Methods of thematic mapping

Geographers use many methods to create thematic maps, but five techniques are especially noted.

Choropleth

The most commonly used method of thematic mapping. Choropleth maps are particularly suited for charting phenomena that are evenly distributed within each enumeration unit (set area).

Raw data , e.g. population distribution, should not be mapped with this technique. If a derived values can be otained from raw data, (such as population densities), then the choropleth can apply.

Proportional symbol

Also known as graduated symbols, these maps represent data associated with point locations (i.e., cities or counties). The data is displayed with proportionally sized symbols to graphically represent a realistic difference in occurrence. If the raw data cannot be dealt with as a ratio or proportion, then they should be portrayed with the proportional symbol technique.

Isarithmic

[[Image:January 17, 1982 500-Millibar Height Contours.png|thumb|Isarithmic map of barometric pressure.]]

These maps, also known as contour maps, depict smooth continuous phenomena such as precipitation. They are also well-suited to displaying three-dimensional values such as elevation i.e; on topographic maps.

Dot

A map using dots to show the presence of a feature or occurrence and display a spatial pattern. Dr. Snow used this method in his famous map. One dot represented one death. Note, though, that a dot is not required to represent a single unit and may indicate any number of entities; 14 armadillos, seven dwarves, 100 voters.

Dasymetric

These maps utilize areal symbols. However, although boundaries are displayed on dasymetric maps, these geographic units may span multiple theme values. Plots often represent extremes in the data sets, without much coverage in between. For that reason, and because they can be difficult to generate, dasymetric maps are not very common.

References

• Muehrcke, P...et al. Map Use, The University of Chicago Press, 4th Edition, 2001
• Petchenik, B. B. From Place to Space: The Psychological Achievement in Thematic Mapping, American Cartographer 1, 1979
• Robinson, A. Early Thematic Mapping in the History of Cartography, The University of Chicago Press, 1982
• Robinson, A...et al. Elements of Cartography, Wiley, 6th Edition, 1995
• Slocum, T....et al. Thematic Cartography and Geographic Visualization, Prentice Hall, 2nd Edition, 2005
• Thrower, N. Maps and Civilization: Cartography in Culture and Society, The University of Chicago Press, 1996

Methods of thematic mapping courtesy of wikipedia.org