Francis Amasa Walker's Statistical Atlas of the United States, published in 1874, was unlike anything that most Americans had ever seen. But he soon realized that these new forms of cartography would require training to be understood. Emma Willard created maps of the past to convey a sense of sovereignty and identity to her fellow citizens, giving tangible evidence of the new nation's coherence and stability. Similarly, Johann Georg Kohl spearheaded an effort to collect and preserve old maps as evidence of America's territorial legitimacy. By giving the nation's history a cartographic dimension, Kohl and Willard helped establish many of the models of historical mapping that endure today. Whereas Kohl and Willard mapped the nation's past, others, such as Alexander von Humboldt, focused on mapping its present. Techniques were developed to map slavery, climate, and epidemics. The twentieth century witnessed how the use of thematic maps continued to flourish—in part due to the advent of geographic information systems (GIS)—in areas ranging from etiology to urban planning, social research, marketing, and political strategy.