I've discovered that the world isn't what it appears to be. When National Geographic, for instance, publishes a map, they use Mercator projection, which distorts the physical size of many countries. Although it looks like Greenland and Africa are the same size on a Mercator map, Africa is actually nine times as large! This is due to the fact that they have placed the equator two-thirds of the way down the page, elongating the northern hemisphere and shrinking the southern. Is this a snide western plot to diminish the physical impact of continents like Africa and South America?
Gina B., Montreal, Canada
Don’t organize a hunger strike just yet, love. For starters, National Geographic hasn’t used Mercator maps since 1922. Until 1988 it used mostly Van der Grinten projection, and today uses Robinson projection, both of which are substantially different from Mercator. Second, distortion in these maps isn’t caused by placing the equator two-thirds of the way down the page; on the contrary, in all three projections, the equator is a straight line in the exact center. Distortion arises from the way all maps deal with the essential mapmaking problem, namely, trying to make a flat representation of a spherical surface.
On a globe, the lines of longitude converge as they approach the poles; on a Mercator map, to use the extreme example, the lines of longitude are parallel. Making such a map means taking each equator-to-pole globe segment, which is roughly triangular, and stretching it into a rectangle. Obviously the stuff near the poles gets stretched the most, greatly exaggerating its size. For that reason Mercator projection isn’t seen much anymore in general purpose maps of the world.
Still, Mercator maps are useful in navigation. If you draw a line between you and your destination on a Mercator map and then calculate the angle relative to north, you’ll get the compass bearing needed to get you where you’re going (though it won’t necessarily be the shortest route). Other methods of map projection such as the Van der Grinten and Robinson reduce the distortion of land area but aren’t anywhere near as handy for charting a course.
A word from the Bureau of Map Deviance
You recently discussed navigation with compass and Mercator map. With this information in hand I plan my pilgrimage to the land of Cecil. Since I live in Washington, D.C., I figure Chicago is approximately 292° off north on my map and about 695 miles away. I follow my compass heading for that distance (ignoring the fact that roads are not straight lines) and find myself near Normal, Illinois. As you are used to hearing, this is one Teeming Millionth that doesn’t want to be anywhere near Normal. (You take the jokes from there.) Why? Because on a Mercator map north is true north. My compass shows magnetic north. From my starting point there is a variance of about 9°. You must differentiate between true and magnetic bearings and compensate, oh formerly perfect one, or you will be one lost bastion of wisdom. Having thus nailed you to the wall, I await your verbal assault.
— David Alexander, Washington, D.C.
If you don’t mind, David, I’ll save my ammo for a more worthwhile target. The difference between true north and magnetic north is called declination. It varies from place to place and is routinely corrected for when plotting a course.
Send questions to Cecil via firstname.lastname@example.org.