Our article describes the work of the Digital Harrisburg Project in placing the population of Pennsylvania's capital city on geocoded historical maps in 1900–1930. We argue that geocoded census data—population tied to precise locations in a GIS—marks a game-changer for creating fine-grained historical pictures of human mobility and changing urban diversity. Because historical federal census tables recorded information about race, immigration, occupation, property value, and home address, the historian has the power to study patterns of residence that relate to complex forces such as regional and global immigration, economic change, and urban reform. Census records and historical maps are hardly unproblematic, however, and require care in analysis and interpretation. The article highlights the digital project and data, the challenges of digitizing demographic records and geocoding urban space, and potential applications for rethinking historical problems such as City Beautiful.