Cartography is "the science or art of making maps" (Merriam-Webster 2020). The broad objectives of cartography are:
- Creating maps that are useful
- Creating maps that are beautiful
The balance between those two objectives is dependent upon the intentions of the cartographer. There are maps that are created entirely as works of art with no clear utility, and there are maps that are designed to be entirely functional with little aesthetic consideration. However, most cartographers strive to achieve both objectives within the needs and constraints of a particular project.
The determination of whether a map is "good" or not is subjective and evaluated within the norms of the community where the map is created and used. However, there are cartographic techniques and conventions that have been developed by cartographers over hundreds of years of map making that are commonly accepted in the cartographic community. This tutorial will cover some high-level cartographic design concepts.
This tutorial draws heavily on chapter 12 from Slocum et al. (2009) Thematic Cartography and Geovisualization, as well as David Lauer and Stephen Pentak (2000) Design Basics. You may consider consulting those texts if you want more detail on the concepts introduced here.
The layout of a map refers to how the map content is presented on a printed page or electronic display. The figure below shows some layout elements common to a variety of map designs.
Maps for figures in documents such as reports or articles have less space to present information, so cartographers either omit elements not essential to effective communication, or present that information at different points in the document, such as captions, the text of the article, or the bibliography.
This example with minimal map elements is from an article by in the Annals of the American Association of Geographers. While the map is in color, the thematic symbols vary by lightness, which will make it legible in the grayscale (black and white) printed version.
This example with a fuller set of map elements is also from the the Annals of the American Association of Geographers. Note that the cartographers have chosen to create this graphic in grayscale to avoid any grayscale printing compatibility issues.
Web maps often retain many of the map elements of traditional print cartography, but the interactive nature of web maps and the varying display sizes (smartphones to large-format monitors) creates opportunities and limitations.
This is an example of a web "dashboard" containing a map that was a popular type of website during the COVID-19 pandemic.
Map layouts utilize a design concept called grids, which organize the placement of map elements relative to each other. This usage of the term grid should not be confused with the grid lines that show lines of latitude and longitude on a map.
Field (2016) defines two broad types of grids:
- Skeletal grids keep groups of map element in sections in fixed-size rows and / or columns.
- Interval grids keep groups of map elements in varying-sized sections separated from each other by intervals.
A thematic map "is used to display the spatial pattern of a theme or attribute" (Slocum et al. 2009, 1). For example, this is a city planning map of different land use types in Spokane, WA from their 2017 comprehensive plan.
Thematic maps exist in contrast to general-reference maps (often shortened to reference maps) that are focused on areas rather than themes, and provide general information rather than information about a specific characteristic or type of feature.
On a thematic map, graphical characteristics of thematic symbols that can be varied based on the variables in the data are called visual variables. Although area maps are used below for examples, these visual variables exist in point, line, and raster visualizations. They can also be combined, such as symbols that vary both in color and shape.
For quantitative maps, Slocum et al. (2009, 82) suggests that there are six different visual variables.
The four visual variables for qualitative data are similar, although some (like spacing, lightness, and saturation) are omitted because they do not offer adequate contrast between different categories.
Intellectual and Visual Hierarchy
Maps tell stories about the world and the relationship between aspects of that world in space. Accordingly, there is a hierarchy to the different elements in a map. Some elements are important to the story and need to be emphasized, while other elements are supporting characters that need to stay in the background visually.
Monmonier (1993) asserts that there is a intellectual hierarchy or scale of concepts that governs what should be considered most and least important on a map (Slocum et al. 2009, 214).
The size, content, and arrangement of layout and thematic elements can be adjusted to increase or decrease visual weight.
The choice of which map elements get greater or lesser visual weight communicates a visual hierarchy about what is most and least important on a map.
One definition of good basic cartographic design is that the visual hierarchy in the design reflects the intellectual hierarchy. Monmonier's hierarchy (from most to least important) is:
- Thematic symbols, and labels directly related to the theme
- The title, subtitle, and legend
- Base map information (boundaries, roads, place-names, etc.)
- Scale bar and north arrow
- Data source information
- Frame and neat lines
Design is "the arrangement of elements or details in a product or work of art" (Merriam-Webster 2021). Lauer and Pentak (2000, 4) refer to design as "visual organization" that is the result of a design process.
Lauer and Pentak (2000) define a fundamental set of design principles that guide the design process. In cartography, these principles follow from the central purpose for the map, and the story the cartographer is trying to tell. The design principles then inform choices made with design elements that are used by the cartographer to translate the intellectual hierarchy into a visual hierarchy that allows the map realize its purpose. These principles include:
- Emphasis / focal point
- Scale / proportion
Emphasis / Focal Point
Thematic maps often have a central focal point that stands out among all the other elements on the page. This represents the highest element in the visual hierarchy There can be secondary points of emphasis called accents.
With thematic maps, the concept of figure-ground relationship follows from the principle of emphasis. The thematic symbols are high in the visual hierarchy (figure) while all the other elements exist to give context (ground). Accordingly, the figure should stand out over the ground.
For example, in the pair of maps below, the map on the left has weaker focal point emphasis. While the state counties are in the center of the layout, overlapping features and indistinct colors obscure the central point of the map as a display of election results.
In contrast, the bold colors for the features (figure) in the map on the right and the more muted choices for the supporting elements (ground) draw focus more clearly to the features and make the visual hierarchy reflect the intellectual hierarchy.
Reference maps often do not have as clear a set of emphasized elements since their purpose is to provide general information. However, reference maps like highway maps emphasize roads with heavier lines and bolder colors over features like city boundaries or waterways.
Scale / Proportion
Scale refers to size, and proportion refers to size in relation to other objects.
The concept of proportion with maps might also be extended to location relative to features on the surface of the earth. One way in which proportion is communicated with thematic maps in in the use of a base map that sits under the thematic symbols and gives locational context to the features.
For well-known geographic areas (such as world countries, or US states as a whole), no base map may be necessary and may add clutter. However, when mapping a local area that may be unfamiliar to the map users, a base map can provide useful geographic context to understand the relationship between the phenomena being depicted, and the real-world locations of that phenomena on the surface of the planet.
In our bad map / good map example, the map on the left provides no visual indication of where the areas are, or even what they are since Wyoming is a rectangle and no naming is given.
In contrast, the map on the right, provides the names and borders of adjacent states, as well as a name in the subtitle. Although not particularly relevant for this thematic map, a scale bar would provide additional scale information for evaluating the amount of area covered by the different counties.
Balance is the distribution of visual weight across the page (Lauer and Pentak 2000, 76). More specifically to maps, Slocum et al. (2009, 218) define balance as "the organization of map elements and empty space that results in visual harmony and equilibrium"
Balance implies an efficient use of available layout space while avoiding clutter.
While balance can also imply symmetry, there can be balanced assymetry where multiple small elements on one side of the page balance with a large element on the other side of the page.
In our bad map / good map example, the map on the left clumps and overlaps elements, while also leaving unused space on the right side of the layout.
In contrast, the map on the right the titles are centered while the legend and credit information asymmetrically balance each other visually.
Rhythm in design refers to patterns of repetition (Lauer and Pentak 2000, 100).
Rhythm in map layout is represented by layout elements like legends (with consistently styled entries) or grids that align elements in a regular pattern along the margins of the map.
Mapped thematic features often have their own rhythm embedded in autocorrelation (clustering or dispersion). These rhythms can be in counterpoint to the regular arrangement of latitude and longitude lines in a graticule.
Making such rhythms visible requires feature contrast, which refers to "visual differences between map features that allow us to distinguish one feature from another" (Slocum et al. 2009, 215). Clear feature contrast facilitates clear communication of quantitative and qualitative information, and adds aesthetic appeal by providing visual variety.
In the example bad map on the left, the clumping of layout elements in the bottom left corner disturbs the visual rhythm of the layout grid. The almost indistinguishable colors of blue for the features gives poor feature contrast that obscures any rhythms in the patterns of distribution.
In contrast, the map on the right adheres to a layout grid with the title/subtitle, map area, and bottom marginalia forming a clear vertical pattern, while the legend and credits text form a clear horizontal pattern. The outlines of the surrounding states provide a somewhat regular pattern that contrasts with the irregularity of vote distributions across Wyoming's counties.
Lauer and Pentak (2000, 20) assert that, "Unity, the presentation of an integrated image, is perhaps as close to a rule as art can ap proach. Unity means that a congruity or agreement exists among the elements in a de sign; they look as though they belong together, as though some visual connection beyond mere chance has caused them to come together."
Since unity relates to an overall impression, it is subjective and encompasses the other principles given above. Slocum et al's (2009) concepts of harmony and equilibrium are similarly subjective, and achieving those qualities in design often require some level of experimentation with a particular map design.
In the bad map example on the left, the blue / black palette does provide some visual unity, albeit at the expense of other principles described above. However, the inexplicable use of different fonts and inconsistencies between the intellectual and visual hiearchy gives the map a disunited feel.
In contrast, while the map on the left uses a diverging color scheme, the colors compliment each other. Font styles and sizes are used consistently across the map. While not particularly compelling, the map does project a unity of purpose.
As seen in the critiques above, the design principles are embodied in design choices made with design elements. Lauer and Pentak (2000) define seven different design elements:
- Shape / Volume
- Illusion of space
- Illusion of motion
Cartographic Design Procedure
Creation of maps that utilize these design principles can be facilitated by a systematic approach to each project that forces consideration of those general concepts in the context of the specific requirements of each particular project.
The classical waterfall technique is a requirements-driven approach that involves clear definitions of the specification for the project up front, with design following only after those specifications are codified. This approach is most effective where the data and the ultimate objectives are clear from the beginning, and the cartographic layout will be complex, such as for an urban planner preparing a map of proposed zoning changes for a public meeting. This approach dates from a time when data was scarce, and cartography required difficult, time-consuming manual drafting work to create compelling visualizations.
However, there are commonly situations (notably in research) where a complete understanding of the data or the ultimate objectives are not clear from the beginning of the process. Exploratory data analysis is an iterative, data-driven approach that integrates cartography with the research process. This approach is possible because of the availability of comparatively fast and inexpensive GIS technology, as well as increased availability of massive amounts of geospatial data.
In practice, elements of both approaches are often used, and thinking through your projects before expending the often extensive amount of time needed to create a complex cartographic design is usually a preferred approach that ultimately can save time and effort.