Spatial Thinking in Planning Practice: An Introduction to GIS

32 usually sorted into four to eight ordinal level categories. ArcMap calls these categories classes. Users can adjust the number of classes, the class break values that separate the classes, and the colors used to symbolize the class- es. Users may choose a group of prede!ned colors, known as a color ramp, or they may specify their own custom colors. Color ramps are sequences of colors that vary from light to dark, where the darkest color is used to rep- resent the highest value range. Most textbook cartographers would approve of this, since they have long argued that it is the lightness and darkness of colors, not di#erent color hues, that most logically represent quantitative data. Logically or not, people prefer colorful maps. For this reason some might be tempted to choose ArcMap’s Unique Values option to map rates, densities, or even counts. "is option assigns a unique color to each data value. Colors vary in hue as well as lightness. "is symbolization strategy is designed for use with a small number of nominal level data categories. As illustrated in the map below (Figure 4.8), the use of an unlimited set of color hues to symbolize unique data values leads to a confusing thematic map. Figure 4.8. A “unique values” map that depicts density data. Note that the legend, which in the original shows one category for each state, is trimmed o#. Source: G Htchard. https://www.e-education.psu.edu/geog482fall2/ c3_p16.html DATA CLASSIFICATION As discussed earlier, all maps are abstractions. "is means that they depict only selected information, but also that the information selected must be generalized due to the limits of display resolution, comparable limits of human visual acuity, and especially the limits imposed by the costs of collecting and processing detailed data. What we have not previously considered is that generalization is not only necessary, it is sometimes bene!cial; it can make complex information understandable. Consider a simple example. "e graph below (Figure 4.9) shows the percent of people who prefer the term “pop” (not soda or coke) for each state. Categories along the x axis of the graph represent each of the 50 unique percentage values (two of the states had exactly the same rate). Catego- ries along the y axis are the numbers of states associated with each rate. As you can see, it’s di&cult to discern a pattern in these data; it appears that there is no pattern. Chapter 4: Mapping People with Census Data