is to provide accurate, unbiased and honest descriptions and understanding of the world as we know it. If we don’t, or if the perception is that we don’t, then our value to society is diminished. And that diminishes society’s ability to make evidence-based decisions. By the same token, it matters to the public as people try to make informed decisions for themselves. If you know that a Mercator projection makes a bad thematic map, you can either try to compensate for the distortion on your own, or better, demand a better projection.
And that brings me to the solution.
“U.S. Geological Survey Professional Paper 1453″ is filled with all kinds of projections. To a map-lover like me, it’s endlessly fascinating, but for people just looking for reliable information, it presents many projections that distort area far less than Mercator’s, or even not at all. No flat map can be free of all distortion, but, for example, the Eckert V and VI projections still look familiar, like the map at the beginning of the article — some of the projections are almost fantastical — and yet distort area far less than Mercator’s.
The point here is not that you should memorize the limitation of every projection, but for you to learn to look at thematic maps (e.g., of climate, population density, disease incidence and so on) critically. Do Norway and Sweden look bigger than Egypt or Nigeria? Does Iceland look bigger than Florida? If so, then take the map with a grain of salt, or get a better map. We can’t all be experts in all fields, but we all can evaluate information critically and ask critical questions.
David Policansky is a retired scientist who worked in the Division on Earth and Life Studies at the National Academy of Sciences.