No one knew how cholera spread or how to stop it. But one doctor, John Snow, had an idea. Snow convinced officials to remove the handle from a water pump on Broad Street so that no one could use the pump. Soon the cholera stopped spreading. Snow had guessed correctly that the disease was spread through polluted drinking water.
Snow had, in effect, solved the mystery by using the skills of the geographic inquiry process. First he asked a question: How was cholera spread? Next he gathered information by going door to door to find out where people were dying. He then organized his information on a neighborhood map, marking the house of each cholera death. His next step was to analyze his map. He was able to answer his question: The Broad Street pump must be spreading the disease, because most of the deaths were clustered around it.
John Snow's map is an example of a thematic map. A thematic map presents information related to only one theme or topic. In this chapter, you will learn how to identify and read different types of thematic maps. You will see how geographers use these tools to make sense of Earth's physical and human features.