Spatial Thinking in Planning Practice: An Introduction to GIS

27 CHAPTER 4: MAPPING PEOPLE WITH CENSUS DATA WHY CENSUS? Some of the richest sources of attribute data for thematic mapping, particularly for choropleth maps, are national censuses. In the United States, a periodic count of the entire population is required by the U.S. Constitution. Ar- ticle 1, Section 2, rati!ed in 1787, states (in the last paragraph of the section shown below) that “Representatives and direct taxes shall be apportioned among the several states which may be included within this union, accord- ing to their respective numbers ... "e actual Enumeration shall be made [every] ten years, in such manner as [the Congress] shall by law direct.” "e U.S. Census Bureau is the government agency charged with carrying out the decennial census. Figure 4.1: A portion of the Constitution of the United States of America (preamble and !rst three paragraphs of Article 1). Credit: Obtained from : http://www.archives.gov/exhibits/charters/charters_downloads.html "e results of the U.S. decennial census determine states’ portions of the 435 total seats in the U.S. House of Rep- resentatives. "ematic maps can show states that lost and gained seats as a result of the reapportionment that fol- lowed the 2000 census (Figure 4.2). By focusing on the U.S. state-by-state, we develop a variant on a choropleth map. Rather than using color !ll to depict quantity, color depicts only change and its direction, red for a loss in number of Congressional seats, gray for no change, and blue for a gain in number of Congressional seats. Num- bers are then used as symbols to indicate amount of change (small -1 or +1 for a change of 1 seat and larger -2 or +2 for a change of two seats). "is scaling of numbers is an example of the more general application of “size” as a graphic variable to produce “proportional symbols” – the topic we cover in detail in the section on proportional symbol mapping below.