Racial Dot Maps Based on Dasymetrically Modeled Gridded Population Data

Anna Dmowska,Tomasz F Stepinski

doi:10.3390/socsci8050157

Abstract

Racial geography, mapping spatial distributions of different racial groups, is of keen interest in a multiracial society like the United States. A racial dot map is a method of visualizing racial geography, which depicts spatial distribution, population density, and racial mix in a single, easy-to-understand map. Because of the richness of information it carries, the dot map is an excellent tool for visual analysis of racial distribution. Presently-used racial dot maps are based on the Census data at the tract or the block level. In this paper, we present a method of constructing a more spatially-accurate racial dot map based on a sub-block-resolution population grid. The utility of our dot maps is further enhanced by placing dots on the map in random order regardless of the race they represent in order to achieve a more accurate depiction of local racial composition. We present a series of comparisons between dot maps based on tract, block, and grid data. The advantage of a grid-based dot map is evident from the visual comparison of all maps with an actual image of the mapped area. We make available the R code for constructing grid-based dot maps. We also make available 2010 grid-based racial dot maps for all counties in the conterminous United States.

Highlights

Residential racial segregation and racial diversity of population have been traditionally studied using a non-spatial statistical analysis with the results reported in the form of a table listing segregation/diversity indices (Farrell 2008; Frey and Farley 1996; Frey and Myers 2005; Iceland 2004; Iceland et al 2002; Johnston et al 2007; Lichter et al 2007; Logan et al 2004; Massey and Denton 1987) for different metropolitan areas, counties, etc
We described the construction of a grid-based racial dot map and demonstrated that such maps are more spatially accurate than racial dot maps currently in use
R scripts for calculating dot maps from a grid. This adds to a set of tools that social scientists and others can use to visualize the geography of racial segregation and/or diversity

Summary

Introduction

Residential racial segregation and racial diversity of population have been traditionally studied using a non-spatial statistical analysis with the results reported in the form of a table listing segregation/diversity indices (Farrell 2008; Frey and Farley 1996; Frey and Myers 2005; Iceland 2004; Iceland et al 2002; Johnston et al 2007; Lichter et al 2007; Logan et al 2004; Massey and Denton 1987) for different metropolitan areas, counties, etc. The same problem remains even if smaller Census units such as block groups or blocks are used instead of tracts Another artifact is the abrupt change of racial character at the boundaries between tracts; this is especially noticeable in a dot map (Panel C). These artifacts (and other problems, like, for example, incompatibility of Census area boundaries between different years) transcend segregation/diversity studies and apply to all Census-based maps. The racial dot map constructed from the population grid has the same advantage over the dot map constructed from the Census tracts as the race percentage map and the racial diversity map do (see Figure 1F). All three types of dot maps are shown, and their spatial accuracy can be visually assessed by comparison to an image of the mapped area

Methods

Gridded Data

Grid-Based Dot Map

Examples

Discussion and Conclusions