Thematic Maps

As you learned in the Intro to Mapmaking with QGIS, there are two main kinds of maps: reference maps and thematic maps. While reference maps are contextual, providing a ‘lay of the land’ as it were, thematic maps visualize the results of some kind of spatial analysis.

Writes Statistics Canada: “A thematic map shows the spatial distribution of one or more specific data themes for standard geographic areas.” Thematic maps render the results of spatial anlaysis. QGIS defines spatial analysis as:

the process of manipulating spatial information to extract new information and meaning from the original data. Usually spatial analysis is carried out with a Geographic Information System (GIS). A GIS usually provides spatial analysis tools for calculating feature statistics and carrying out geoprocessing activities such as data interpolation.

Below are examples of different thematic maps, all visualizing chestnut street trees by Vancouver neighborhoods.

Choropleth maps

Choropleth maps are useful to visualize and compare the density, frequency, or quantity of a value generalized across standardized geographic areas (such as zip-codes, provinces, or countries). See Axis Map’s guide to choropleth maps for more. Unless you specifically want to emphasize differences in total number of events/data points, it is best practice to normalize your data when choropleth mapping. Normalization is when you divide the values for each geographic area by something like the area in square kilometers or total population of that area. For instance, mapping winter flu cases across census tracts in British Columbia, you’d want to normalize the total cases in each census tract by that tract’s total population. Normalization enables better comparison across multiple geographic areas. See Axis Maps for more.

The map below shows total chestnut street trees per Vancouver neighborhood.

chropleth map

The map below shows chestnut street trees as a fraction of total street trees per Vancouver neighborhood.

chropleth map normalized

Each map serves a purpose. It’s simply important to consider what information you want to convey with your map.

Proportional Symbol maps

Choropleth maps use a color gradient to convey value differentials, whereas proportional symbol maps use symbol size. Proportional symbol maps are useful to visualize the quantity of something across respective locations. Proportional symbols are quite intuitive, and can be combined with other parameters like color and lettering size to provide rich spatial information. Proportional symbols can even be layered atop a choropleth map. See Axis Map’s guide to Proportional Symbols or School of Cities for more.

prop symbol map

In most cases you do not normalize values when using proportional symbols, as that would reduce the range in difference. If anything, it can be useful to exaggerate the range slightly. While Absolute Scaling renders symbols increasingly larger along a linear scale, Perceptual/Apparent Scaling compensates for the eye’s tendency to reduce difference in sizes close together. See here for more.

Dot Density maps

Dot density maps are useful in visualizing the concentration and distribution of discrete incidents. Each dot can represent an event (e.g., an earthquake), or a multiple such as 10. Dot Density maps can over-simplify. See Axis Map’s guide to Dot Density Maps for more.

dot density map

Heatmaps

Heatmaps are useful in visualizing the intensity or frequency of occurrence. Heatmaps can be thought about as generalized dot density maps.

heat map

Cartograms

Cartograms distort area to emphasize the value associated with a geographic region. When using cartograms, it’s important to consider whether your audience is already familiar with the un-distorted geography, otherwise they might not glean the added information.

cartogram map

There is a case to be made that all maps are thematic, as the definition of boundaries, borders, names, etc. is a political - and almost always contested - act. In other words, there are no neutral maps that simply, impartially, represent an objective reality or truth. See Crampton and Krygier (2006) or Harley (1992) for a seminal introduction to critical cartography, or Wang and Liu (2022) for an overview of critical cartography and GIS through the last several decades. See also the classic by Denis Wood, The Power of Maps. For an excellent read on the power of Google Maps, see Digital territories: Google maps as a political technique in the re-making of urban informality.


Static or dynamic?

While outside the remit of this workshop, may be important to you is whether your map is static or dynamic. Both reference maps and thematic maps can be either static or dynamic. Static maps tell a spatial story at a single scale. Static maps can be exported/stored/formatted as an image (.jpeg or .png), can be exported as a pdf, printed or embedded digitally into website or online publication. They can also be included in an academic paper, poster, or flyer. Dynamic maps, on the other hand, allow the user to interact with your spatial story. Dynamic display data in an interactive fashion, allowing viewers to pan around and zoom in and out to reveal more information at different scales. This workshop focuses on static reference maps. For more on making web maps, please see our Research Commons Workshop on Webmapping.



Resources for Thematic Mapping

View in GitHub

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