Extract
Dasymetric Mapping and Areal Interpolation: Implementation and Evaluation.
Introduction
A dasymetric map depicts quantitative areal data using boundaries that divide the mapped area into zones of relative homogeneity with the purpose of best portraying the underlying statistical surface. The dasymetric map was conceived as a type of thematic map during the early to mid nineteenth century, the formative years of modern thematic cartography. During their early development, the demand for both dasymetric and choropleth maps was driven by interest in population mapping (McCleary 1969; 1984). By 1900, dasymetric and choropleth mapping methods became more clearly differentiated, with the latter becoming overwhelmingly popular in modern cartography and for general use outside the discipline. In contrast, dasymetric mapping has remained relatively unknown even to most geographers. Consistent with their original purpose, dasymetric maps of population are still the most common type found today. Although dasymetric maps are closely related to choropleth maps, they differ in several ways. First, zonal boundaries on dasymetric maps are based on sharp changes in the statistical surface being mapped, while zonal boundaries on choropleth maps demarcate enumeration units established for more general purposes (e.g., states within the U.S.). The cartographer generates dasymetric zones by using ancillary information. This information can be both objective and subjective, depending on other available data and the cartographer's knowledge of the area. Second, individual dasymetric zones are developed to be internally homogeneous. In contrast, choropleth zones are not defined based on the data and, thus, have varying levels of internal homogeneity. Third, choropleth mapping methods have become standardized (including the development of common classification schemes; Slocum 1999), but the wide range of dasymetric procedures have been under researched. Surprisingly little literature exists on dasymetric mapping. At a theoretical level, MacEachren (1994) placed dasymetric maps in the continuum between isopleth and choropleth maps, suggesting that dasymetric maps represent data half way between smooth and stepped statistical surfaces. In preparation for this research, we studied the two most frequently cited early pieces by Wright (1936) and McCleary (1969). More recent implementations of dasymetric mapping methods within GIS include Gerth (1993), Holloway et al. (1996), and Charpentier (1997). Their work relies heavily on Wright and McCleary and offers insight into the practical challenges of produc...See the full content of this document
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