Atlas of Historical County Boundaries: Conversion to Topology

The Atlas of Historical County Boundaries is a resource created by The Newberry Library, Chicago, Illinois, to document changes in county and county-equivalent boundaries in the United States of America over time. In 2019, The Newberry Library donated the project shapefiles to the Library of Congress and released them under a Creative Commons CC0 "No Rights Reserved" License.

Each multipolygon feature in the project shapefile data represents a particular government unit (or non-county or disputed area) for a particular time period. In order to avoid duplication of shared boundaries, the scripts in this repository convert these features into least common geometries,¹ which, once further converted to a PostGIS topology and exported as edges (or "ways"), can be imported into other projects with topological data models, like OpenHistoricalMap.

The files that were exported as a ZIP archive by the scripts are included in the release in the output folder. For additional information, see the README.md file.

1. Martina De Moor and Torsten Wiedemann, "Reconstructing Territorial Units and Hierarchies: A Belgian Example," History and Computing 13, no. 1 (March 2001): 71-98, https://doi.org/10.3366/hac.2001.13.1.71.