Mapping the past: dynamic local georeferencing for historical cartography

Georeferencing is crucial for integrating historical and pictorial maps into digital environments, enabling their application in research, education, and public engagement. However, traditional GIS-based georeferencing workflows often fall short when applied to these maps, as global image transformations can distort the unique and non-linear representations that are essential to the maps’ historical value. This paper introduces Dynamic Local Georeferencing (DLGR) as an approach to this challenge by employing piecewise affine transformations based on a network of user-defined control points. Although each local transformation is affine within a small region, their combination approximates non-uniform, non-linear distortions while preserving the original map’s design integrity. DLGR Mapper, a web-based tool implementing DLGR, provides an interface for creating, editing, and refining control points for bidirectional coordinate transformation between maps, facilitating nuanced comparisons. Key features include real-time visualization of dynamically updated Triangulated Irregular Networks (TINs), grid overlays, and highlighting of reversed triangles to guide iterative refinement. The paper details the functionality of DLGR Mapper, including its support for progressive map exploration, multi-map workflows aided by control point inference, and interactive feature mapping. Case studies involving historical bird’s-eye-view maps by Hatsusaburo Yoshida and a multi-stage analysis of maps related to a traditional building preservation area demonstrate the potential effectiveness of DLGR Mapper in enhancing historical cartographic analysis and local geographic studies. The paper concludes with a discussion of the advantages of DLGR Mapper’s iterative and exploratory workflow, its potential for collaborative georeferencing, and future research directions such as usability testing and collaborative platform development.