Visualizing patterns in U.S. Urban population trends

Abstract

With the completion of the U.S. National Historical Geographic Information System (NHGIS), it is now feasible to assemble a large dataset of historical census tract population statistics and boundary data in order to investigate patterns in long-term urban population trends. The present study makes use of this new resource to achieve a broad but concise overview of population trend patterns throughout major U.S. urban areas since 1950. This work thereby makes both methodological and substantive contributions to multiple fields of research, with much of the work dedicated to the development and assessment of new techniques to address two key methodological challenges. The first challenge is to construct a time series of census tract data, which requires linking data through time even where tract boundaries have changed. I present a few relatively simple areal interpolation techniques that can be used to address this problem. Two case studies indicate that a novel technique, cascading density weighting, should be effective both in the present setting and potentially elsewhere. The second methodological challenge is to identify an effective visualization strategy for investigating patterns in long-term trends. I present here a new conceptual framework that identifies a group of mapping techniques--trend summary maps--that should be most useful for visualizing patterns in trends. I provide an overview and assessment of several types of trend summary mapping techniques, and I introduce a novel technique, bicomponent trend mapping, which combines principal component analysis with bivariate choropleth mapping. This technique has several useful advantages not only for visualizing urban population trends but potentially in many other settings of spatio-temporal data visualization as well. Applying the new techniques to historical census tract data enables the central substantive contribution of this research: an overview of population trend variations throughout major U.S. urban cores. This overview supports the standard narrative of recent urban population dynamics--growth on the outskirts, decline in the cores, and some regrowth in centers--but it also reveals many regionally and locally unique patterns, indicating both divergence among cities and increasing heterogeneity within them.