Normally however, all thematic maps use maps with coastlines, city locations and political boundaries as their base maps. The map's specific theme is then layered onto this base map via different mapping programs and technologies like a geographic information system (GIS).
History of Thematic MapsThematic maps did not develop as a map type until the mid-17th Century because accurate base maps were not present prior to this time. Once they became accurate enough to display coastlines, cities and other boundaries correctly, the first thematic maps were created. In 1686 for example, Edmond Halley, an astronomer from England, developed a star chart. In that same year, he published the first meteorological chart using base maps as his reference in an article he published about trade winds. In 1701, Halley also 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 the study of the physical environment. In 1854, John Snow, a doctor from London 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 all streets and water pump locations. He then mapped the locations where people died from cholera on that base map and was able to find that the deaths clustered around one pump and determined that the water coming from the pump was the cause of cholera.
In addition to these maps, the first map of Paris showing population density was developed by a French engineer named Louis-Leger Vauthier. It used isolines (a line connecting points of equal value) to show population distribution throughout the city and was believed to be the first use of isolines to display a theme that did not have to do with physical geography.
Thematic Map ConsiderationsWhen cartographers design thematic maps today, there are several important things to consider. The most significant though is the map's audience. This is important because it helps determine what items should be included on the thematic map as reference points in addition to the map's theme. A map being made for a political scientist for example would need to have political boundaries, whereas one for a biologist might instead need contours showing elevation.
The sources of a thematic map's data are also important and should be carefully considered. Cartographers must find accurate, recent and reliable sources of information in a wide range of subjects- from environmental features to demographic data to make the best possible maps.
In addition to making sure a thematic map's data is accurate, there are various ways to use that data and each must be considered with the map's theme. Univariate mapping for example is a map dealing with only one type of data and therefore 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 is mapping with two or more data sets. A multivariate map could look at rainfall, elevation and the amount of vegetation relative to both for example.
Types of Thematic MapsAlthough cartographers can use these datasets in many different ways to create thematic maps, there are five thematic mapping techniques that are used most often. The first and most commonly used of these is the choropleth map. This is a map that portrays quantitative data as a color and can show density, percent, average value or quantity of an event within a geographic area. Sequential colors on these maps represent increasing or decreasing positive or negative data values. Normally, each color also represents a range of values.
Proportional or graduated symbols are the next type of map and represent data associated with point locations such as cities. Data is displayed on these maps with proportionally sized symbols to show differences in occurrences. Circles are most often used with these maps but squares and other geometric shapes are suitable as well. The most common way to size these symbols is to make their areas proportional to the values to be depicted with mapping or drawing software.
Another thematic map is the isarithmic or contour map and it uses isolines to depict continuous values like precipitation levels. These maps can also display three-dimensional values like elevation on topographic maps. Generally data for isarithmic maps is gathered via measureable 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 is a high and low side in relation to the isoline. For example in elevation if the isoline is 500 feet (152 m) then one side must be higher than 500 feet and one side must be lower.
A dot map is another type of thematic map and uses dots to show the presence of a theme and display a spatial pattern. On these maps, a dot can represent one unit or several, depending on what is being depicted with the map.
Finally, dasymetric mapping is the last type of thematic map. This map is a complex variation of the choropleth map and works by using statistics and extra information to combine areas with similar values instead of using the administrative boundaries common in a simple choropleth map.
To see various examples of thematic maps visit World Thematic Maps