The Atlas of Historical County Boundaries is an ongoing project undertaken by the Newberry Library in Chicago. When fully completed, the effort will result in the presentation of all United States county boundary changes from their creation during colonial times through to the 21 st Century. The Newberry Library explains why they decided to focus on counties:
…the judicial system is administered by counties. Counties administer most state laws and, in the process, they collect large quantities of important information, such as records of births, marriages, deaths, probated wills, real-estate transfers, and voting. Counties have been the principal geographic units for the collection and aggregation of data from colonial/territorial, state, and federal censuses, especially counts of population. Outside densely populated cities, counties have served as the building blocks for congressional districts and for colonial, territorial, and state legislative districts.
This is a highly complicated and laborious effort that has been in process for several years. Data are published in book form, in shapefiles for use with GIS programs, and via an interactive map available on their website. Many states have been completed while others are in various stages of production.
These maps are of great value to professional historians and researchers, and even to amateurs with an interest in local history or genealogy. As an example, suppose a genealogist knew that her ancestors lived in Wisconsin when it became a state in 1848. One good source of information would be the 1850 United States Census, the first national census to provide names and other identifying information for every member of the household rather than just the head of the family. Say those early pioneers moved further north into the state around 1849 and were among those that first settled near what would become the town of Waupaca, which today is the government seat of a county with the same name. Where would a researcher look for those valuable 1850 census records? Waupaca County didn’t exist until 1851.
The Atlas of Historical County Boundaries would quickly show that research should focus on census records for Brown County. That wouldn’t be so obvious without the Atlas, however. Modern maps clearly show that Brown County isn’t even contiguous with Waupaca County. Researchers require a crosswalk that delineates boundary changes over time, and here the Atlas becomes a particularly useful tool. It provides a number of layers such as current county boundaries and seats that can be applied to the base map of a past date to ferret out the answer being investigated (and which I’ve highlighted by adding the red circles in the example above).