Circle radii are a function of the square root of data values. A slightly larger value ("Flannery's Constant") is used in some programs to take into account visual underestimation of areas (Campbell,
1993, p. 272). This map was custom designed and executed by cartography students under the supervision of Thomas Rabenhorst, Department of Geography and Environmental Systems, University of Maryland, Baltimore County.
This is sometimes referred to as a "stepped" surface map as compared with a "smoothed" surface. The former is based on choropleth area subdivisions; the latter is not.
For an interesting early example of
isolines applied to crime mapping, see Brassel and Utano (1979).
A map of this type, which has probably never been produced, would demand data from a global positioning system.
SPSS is described at http://www.spss.com. Another widely used package, the Statistical Analysis System (SAS), is described at http://www.sas.com.
San Bernardino County, California, about the size of Denmark, is an extreme example. The urbanized part is small, and most of the county is part of the sparsely inhabited Mojave Desert. Yet a choropleth map of population density will show the entire county as having the same density (the average value), giving the visual impression that the desert is populated.
Dasymetric mapping ameliorates the areal averaging of the choropleth map by recognizing the internal variation inherent in the subdivisions. The result resembles an isoline map with political or administrative subdivisions superimposed. (For additional information, see Campbell 1993, p. 218.) For an illustration of what Tufte calls the "mesh" map alternative to the choropleth map, see Tufte, 1990, pp. 40-41.
Some police departments rely on a
proprietary map system for navigation; embedding that coordinate system in their current mapping applications could be helpful. In Baltimore County, for example, locational references are to the grid of maps published by ADC of Alexandria, Inc.
This assumption is culturally conditioned by the way the person reads, which is itself language dependent. (Arabic is not read in the same way as English, for example.)
Take into account the size of the room and how far the back row of seats is from the image. Similar considerations apply if you have a poster-size map to be displayed to an audience.
Mimetic symbols mimic the appearance of the real object. (See MacEachren, 1994, pp. 56-57; 1995, pp. 258-259.)