A cartogram is a map, but a special type of map which attempts to convey a general sense of geography in addition to one or more other important statistical elements. Most maps attempt to portray an accurate image of the Earth’s surface, superimposed with other information such as political borders or place names. A cartogram, by contrast, is primarily concerned with accurately portraying a unique data set, while secondarily doing the best it can to stay true to geographic accuracy.
The Problem With Most Common Maps
Let us take a real-world example. One of the cartograms in this book attempts to show the number of AIDS (Acquired Immuno-Deficiency Syndrome) cases worldwide. Take a look at a chart that appears on the website of the Joint United Nations Programme on HIV/AIDS to illustrate the number of worldwide AIDS cases in 2001:
The second map uses that exact same raw data, but this time displays it so that the size of each country is directly proportional to the number of cases it has, while actual geography is secondary but still close enough to the real world to be recognizable.
The second map allows the reader to more immediately gain an accurate sense of the relative numbers of AIDS cases in each country, rather than the general sense given by the first. The first map color codes the area of each country, which has no relevance at all to the data. What matters instead is the number of cases. In the second cartogram, all the eye sees in red are actual cases. It is harder to identify each country because they are not in their most familiar shapes, but their correct relative positioning and labeling compensates for the stylized borders while the key data–the number of cases–is directly comparable across countries.
The map allows easy and direct visual comparisons among countries, as well as highlighting the staggering primacy of Africa in the AIDS epidemic. In fact, South Africa alone had more cases than Western Europe and the United States combined. The devastating impact of the disease on Africa is simply much more readily apparent in the second graphic than in the first. India, to use another example, had the second largest number of cases in the world but this fact is relatively hidden in the first map by the way the data is shown: India’s land area is small and its rate of infection is low, so the light shading in a limited small area does not cause India to visually stand out as a place where there is a serious AIDS problem. The second cartogram shows the magnitude of the Indian cases far more clearly and accurately relative to other countries. So why aren’t cartograms used more often? Because making them is hard.