Dasymetric Maps

Dasymetric mapping is a cartographic technique with roots in choropleth mapping. Both techniques originated in the 1800s when cartographers and others began to design maps of population distributions. Like the choropleth map, the dasymetric map uses area symbols to show variations in topics collected by standard political units (e.g., census tracts, county boundaries). The advantage of this particular representation lies in its treatment of the boundaries of the political units. A choropleth map of county population density, for example, would suggest uniform densities within each county, with possible abrupt changes occurring between adjacent counties due to the geography's artificial nature. The mapping of the topic with this technique is heavily influenced, then, by the geography of the political units used; displaying population densities by census tracts may result in quite different density patterns when compared with a similar map using counties as the geographic foundation (Figure 1).

The dasymetric version of the same data uses ancillary data sets, such as land use, as the boundaries for representing changes in population density (Figure 2).

The density data from each county are reassigned to land use areas within the county using formulas to determine how many people should be assigned to each inner-county area. The result is a more accurate mapping of the topic; areas of abrupt change follow more closely the real-world distribution because they are tied not to political [Page 670]boundaries but to natural boundaries. The initial interest in dasymetric mapping seems to have originally peaked during the 1920s and 1930s, around the time that J. K. Wright published his seminal work on the subject. It is back at the forefront of cartographic research today, largely because of its more precise representation potential and its ease of implementation with current state-of-the-art software.

Figure 1 Population density of Guilford County, North Carolina, by census tract and block group

Source: Map created by author based on data from ESRI Data and Maps, 2000; Census 2000 Summary File 1 (SF 1) 100-Percent Data, U.S. Bureau of the Census, 2000; and National Land Cover Database, Multi-Resolution Land Characteristics Consortium, 2001.

Figure 2 Population density of Guilford County, North Carolina, using dasymetric mapping

Source: Map created by author based on data from ESRI Data and Maps, 2000; Census 2000 Summary File 1 (SF 1) 100-Percent Data, U.S. Bureau of the Census, 2000; and National Land Cover Database, Multi-Resolution Land Characteristics Consortium, 2002.