The Harrison Public Library in Westchester County, New York operates two library buildings in Harrison: The Richard E. Halperin Memorial Library Building (the library’s main building, located in downtown Harrison) and a West Harrison branch location. As part of its latest three-year strategic plan, the library sought to use existing resources to improve understanding of its cardholders at both locations.

To do so, we needed to link the circulation data in our integrated library system, Evergreen, to geographic data and demographic data. We decided to build a geodemographic heatmap that incorporated all three aforementioned types of data. Using Evergreen, American Community Survey (ACS) data, and Google Maps, we plotted each cardholder’s residence on a map, added census boundaries (called tracts) and our town’s borders to the map, and produced summary statistics for each tract detailing its demographics and the library card usage of its residents. In this article, we describe how we acquired the necessary data and built the heatmap. We also touch on how we safeguarded the data while building the heatmap, which is an internal tool available only to select authorized staff members. Finally, we discuss what we learned from the heatmap and how libraries can use open data to benefit their communities.