Lab #5: Map Projections

Six map projections and their planar distances between Washington D.C and Kabul, Afghanistan.

My favorite example so far of the huge variation in map types and functions is the one Professor Sheng gave in lecture about BART. What are the important features on a BART map? Is it the distance between stations? The coordinates of each station? The shape of each station? It isn't any of those really. As a Bay Area native, I can say for certain that nobody on the Peninsula or the East Bay cares minutes and degrees when they're going into the city (save the degree of fog)--not even distance.

What is important is the relative location--the order. Which stop do I get off at? When the lovely Bay Area Transit designers were making the BART map, they probably didn't worry too much about whether the scale was spot on. There are plenty of instances, though, in which precision is much more important. Say you want to know the distance between New York and LA--or hey, Kabul and Washington D.C? How far apart are they? An equidistant map projection, like the two shown above, might give you a good number. Some other map projections distort the distance between locations in favor of preserving other qualities, like area (See Bonne and Eckhert IV) or shape and angles (See Hotine Oblique Mercator and Stereographic).

This has positives and negatives. On the one hand, we have a lot of map projections at our fingertips, increasing the likelihood that there is one out there that measures what we want it to measure. If you look at the Equidistant maps above, you'll see that they have two very different functions.The Azimuthal Equidistant map preserves distances from the poles outwards, while the Equidistant Cyllindrical projection preserves distances across meridians and equators. That is why the Azimuthal produces a distance of over 8,000 miles, while the Cyllindrical produces one closer to 5,000. It turns out that in the case of Kabul and Washington D.C., neither of these is perfect, although the Cyllindrical map comes closer, since Kabul and D.C are at similar latitudes.

The downside is that map projections can be misleading and manipulative. Distortion isn't always apolitical. During the Cold War, for example, the United States used Mercator projections because they distorted the size of the Soviet Union, making it monstrously larger than it really was. What the Mercator does is distort areas based on their distance from the equator. So Greenland, for example, is larger than Africa, even though Africa is at least 10 times as huge. Maps shape the way we think about the world. They can shape the way we feel. Africa is a good example. A lot of people don't realize that the United States can essentially fit inside the Sahara Desert alone. Maybe if we had fewer cheeseburgers and better map projections growing up, we wouldn't be such big-headed Americans.