Mercator Projection in AP Human Geography

Why sailors could plot a Mercator course with a ruler

Mercator built the map for navigation. On it, the lines of longitude run straight up and down instead of meeting at the poles, and the lines of latitude get spaced farther apart the closer they are to the poles. That spacing is calculated so angles stay true.

The payoff is that any straight line drawn on a Mercator map is a line of constant compass bearing (called a rhumb line). A navigator could draw a line from port to port, read the angle, and hold that heading the whole way. That makes absolute direction easy to read straight off the map.

The projection is called conformal for a related but separate reason. It preserves local angles, so the shapes of small areas look correct.

One catch: that straight line usually isn't the shortest route. The shortest path between two points on a globe is a great circle. On a Mercator map, great-circle routes generally show up as curves. The exceptions are routes along the equator or along a meridian, which appear as straight lines. So Mercator gets direction right but gets distance wrong.

Greenland vs. Africa: reading the area distortion

The stretching that keeps angles true also inflates area, and the inflation grows with latitude. Places near the equator look about right. Places near the poles balloon, and the poles themselves can't be shown at all because they'd be stretched infinitely.

The famous comparisons are worth knowing. Greenland looks about the size of Africa, but Africa is roughly 14 times larger. Greenland also looks bigger than Australia, even though Australia is about 3.5 times larger. Canada, Russia, and Antarctica all look oversized for the same reason.

This is the core of Topic 1.1 (Introduction to Maps and Types of Maps) in Unit 1. The course says all maps are selective and that projections inevitably distort spatial relationships in shape, area, distance, and direction. Mercator is the clearest case because you can see its trade-off just by looking at it. It picks direction and local shape, and it gives up area and distance.

Mercator, Peters, and the argument that maps carry power

The Peters projection (also called Gall-Peters) is the usual comparison. It's an equal-area projection, so Africa and South America appear at their correct relative sizes. The cost is shape, since continents near the equator look stretched tall and those near the poles look squashed. Neither map is correct. Each one keeps a different property.

Critics argued that Mercator makes higher-latitude regions, including much of Europe and North America, look larger relative to equatorial regions, many of which were colonized by European powers. Which region sits in the center of a map is a separate choice that depends on how the map is laid out, and the projection doesn't decide it. That links the projection to Unit 4, Topic 4.2 (Political Processes), where colonialism and imperialism shape the present-day political map. The takeaway for the course is that choosing a projection is a choice about what a map emphasizes.

Mercator also connects to Topic 1.2 (Geographic Data). Many online mapping tools use a version called Web Mercator, so the distortion shows up in the online mapping and visualization the course lists among geospatial technologies.

Answering Mercator trade-off questions

If Mercator appears in a multiple-choice question, you might encounter a map or a description of the projection. Possible practice questions include:

• Which characteristic of all projections the Greenland distortion shows (distortion is unavoidable).
• What Mercator prioritizes over accurate area (direction and shape).
• Why a navigation company or pilot would still use it (a straight line is a constant compass bearing, which makes absolute direction easy to follow).
• What an equal-area map like Peters gives up in return (shape).

In the FRQ section, a projection could come up in Question 2 (One-stimulus question) or Question 3 (Two-stimuli question) whenever a world map is the stimulus. A strong answer names the specific property the map keeps and the one it distorts, ties the choice to the map's purpose, and explains how that distortion could change a reader's sense of a region.

A useful distinction is that Mercator preserves local angles and shapes but does not preserve area or the overall appearance of large regions. That's why a big landmass near the poles can look warped even though the projection is conformal. The property it distorts most dramatically is area. Also keep in mind that a straight line on Mercator is the constant-bearing route, which usually isn't the shortest path. Great-circle routes generally appear curved on the map, except along the equator or a meridian.