5. The Cartographer's Palette: The Semiotics of Cartography

Cartographers employ symbols to represent location, direction, distance, movement, function, process, and correlation. These features of the real world are abstracted and symbolized on maps as points, lines, and areas. A tremendous amount of practice and skill is involved in choosing effective strategies for symbolization. One of the best ways to learn about these strategies is to consider the sorts of visual resources available to the cartographer.

5.1 The range of visual resources

As cartographers reduce the world to points, lines, and areas, they use a variety of visual resources. Jacques Bertin in his book The Semiology of Graphics (1983), inventories these resources using the categories of size, shape, value, texture or pattern, hue, orientation, and shape.

Cartographers can apply these resources to points, lines, and areas in a variety of ways.

Each of these resources can be used individually to draw attention to map features, or they may be used in combination. Sometimes cartographers deploy several of these resources simultaneously to stress particularly important information or to improve its legibility.

5.2 Strategies of symbolization

With such a range of resources available, the question arises as to what strategies to use in assigning these resources to points, lines, and areas. The strategies employed vary greatly depending on the nature of the phenomena being mapped. We distinguish among four levels of measurement in describing real-world phenomena:
Nominal data is information that is simply grouped into categories on the basis of qualitative considerations: a road distinguished from a river or a forest distinguished from an open field.

Ordinal data is grouped by rank on the basis of some quantitative measure: small, medium, and large cities or single-lane, double-lane, and four-lane roads.

Interval data is information that can be arranged using a standard scale along which operations of addition and subtraction have meaning. Temperature is an example of an interval measure.

Ratio data is information that, like interval data, can be arranged along a scale but, in addition, the scale begins at a non-arbitrary zero point. At the zero point, no features are present. The operations of multiplication and division can be employed with ratio data to consider proportions and magnitudes. Elevation above sea level, precipitation, and population are all examples of ratio data.

These resources can be used in a wide variety of ways and several may be used at once to highlight or reinforce a single relationship. Thus, for example, if only a single ordinal variable is being mapped, several visual resources may be deployed at once. If several ordinal variables are being displayed, the visual resources may have to be used more selectively.
USGS map symbols

5.3 Typography and lettering

Adding text to a map is one of the cartographer's most demanding tasks. The cartographer must be concerned both with the content and the form of the text, that is the wording and the way the text is displayed on the map. Indeed, the verbal content of a map is one of the most powerful communicational resources available to the cartographer. Its effective use is often the point of difference between high-quality, professional maps and less skillfully executed maps. Compare the following maps and notice how much difference lettering makes to overall readability. Pay special attention to the differences--the size of the lettering, how it is aligned, and the fonts and cases used.
A. Content

Concise, carefully formulated captions and annotations make a tremendous difference to a map. Compare:

One way to test the effectiveness of your text is to ask others to read and interpret a draft of your map. If they are confused or misinterpret the theme, rewording is essential. You should also check your map for redundancies so as to avoid repeating yourself in the title, legend, and annotations. That is, consider all of the map text together and balance its content to convey your message without being repetitious. Finally, avoid using abbreviations unless you are certain your audience will be familiar with their meaning.
B. Form

Bertin's semiotic resources apply to text just as they do to points, lines, and areas. However, with respect to text, some of these resources are given special names. Font refers to the shape and pattern of letters. Hundreds of fonts have been created since the invention of the printing press and are available using automated systems. Fonts are often grouped into several broad categories. A distinction is made between  and  fonts, as well as between Roman and Italic fonts. The weight of text is classified as , or .

fonts can be scanned more rapidly by most readers, although less information seems to be retained than would be the case if the same text was displayed with a  font. Italic fonts are used for the titles of books and journals and for some proper names. Since it was impossible to print italic characters with typewriters,  was used to indicate the placement of italic characters. It is no longer necessary to employ this convention with automated systems capable of printing italic characters. Traditionally the size of lettering was measured in picas or point size. Increasingly, automated systems measure size using conventional units such as inches and millimeters. The horizontal and vertical distances between lettering are traditionally referred to as leading , but increasingly today as inter-character and inter-line spacing.

Although automated systems offer a wealth of fonts and sizes, good practice dictates that these resources be used sparingly. Too many fonts (and sizes) can potentially confuse the reader. Traditionally, typographers try to use no more than four fonts or font sizes on a given page of print. Apply the same principle to your maps. Use different fonts and sizes only when you have a compelling reason to do so.

Lettering is also distinguished by case: UPPER CASE, lower case, and Mixed Case.

If you consider professionally produced maps, you find that font, size, and case are used very carefully to encode text. In effect, the text is used to group information into useful categories that reflect the theme of the map.

Special attention must be paid to the orientation of text with respect to the features being labeled. In this respect, text can be used as an important cue to different map features.

Point features--lettering "points" to feature, but try to avoid lettering across boundaries.
Linear features--lettering shows shape, but watch out for ambiguities.
Area features--lettering occupies the area.

In following these guidelines, conflicts will always emerge. On maps crowded with information it is nearly impossible to arrange all text without striking compromises among these principles. The point is to follow the guidelines as much as possible and, when conflicts do arise, consider options carefully in light of the map theme and competing cartographic elements.

The arrangement of letters can also be used to convey quite subtle distinctions. For example, in the following map, lettering pertinent the map theme is aligned with the map frame whereas lettering that describes the background features is aligned with the graticule.

Quite apart from labeling features, text is used for convey other information about the map--sources, date, methods of compilation, projection, and cartographer. This ancillary information is usually placed in a subordinate position within the frame of the map. However, the readability of your map will be improved if you position this text in relation to the major map elements. That is, your map can be composed with implicit margins and tabs that can be used as a means of alignment for subsidiary text.

5.4 Foreground-background (figure-ground) relationships
Good cartography involves bringing the most important map information into the foreground of the reader's attention, even though other detail must be displayed in the background to make the map intelligible. Differences between the foreground and background are critical to some maps where clear distinctions must be made between certain types of features like the boundaries between land and water bodies. Cartographers employ a number of devices to make it easier for readers to distinguish between these features and to sort out a map's most important message.

The key to resolving foreground-background relationships is to use color, value, and patterning to "raise" some map features into the foreground. In some respects this is like using color, value, and patterning to produce a sort of three-dimensional effect in which certain key features appear to rise off the map's two-dimensional surface. In fact, these resources can be used to create a sort of three-dimensional visual hierarchy.

This idea can be put to practical use in a wide range of situations where the reader would otherwise have difficulty sorting out information displayed on a map.

Understanding the dynamics of foreground-background relationships can also help resolve certain ambiguities that sometimes arise in mapmaking. Consider the following example:

In this map it is difficult to distinguish between water and land because both are arrayed at the same level of the visual hierarchy. This ambiguity can be resolved in several ways: shading, adding a graticule, vignetting, and lettering.

Depending the map and mapping system you employ, any of the options might be used either singly or in combination.

5.5 Highlighting the theme

The theme of your map must rise to the foreground. Now that you are aware of the cartographic resources available, you can use them to highlight the most important thematic information. This may involve using a vivid or saturated color to bring the information forward, using heavier line weights, or a contrasting, bold pattern. These resources can, of course, be used in combination.