Spatial Thinking in Planning Practice: An Introduction to GIS

28 Figure 4.2: Reapportionment of the U.S. House of Representatives as a result of the 2000 census. Source: Smith, JM., 2012. Department of Geography, "e Pennsylvania State University; A$er !gure in Chapter 3, DiBiase. Congressional voting district boundaries must be redrawn within the states that gained and lost seats, a process called redistricting . Constitutional rules and legal precedents require that voting districts contain equal popula- tions (within about 1 percent). In addition, districts must be drawn so as to provide equal opportunities for rep- resentation of racial and ethnic groups that have been discriminated against in the past. Further, each state is al- lowed to create its own parameters for meeting the equal opportunities constraint. Whether districts determined each decade actually meet these guidelines is typically a contentious issue and o$en results in legal challenges. Beyond the role of the census of population in determining the number of representatives per state (thus in providing the data input to reapportionment and redistricting), the Census Bureau’s mandate is to provide the population data needed to support governmental operations, more broadly including decisions on allocation of federal expenditures. Its broader mission includes being “the preeminent collector and provider of timely, rel- evant, and quality data about the people and economy of the United States”. To ful!ll this mission, the Census Bureau needs to count more than just numbers of people, and it does. THEMATIC MAPPING Unlike reference maps, thematic maps are usually made with a single purpose in mind. Typically, that purpose has to do with revealing the spatial distribution of one or two attribute data sets. In this section, we will consider distinctions among three types of ratio level data, counts, rates, and densities. We will also explore several dif- ferent types of thematic maps, and consider which type of map is conventionally used to represent the di#erent types of data. We will focus on what is perhaps the most prevalent type of thematic map, the choropleth map. Choropleth maps tend to display ratio level data which have been transformed into ordinal level classes. Finally, you will learn two common data classi!cation procedures, quantiles and equal intervals. MAPPING COUNTS "e simplest thematic mapping technique for count data is to show one symbol for every individual counted. If Chapter 4: Mapping People with Census Data