Choosing Geographic Units for Choropleth Rate Maps, with an Emphasis on Public Health Applications

Abstract

Choropleth maps are the most widely used map type for mapping rates, such as those involving disease, crime, and socioeconomic indicators. The essential step of choosing a geographic unit to map is often made in an ad hoc manner. Among the desirable characteristics of choropleth mapping units are high degree of resolution, homogeneity of population size, homogeneity of land area, observation of minimum population thresholds and land area thresholds, temporal stability and currency, compactness of shape, audience familiarity, data availability, and the functional relevance of the unit to the phenomena mapped. Because of the uneven distribution of human populations, no single geographic unit can meet all of these characteristics in practice, and a well designed choropleth map necessarily involves some compromise. We present guidelines for choosing geographic units that take into account the above criteria, considering 12 geographic units ranging from census blocks to states. Even allowing for differences in scale and purpose, some units confer clear advantages over others.

Open source GIS for HIV/AIDS management

“…Choropleth mapping is an effective tool for viewing patterns within a data set and are commonly used for visualizing socio-economic patterns, disease and various other human geographic variables. In addition to these characteristics choropleth maps have the added advantage that they are familiar to a wide audience [ 66 ], increasing the appeal and usability of the GIS prototype.…”

“…Boscoe and Pickle [ 66 ] identify a number of desired characteristics of choropleth maps, including a high degree of spatial resolution, allowing for more details and potentially more patterns to be presented on the map. A second desirable feature of choropleth maps in that the population and area sizes are homogenous, so that the number of observations is equally spread over equal areas on the map.…”

Evaluating web-based static, animated and interactive maps for injury prevention

“…The FSA is an unusual unit of geography, particularly for the target end users who are more accustomed to seeing maps of Toronto delineated by census tracts or neighbourhoods. Using an "areal" unit that is familiar to the target audience is important (Boscoe and Pickle, 2003). However, using FSAs was unavoidable as the injury dataset was only available to be mapped at that level because of the data provider's concern for privacy issues.…”

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“…Specifically, disease rates in areas with smaller numbers of cases are more variable than those in areas with larger numbers of cases, a property that has also been termed "variance instability" [31]. Variance instability is particularly pervasive on maps, since it is extremely difficult to design a map that is not visually biased toward either sparsely populated or densely populated areas [32,33]. A simple example is the tendency for rural counties to contain disproportionate numbers of unusually high or unusually low disease rates and thus visually dominate a choropleth map.…”