A map showing the distribution of a phenomenon by graded shading to indicate the density per unit area of that phenomenon; the greater the density of shading, the greater the density in reality. Choropleths give a clear, but generalized, picture of distribution, but may mask finer details. The choice of values for the classes, and the areal units for which data are available, will affect the visual picture; see scale. See Goldsberry et al. (2004) Procs Annual Meeting NACIS on classification choice and animated choropleths; see also Murray in P. Forer et al., eds (2007) on geographic space and choropleth display.