Uses of Thematic Maps in Geography

These maps display data including population, rainfall, and epidemics

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A thematic map emphasizes a theme or topic, such as the average distribution of rainfall in an area. They're different from general reference maps because they don't just show natural and manmade features such as rivers, cities, political subdivisions, and highways. If these items appear on a thematic map, they're reference points to enhance one's understanding of the map's theme and purpose.

Normally, thematic maps use coastlines, city locations, and political boundaries as their basis. The map's theme is then layered onto this base map via different mapping programs and technologies such as geographic information systems (GIS).

History

Thematic maps didn't develop until the mid-17th century, because accurate base maps didn't exist before then. Once maps became accurate enough to correctly display coastlines, cities, and other boundaries, the first thematic maps were created. In 1686, for example, English astronomer Edmond Halley developed a star chart and published the first meteorological chart using base maps as his reference in an article he wrote about trade winds. In 1701, Halley published the first chart to show lines of magnetic variation, a thematic map that later became useful in navigation.

Halley's maps were largely used for navigation and study of the physical environment. In 1854, London doctor John Snow created the first thematic map used for problem analysis when he mapped cholera's spread throughout the city. He began with a base map of London's neighborhoods that included streets and water pump locations. He then mapped locations where people had died from cholera on that base map and found that the deaths clustered around one pump. He determined that the water coming from the pump was the cause of cholera.

The first map of Paris showing population density was developed by Louis-Leger Vauthier, a French engineer. It used isolines (lines connecting points of equal value) to show population distribution throughout the city. He is believed to have been the first to use isolines to display a theme that didn't have to do with physical geography.

Audiences and Sources

The most significant factor to consider when designing thematic maps is the map's audience, which helps determine what items should be included on the map as reference points in addition to the theme. A map being made for a political scientist, for example, would need to show political boundaries, whereas one for a biologist might need contours showing elevation.

The sources of thematic maps' data are also important. Cartographers must find accurate, recent, reliable sources of information on a wide range of subjects, from environmental features to demographic data, to make the best possible maps.

Once accurate data is found, there are various ways to use that data that must be considered with the map's theme. Univariate mapping deals with only one type of data and looks at the occurrence of one type of event. This process would be good for mapping a location's rainfall. Bivariate data mapping shows the distribution of two data sets and models their correlations, such as rainfall amounts relative to elevation. Multivariate data mapping, which uses two or more data sets, could look at rainfall, elevation, and the amount of vegetation relative to both, for example.

Types of Thematic Maps

Although cartographers can use data sets in different ways to create thematic maps, five thematic mapping techniques are used most often:

  • The most common is the choropleth map, which portrays quantitative data as a color and can show density, percent, average value, or quantity of an event within a geographic area. Sequential colors represent increasing or decreasing positive or negative data values. Normally, each color also represents a range of values.
  • Proportional or graduated symbols are used in another type of map to represent data associated with locations, such as cities. Data is displayed on these maps with proportionally sized symbols to show differences in occurrences. Circles are most often used, but squares and other geometric shapes are also suitable. The most common way to size these symbols is to make their areas proportional to the values to be depicted using mapping or drawing software.
  • Another thematic map, the isarithmic or contour map, uses isolines to depict continuous values such as precipitation levels. These maps also can display three-dimensional values, such as elevation, on topographic maps. Generally, data for isarithmic maps is gathered via measurable points (e.g. weather stations) or is collected by area (e.g. tons of corn per acre by county). Isarithmic maps also follow the basic rule that there are high and low sides in relation to the isoline. For example, in elevation, if the isoline is 500 feet, then one side must be higher than 500 feet and one side must be lower.
  • A dot map, another type of thematic map, uses dots to show the presence of a theme and display a spatial pattern. A dot can represent one unit or several, depending on what is being depicted.
  • Finally, dasymetric mapping is a complex variation on the choropleth map that uses statistics and additional information to combine areas with similar values instead of using the administrative boundaries common in a simple choropleth map.