Communications - June 2010

knowingly distort or abstract geographic features to tell a richer story or highlight specific data.

Flow Maps. By placing stroked lines on top of a geographic map, a flow map can depict the movement of a quantity in space and (implicitly) in time. Flow lines typically encode a large amount of multivariate information: path points, direction, line thickness, and color can all be used to present dimensions of information to the viewer. Figure 3a is a modern interpretation of Charles Mi-nard’s depiction of Napoleon’s ill-fated march on Moscow. Many of the greatest flow maps also involve subtle uses of distortion, as geography is bended to accommodate or highlight flows.

Choropleth Maps. Data is often collected and aggregated by geographical areas such as states. A standard approach to communicating this data is to use a color encoding of the geographic area, resulting in a choropleth map. Figure 3b uses a color encoding to communicate the prevalence of obesity in each state in the U.S. Though this is a widely used visualization technique, it requires some care. One common error is to encode raw data values (such as population) rather than using normalized values to produce a density map. Another issue is that one’s perception of the shaded value can also be affected by the underlying area of the geographic region.

Graduated Symbol Maps. An alternative to the choropleth map, the graduated symbol map places symbols over an underlying map. This approach avoids confounding geographic area with data values and allows for more dimensions to be visualized (for example, symbol size, shape, and color). In addition to simple shapes such as circles, graduated symbol maps may use more complicated glyphs such as pie charts. In Figure 3c, total circle size represents a state’s population, and each slice indicates the proportion of people with a specific BMI rating.

Cartograms. A cartogram distorts the shape of geographic regions so that the area directly encodes a data variable. A common example is to redraw every country in the world sizing it proportionally to population or gross domestic product. Many types of cartograms have been created; in Figure 3d we use the Dorling cartogram, which represents