Thematic map
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A thematic map displays spatial pattern of a theme or series of attributes. In contrast to reference maps which show many geographic features (forests, roads, political boundaries), thematic maps emphasize spatial variation of one or a small number of geographic distributions. These distributions may be physical phenomena such as climate or human characteristics such as population density and health issues. These types of maps are sometimes referred to as graphic essays that portray spatial variations and interrelationships of geographical distributions. Location, of course, is also important to provide a reference base of where selected phenomena are occurring. Barbara B. Petchenik described the difference as "in place, about space." While general reference maps show where something is in space, thematic maps tell a story about that place.
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[edit] Development
An important cartographic element preceding thematic mapping was the development of accurate base maps. Improvements in accuracy proceeded at a gradual pace, and even until the mid-17th century, general maps were usually of poor quality. Still, base maps around this time were good enough to display appropriate information, allowing for the first thematic maps to come into being.
[edit] Examples of early thematic cartographers
[edit] John Snow
The most widely touted example of early thematic mapping comes from London physician John Snow. Though disease had been mapped thematically, Snow’s cholera map in 1855 is the best known example of using thematic maps for analysis. Essentially, his technique and methodology anticipate principles of a geographic information system (GIS).
Starting with an accurate base map of a London neighborhood which included streets and pump locations, Snow mapped out the incidents of cholera death. The emerging pattern centered around one particular pump on Broad Street. At Snow’s request, the handle of the pump was removed, and new cholera cases ceased almost at once. Further investigation of the area revealed the Broad Street pump was near a sewer line.
[edit] Uses of thematic maps
Thematic maps serve three primary purposes. First, they provide specific information about particular locations. Second, they provide general information about spatial patterns. Third, they can be used to compare patterns on two or more maps. Common examples are maps of demographic data such as population density. When designing a thematic map, cartographers must balance a number of factors in order to effectively represent the data. Besides spatial accuracy, and aesthetics, quirks of human visual perception and the presentation format must be taken into account.
Of equal importance is audience. Who will “read” the thematic map and for what purpose helps define how it should be designed. A political scientist might prefer having information mapped within clearly delineated county boundaries (choropleth maps). A state biologist could certainly benefit from county boundaries being on a map, but nature seldom falls into such smooth, man-made delineations. In which case, a dasymetric map charts the desired information underneath a transparent county boundary map for easy location referencing.
[edit] Displaying data
In constructing any type of thematic map (or any map for that matter) it is understood that location is a key feature. After selecting the physical area to examine, the next step is collecting data sets.
Data dealing with one subject is called univariate, which examines occurrences of a single type of event. The distribution of population, cancer rates, and rainfall are all examples of univariate data.
Bivariate mapping shows the distribution of two sets of data to explore possibilities of correlations. For example, we can examine population density in relation to textile manufacturing. Other examples could be cancer rates and population density, or rainfall and elevation.
More than two sets of data leads to multivariate mapping. Taking three or more data sets and displaying the result on a map helps determine possible correlations between different phenomena. For instance, our bivariate example maps two data sets, rainfall and elevation. If we add another variable such as population density, our map becomes multivariate rather than bivariate.
Map makers must be careful in designing thematic maps that display too much information or suggest phenomenon have a correlation when in fact they do not.
[edit] Methods of thematic mapping
Geographers use many methods to create thematic maps, but five techniques are especially noted.
[edit] 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.
[edit] 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.
[edit] 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.
[edit] 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.
[edit] 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.
[edit] 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
[edit] See also
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