Automated Generalization of Historical U.S. Census Units

This paper investigates as part of the National Historical Geographic Information System project (http://www.nhgis.org/) the creation of a multi-scale database of historical US census units. This database will include, at a minimum, three different scales (1:150,000, 1:400,000 and 1:1,000,000) and boundary data for all documented census since 1790. Besides the commitment to the production need, the main challenge in the generalization of these spatio-temporal data is the maintenance of geometric and topological consistency both within a dataset and between datasets for one target scale. We propose to address this challenge through: (1) a generalization framework based on the constraint based generalization paradigm and the active object approach; and (2) a topological data model linking all datasets, which represent different census years, for one target scale. The framework is implemented in ESRIs ArcGIS environment using ArcGIS 9.0, Oracle, C# and ArcObjects. The implementation of the model generalization process was completed and successfully tested for the three target scales of the final database. Model generalization accomplishes the removal of redundant points and the removal of boundary-change sliver polygons. The implementation of the cartographic generalization process is still on-going and has focused, until recently, on different approaches for the enlargement and elimination of too small census units or detached parts of a census unit as well as on the reduction of the outline granularity of census units boundaries. Results that were automatically generalized with the current version of the prototype exhibit satisfying quality based on a preliminary visual evaluation.