A thematic map is a specialized map made to visualize a particular subject or theme about a geographic area. Thematic maps can portray physical, social, political, cultural, economic, sociological, or any other aspects of a city, state, region, nation, continent, or the entire globe. A thematic map is designed to serve a special purpose or to illustrate a particular subject, in contrast to a general map, on which a variety of phenomena appear together, such as landforms, lines of transportation, settlements, and political boundaries. This is in direct contrast to a reference map or Topographic map, which are designed to show the location of visible features of the landscape with minimal interpretation and intended to be used for a wide variety of purposes. Thematic maps also portray basic features such as coastlines, boundaries and places, but they are only used as a point of locational reference for the phenomenon being mapped.
Thematic maps also emphasize spatial variation of one or a number of geographic distributions. These distributions may be physical phenomena such as climate or human characteristics such as population density and health issues. Barbara 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 based on spatial patterns. Thematic maps are sometimes referred to as graphic essays because they display spatial variations and interrelationships of geographical distributions that can be interpreted.
An important cartographic element preceding thematic mapping was the development of accurate base maps. Mapping accuracy improved at a gradual pace, and even until the mid-17th century, general maps were usually 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.
One of the significant early contributors to thematic mapping was the English astronomer Edmond Halley (1656–1742). His first significant cartographic contribution was a star chart of the constellations of the Southern Hemisphere made during his stay in St. Helena and published in 1686. That same year, he published his first terrestrial map in an article about trade winds, and this map is called "the first meteorological chart".  In 1701 he published the "New and Correct Chart Shewing the Variations of the Compass", the first chart to show lines of equal magnetic variation.
Another example of early thematic mapping comes from London physician John Snow's early dot density map. Though disease had been mapped thematically, Snow’s cholera map in 1854 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 carrying the Cholera bacteria.choropleth map. Later on, Louis-Léger Vauthier (1815–1881) developed the population contour map, a map that shows the population density by contours or isolines.
Thematic Map Design
The primary purpose of a thematic map is to visually portray a non-visual phenomenon, usually the attributes of geographic features (e.g., the median income of a county). A good thematic map clearly shows geographic patterns that mirror patterns in the real-world phenomenon. For example, if a map reader looks at a map of income distribution, he or she should be able to quickly and intuitively identify geographic concentrations of wealth and poverty that would be the same as those seen in the field. Aesthetics is also an important goal: potential map readers are more likely to look at an attractive map and to spend enough time reading it to understand the patterns in the phenomena being represented.
A thematic map will typically consist of three types of information:
- Primary theme: the geographic phenomena that represent the topic being discussed. In a map of population density of a city, this would be population density. Most thematic maps have a single primary theme. Multivariate maps are also possible but are typically more difficult to design well.
- Supporting theme: a layer of information that helps to tell the story, such as those that offer possible explanations for the patterns found in the primary theme. For a city population density map, this could be population attractors such as shopping districts or highways, or exclusionary features such as water bodies or mountains.
- Reference theme: a layer of geographic features that usually have little to do with the theme of the map, but help map readers locate the thematic information in a context of recognizable geography. Roads, administrative boundaries, terrain, and latitude/longitude graticules are common reference layers.
Because these are in a clear conceptual order of importance to map readers (primary theme most important, reference least important), a well-designed thematic map should reflect this order in the visual hierarchy.
In addition, knowing the audience is of equal importance. Asking "Who will read the thematic map and for what purpose?" helps the cartographer decide how a map 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.
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.
Mapmakers must be careful in designing thematic maps that display too much information or suggest phenomena have a correlation when, in fact, they do not.
Qualitative vs. quantitative data plays a role in how the data will be best displayed. Qualitative map data is in the form of some subject, like vegetation type, and shows the presence or absence of the subject on the map. Quantitative map data is expressed in the form of a numerical value, like population values, or elevation in feet.
Methods of thematic mapping
Cartographers use many methods to create thematic maps. Most of these methods use a particular visual variable to visualize the attributes of geographic features. For example, Hue = Geologic Layer, Value = Income, Size = Population. The choice of an appropriate technique for a particular dataset depends on the scale of measurement of the attribute, the dimension of the geographic features (point, line, region), and the geographic pattern of the topic in question.
Choropleth maps are 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, due to Ecological fallacy misinterpretations. If derived values or normalized data can be obtained from raw data (such as population densities), then the choropleth technique can be applied.
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. Size can be a very intuitive visual variable for certain kinds of data: the larger the symbol, the greater the amount of something at a location . This technique is best for ratio-type attributes that represent the total amount of something, in which values less than zero do not occur (e.g., total population). Proportional Symbol Maps can map single variables or multiple values by varying the design of each symbol.
These maps, also known as contour maps, depict smooth continuous fields such as precipitation. They are also well-suited to displaying three-dimensional values such as elevation i.e; on topographic maps.
Isometric and isopleth are the two types of isarithmic maps. In both cases, they can effectively show gradual change over space better than methods like choropleth maps. Isopleth and Isometric maps display two different types of data. Isometric maps show lines of points with true data, meaning that data at that point is accurate (along the line of a topographical map labeled 500 feet, the points that make up that line are all 500 feet in elevation).  Isopleth maps, however, show lines of areal data (not point data like isometric), and therefore the lines approximate phenomenon. Along the isolines, however, the values do not equal exactly what the lines project.  The Space between the lines of defined value allow the map reader to infer the change of value from one line to the next.
Isarithmic maps can also be continuous tone, in which there are no isolines and the values are not classified, so each point is colored according to its exact value. This technique has become much more common as field data is now typically distributed in the raster data model.
Chorochromatic maps are used to map nominal data using various colors, color shades or symbols to distinguish classes. The boundaries of regions are defined by the data rather than arbitrarily as they are in choropleth maps. Examples of chorochromatic maps may include soil type maps, plant hardiness zone maps or language boundary maps.
A dot map uses dots to show the presence of a feature or occurrence and display a spatial pattern. A dot density map is a popular way to use dots in creating a thematic map. When one dot represents one phenomena or one object, it is referred to as one-to-one; if that one dot represents many phenomena or objects, it is referred to as one-to-many. If the dot map is a one-to-one map, it is important to make sure that each point is accurate in its spatial location on the map. If the one-to-one dots are not accurately placed, the map will be misleading and not as accurate as it could be. However, in a one-to-many dot map, the dots are not necessarily placed in exact spatial locations and may be a summary of a much larger or much smaller area than they represent.
Dasymetric Mapping is a hybrid technique in which a choropleth map is refined by ancillary data that provides additional information about the distribution of the topic in question, adjusting boundaries to produce something closer to an isarithmic map. For example, a population attribute organized by census tract might be more accurately portrayed by excluding water bodies, vacant land, and other land-use regions within which it is reasonable to infer that people do not live. Dasymetric 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. 
- Thematic Maps Map Collection & Cartographic Information Services Unit. University Library, University of Washington. Accessed 27 Dec 2009.
- Norman Joseph William Thrower (2007). Maps & Civilization: Cartography in Culture and Society. p.95.
- Barbara Petchenik (1979). From Place to Space: The Psychological Achievement in Thematic Mapping, American Cartographer 1.
- Maps and GIS. Accessed 28 Feb 2009.
- Ball, Laura (2009). Cholera and the Pump on Broad Street: The Life and Legacy of John Snow.
- Michael Friendly (2008). "Milestones in the history of thematic cartography, statistical graphics, and data visualization".
- . Proportional Symbol Maps, Indiemapper. Accessed 16 Nov 2015
- Isometric Line. GIS Dictionary, Esri. Accessed 28 Oct 201
- Isopleth. GIS Dictionary, Esri. Accessed 28 Oct 2011
- Dasymetric Mapping. GIS Dictionary, Esri. Accessed 13 November 2012
| This article needs additional citations for verification.
Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (March 2009)
|Wikimedia Commons has media related to: Maps by theme|
- P. Muehrcke et al. (2001). Map Use, The University of Chicago Press, 4th Edition.
- Arthur H. Robinson (1982). Early Thematic Mapping in the History of Cartography, The University of Chicago Press,
- Arthur H. Robinson, et al. (1995). Elements of Cartography, Wiley, 6th Edition.
- T. Slocum, et al. (2005). Thematic Cartography and Geographic Visualization, Prentice Hall, 2nd Edition.
- N. Thrower (1996). Maps and Civilization: Cartography in Culture and Society, The University of Chicago Press.
- MapsGeek An online free application to build and share thematic maps.