Automated generalization of historical US 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. Thisdatabase 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 productionneed, the main challenge in the generalization of these spatio-temporal data is the maintenance ofgeometric 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 constraintbasedgeneralization paradigm and the active object approach; and (2) a topological data modellinking all datasets, which represent different census years, for one target scale. The framework isimplemented in ESRIs ArcGIS environment using ArcGIS 9.0, Oracle, C# and ArcObjects. Theimplementation of the model generalization process was completed and successfully tested for thethree target scales of the final database. Model generalization accomplishes the removal of redundantpoints and the removal of boundary-change sliver polygons. The implementation of the cartographicgeneralization process is still on-going and has focused, until recently, on different approaches for theenlargement and elimination of too small census units or detached parts of a census unit as well as onthe reduction of the outline granularity of census units boundaries. Results that were automaticallygeneralized with the current version of the prototype exhibit satisfying quality based on a preliminaryvisual evaluation.