Visual variable

A visual variable, in cartography, graphic design, and data visualization, is an aspect of a graphical object that can visually differentiate it from other objects, and can be controlled during the design process. The concept was first developed by Jacques Bertin, a French cartographer and graphic designer, and published in his 1967 book, Sémiologie Graphique. Bertin identified a basic set of these variables and provided some guidance for their usage; the concept and the set of variables has since been expanded, especially in cartography, where it has become a core principle of education and practice.

History

Bertin developed his theory of "retinal variables" while a cartographer at the École pratique des hautes études (EPHE) in Paris. Despite having a background in cartography, and deriving many of his ideas by evaluating maps, he intended for Sémiologie Graphique to be applied to all forms of graphic design and information visualization. By the 1970s, it was gaining international acceptance, and was eventually translated into many languages, with the 1983 English edition probably being the most widely read.

Sémiologie Graphique suggested seven variables: position, size, shape, value, hue, orientation, and texture. As

Each of these variables may be employed to convey information, to provide contrast between different features and layers, to establish figure-ground contrast and a clear visual hierarchy, or add to the aesthetic appeal of the map.

Map symbols commonly employ multiple visual variables simultaneously. This can be used to reinforce the depiction of a single property; for example, a capital city having a symbol that is larger and a different shape than other cities, or a color progression on a choropleth map from pale yellow to dark green, using both hue and value. Alternatively, different visual variables may be used to represent different properties; for example, symbols for cities may be differentiated by size to indicate population, and by shape to indicate provincial and national capitals. Some visual variable can be combined harmoniously to make a map clearer and more informative, while other combinations tend to add more confusion than usefulness. For example, early experiments with using Chernoff faces on maps have been criticized as difficult to interpret correctly.

Position

While the absolute location of a feature on a map is usually unalterable, the position of labels and other information can be controlled, as well as the non-map elements on the page.

Size

A cartogram representing population (a ratio or count-level property) by size.

The size of a label or symbol is how much space it occupies on a map. This commonly refers to the area of point symbols, and the thickness of line symbols. Size differences are relatively easy to recognize, making it a useful variable to convey information, such as a quantitative amount of something, or relative importance. Studies have shown that humans are better at judging relative differences in linear distance (e.g. one road being twice as thick as another) than relative differences in area (e.g., one circle having twice the area of another). Area differences are generally underestimated, but there is a large variation between people in ability to estimate two-dimensional size. Correctly estimating relative volume has proven even more difficult.

Because geographical features have an actual size on the Earth, this cannot always be controlled, and sometimes works against the wishes of a cartographer; for example, it can be difficult to make a world map in which Russia does not stand out. In a cartogram the size of features is purposefully distorted to represent a variable other than area.

Shape

National Park standard point symbols, using shape to represent different types of facilities, a nominal variable.

A shape is a simple design that is used to symbolize an attribute on a map. Shape is most useful for representing nominal categories of features, especially for point features. Some shapes are simple in nature and thus are more abstract, while other shapes are more pictorial and are easy for the reader to comprehend what is trying to be conveyed.

Some aspects of shape are inherent to the phenomenon and may not be easily manipulable, especially in line and region symbols, such as the shape of a road or a country. However, shape can still play a role in line and region symbols, such as a region filled with tree symbols or an arrowhead on a line. Also, the shape of a feature may be purposefully distorted by Cartographic generalization, especially when creating schematic representations such as many transit maps, although this distortion is rarely used to convey information, only to reduce emphasis on shape and location.

Color hue

Main article: Hue

Hue is the visual perceptual property corresponding in humans to the categories called red, green, blue, and others. Maps often use hue to differentiate categories of nominal variables, such as land cover types or geologic layers. Hue is also often used for its psychological connotations, such as red implying heat or danger and blue implying cold or water.

Color value

Population density (a ratio-level variable) represented as color value, with an intuitive correspondence (i.e., dark looks like more people). Value also establishes figure-ground (color vs. white). Hue does not carry information here, but serves an aesthetic purpose.

Main article: Lightness

As an aspect of color, value refers to how light or dark an object appears. Value effectively connotes "more" and "less," an ordinal measure; this makes it a very useful form of symbology in thematic maps, especially choropleth maps. Value contributes strongly to Visual hierarchy; elements that contrast most with the value of the background tend to stand out most (e.g., black on a white sheet of paper, white on a black computer screen).

Color saturation/intensity

The synergy of Saturation (color vs. gray), value (dark vs. light), and position (centrality) to strongly establish figure-ground and visual hierarchy

Main article: Colorfulness

The saturation of a color is its purity or intensity, created by the variety of light composing it; a single wavelength of light is of the highest saturation, while white, black, or gray has no saturation (being an even mixture of all visible wavelengths). Of the three psychological aspects of color, this is the least effective at conveying specific information, but it is very effective at establishing figure-ground and visual hierarchy, with bright colors generally standing out more than muted tones or shades of gray.

Orientation

Orientation refers to the direction labels and symbols are facing on a map. Although it is not used as often as many of the other visual variables, it can be useful for communicating information about the real-world orientation of features. Common examples include wind direction and the direction in which a spring flows.

Texture

Texture (dot density) representing disease incidence (a ratio or count-level variable), which gives the appearance of density. Also, saturation (color vs. gray) is used to create a visual hierarchy, and value (gray vs. white) establishes a figure-ground contrast for Africa.

Main article: Texture (visual arts)

Texture refers to the aggregate pattern made up of many individual symbols. For example, a dense network of street lines could collectively convey the concept urban area. An evenly spaced lattice of green dots could mean Orchard, while a random distribution of the same green dots could mean Forest.

Transparency/opacity

Transparency is used effectively here to indicate overlapping sovereignty claims.

Main article: Opacity (optics)

Main article: Transparency and translucency

A fairly recent addition, the control of opacity has become common in digital cartography. While it is rarely used to convey specific information, it is common to make features translucent to reduce contrast or to retain underlying information.

Non-visual variables

Following on the widespread usefulness of Bertin's variables, cartographers have proposed analogous sets of controllable variables for animated maps, haptic (touch) maps, and even the use of sound in digital maps.