Spatial Thinking in Planning Practice: An Introduction to GIS

31 Some perceptual experiments have suggested that human beings are more adept at judging the relative lengths of bars than they are at estimating the relative sizes of pie pieces (although it helps to have the bars aligned along a common horizontal base line). You can judge for yourself by comparing the e#ect of ArcMap’s Bar/Column Chart option (Figure 4.6). Figure 4.6. A “bar/column chart” map that depicts rate data. Source: G. Hatchard. https://www.e-education. psu.edu/geog482fall2/c3_p16.html Like rates, densities are produced by dividing one count by another, but the divisor of a density is the magnitude of a geographic area. Both rates and densities hold true for entire areas, but not for any particular point location. For this reason, it is conventional not to use point symbols to symbolize rate and density data on thematic maps. Instead, cartography textbooks recommend a technique that ArcMap calls “Graduated Colors.” Maps produced by this method, properly called choropleth maps, !ll geographic areas with colors that represent attribute data values (Figure 4.7). Figure 4.7. A “graduated color” (choropleth) map that depicts density data. Source: G. Hatchard. https://ww- w.e-education.psu.edu/geog482fall2/c3_p16.html Because our ability to discriminate among colors is limited, attribute data values at the ratio or interval level are Chapter 4: Mapping People with Census Data