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Areal cartogram of the United States, showing each county with a size according to its population. Colors refer to the results of the U.S. presidential election, 2004.

A cartogram is a map on which actual land area is substituted by another thematic mapping variable (such as time, cost, AIDS rates, or Gross National Product). It is sometimes referred to as an isodemographic map or a value-by-area map.

Although the term cartogram had been used in Europe in previous decades to refer to any statisical map, America's first cartography textbook, General Cartography by Erwin Raisz (1938), defined the term as it is used today. For more in-depth coverage of the history of cartograms, see Waldo Tobler's Thirty-Five Years of Computer Cartograms.

There are basically two main types of cartograms: central-point and areal cartograms. A central-point cartogram may show travel times from a particular central point to other locations by rescaling the map so that all points that are 10 minutes away are equally distant from the center point. Lines can be drawn on the map to connect all such points. Areal cartograms, by contrast, rescale the entire data unit (a country or state) based on other variables. For example, a population cartogram illustrates the relative sizes of the populations of the countries of the world by scaling the area of each country in proportion to its population; the shape and relative location of each country is retained to as large an extent as possible, but inevitably a large amount of distortion results.

• Campbell, John. Map Use and Analysis. New York: McGraw-Hill, 2001.
• Gillard, Quentin. "Places in the News: The Use of Cartograms in Introductory Geography Courses." Journal of Geography. 78 (1979): 114-115.
• Tobler, Waldo. "Thirty-Five Years of Computer Cartograms." Annals of the Association of American Geographers. 94 (2004): 58-73.

• Cartogram Central
• A population cartogram
• Population & Social/Economic Cartograms
• Multiple cartograms showing the results of the 2004 US Presidential Election
• Tools to draw cartograms
• Worldmapper collection of world cartograms

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