Thematic map

Maps come in a variety of formats. A thematic map (also called a statistical or special purpose map) displays the 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 in the form of geographic distributions. These distributions may be physical phenomena (such as crop yield and climate) or non-physical concepts (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. While general reference maps show where something is in space, thematic maps tell a story about that place.

A Brief History of Maps

Essentially, a map is a “reduced, substitute space for that of reality.” Exactly when and how the first map was created is unknown, but it is reasonable to surmise that mapping has been an integral part of the human experience for perhaps hundreds of thousands of years.

In Western civilization, maps have gone through many progressions and even regressions to develop into the familiar tools we use today. From ancient Babylon to Ptolemy, from Peutinger tables (tabula Peutingeriana), religious T and O maps, to Portolan charts, cartography is a record of how people have identified with the world around them.

Revolutions in mapping “primarily involve shifts or changes in the intellectual aspects of cartography” and the innovations of thematic mapping are no exception. 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.

Examples of Early Thematic Cartographers

Edmond Haley

Though famous for discovering the comet which bears his name, Edmond Halley is also credited with being the first thematic mapmaker with genuine cartographic skill. In 1686, Halley produced a small copper engraving depicting the direction of trade winds in the Atlantic Ocean.

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 reflects principles of 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. Incidentally, further investigation of the area revealed the Broad Street pump was near a sewer line.

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. When designing a thematic map, cartographers must keep in mind certain conventions in order to effectively represent the data. Besides accuracy in location and event occurrences, cartographers should utilize Bertin’s visual variables to enhance the maps legibility and aesthetics.

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 over a county boundary map for easy location referencing.

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 opens up multivariate mapping. Taking three or more data sets and displaying the result on a map helps examine possible correlations between varieties of phenomena. In our bivariate example mapped 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.

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 great to chart phenomena that are even distributed within each enumeration unit (set area). Voting patterns incorporate this method.

Isarithmic: These maps, also known as contour maps, depict smooth continuous phenomena such as rainfall, barometric pressure, etc. They are also well-suited to displaying three-dimensional values such as elevation.

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.

Dot: A map using dots to show the presence of a feature or occurrence and display a spatial pattern. One dot represents a certain amount of some phenomenon’s occurrence. Dr. Snow used this method in his famous map. One dot represented one death. It should be noted that one dot, one occurrence is not a required equation. One dot may represent any set number of an occurrence. Because they are difficult to computerize, dot maps are no longer common.

Dasymetric: These maps utilize areal symbols. However, although boundaries are displayed on dasymetric maps, these units may have one or more values displayed. Plots usually represent extremes in the data sets, without much coverage in between. For that reason, and because they are difficult to computerize, dasymetric maps are not very common.