The Means of Visual Communication
Jacques Bertin proposed a group of 'visual variables' that has since been added on to by other cartographers (such as: Morrison, 1974; McCleary, 1983; and Woodward, 1991). This group of variables constitutes those representational techniques that a cartographer or illustration designer has in the creation of an image. The list includes: location, size, value, hue, intensity, orientation, shape, texture, arrangement, and focus (see Figure 1.7). This list is not necessarily all inclusive, but the list is useful in the design of maps and graphics. Each of the variables can be used to establish visual isolation (difference from surroundings) and visual levels (greater noticeability) (see Figure 1.1 on See Figure 1.1), and according to Bertin, these visual variables have certain levels of measurement that are commonly associated with them, and thus allow representations that convey the character of the data.
Figure 1.7 The visual variables as presented by DiBiase, Krygier, Reeves, MacEachren, and Brenner (1991). All have been done in ARC/INFO 6.0, but some (such as texture for point symbols, orientation for line symbols, and arrangement for area symbols) are more difficult and, thus, less useful than others.
In a static map, the use of location is limited to the signification of the spatial place of an item, although in orthogonal displays, location is a flexible tool because of the possibility of specifying the viewpoint on a simulated three-dimensional surface. In multiple maps, or dynamic mapping, change in position can be used to show movement of a feature. It is inherently an interval/ratio variable, but can be used to depict all levels of measurement.
Size is most often used to depict an ordinal variable, although interval/ratio variables can also be depicted using size. Because a larger symbol is almost always associated with 'more,' this is the context that it should generally be used in.
Of the 'color' variables, Bertin (1983, 42) only identified value and hue. Value, like size, has a distinct range from more to less and is therefore good for mapping ordinal data and can also be used for interval/ratio data. ARC/INFO refers to value as lightness. Hue can be used to depict ordinal or interval/ratio data because the frequencies that constitute hue are ordered, but these orders are not always readily remembered and used; hue (at a constant value and intensity) is therefore best used for nominal data. Intensity, which is also called saturation, has a distinct range from more to less and is therefore good for ordinal data.
As availability increases for the means of creation of high resolution color displays, use of the color visual variables will become an even greater part of cartography. Color can be specified in several ways (Dent 1985). One common and useful way is the Munsell color system (see Figure 1.8); it is based on the human perception of color (and is similar to the Tektronix color cone, and a color specification system in ARC/INFO, Hue-Lightness-Saturation). This system divides color into the three visual variable categories value, hue and intensity.
Figure 1.8 The Munsell color system; for example, an intense yellow would be specifies as 5Y7/14.
Value is the measure of the lightness or darkness of a surface; it is the total amount of light that is reflecting or emitting from a surface measured relative to the human ability to discriminate black from gray up to white. It is the only measure of light along the white-gray-black continuum, because all frequencies of light should be equally present.
Hue is the term most often meant when the word 'color' is used. The Munsell color system records hue as an angular measure around a color space. It represents the modal value of the frequency of light that is reflecting or being emitted from a surface. When a light has no modal frequency, the perceived light is along the white-gray-black continuum, which constitutes the central axis of the Munsell color space.
Intensity is a measure of the purity of the spectral frequency of light. It represents the variance of the frequency of light; greater variance means that a larger range of frequencies of light are being emitted or reflected from a surface--that the surface has more gray in it. A feature of intense colors is that they tend to be more noticeable (stand out more) than less intense colors, even when the total amount of light reflected or emitted may be the same, which makes intensity good for establishing visual hierarchies. Because of the human visual system, the value of the most intense color of different hues varies, with yellow having the highest value for its maximum intensity. This change is accounted for in the Munsell color system; the Tektronix color cone assumes that maximum intensity for any color occurs at the midpoint of the value scale.
For cartographic use in displaying ordered data, color hues are generally arranged to allow ease of interpretation. There are many possible color schemes, many having been suggested for terrain shading. For other data, the spectral ordering of hues may be the most obvious for use in mapping ordered data, but because colors can shift in intensity and value, this may lead to misleading maps--yellow will stand out in the spectral pattern. Two suggestions are of note for remedying this problem. The first is the use of a part-spectral scheme: yellow through orange to red; or, yellow through green to blue. This can allow redundancy in the visual variables of intensity and value, which reinforces the hue progression. The second is the use of hues ordered on the basis of value. This may be a viable alternative, but could prove confusing to those who know the spectral sequence of hues.
The ability to discriminate colors has received some study, particularly in the discrimination of values. The Munsell color system divides the range from black to white into eleven steps, as does the system for black and white photography developed by Ansel Adams (Upton and Upton 1989, 313). For most cartographic use, eleven steps would be both difficult to create and difficult to interpret; most cartographic research indicates that the use of half that range (five or six) is better for map design. If value gives an indication for the amount of discriminability that can be expected for intensities, four intensity values is probably the most that should be used. Discrimination of hues for cartographic use is affected by the apparent changes in intensity and value that occur as hue is changed, but for univariate symbolization, five steps should be easily discriminable in one of the part-spectral sequences.
Colors are produced by two methods: color addition and color subtraction. The color addition process is most commonly seen in color monitors for computer displays and television. It involves the mixture for red, green and blue to create colors on a black surface with all three being used for white. The color subtractive process is used for the production of printed material, such as paper maps. It involves the use of cyan, yellow and magenta (and black) to subtract colors that would reflect from the surface of white paper. Because the overprinting of cyan, yellow, and magenta generally leaves muddy brown, black is often used as a fourth color in the printing process. Colors are obtained by overprinting the four separates, with each offset slightly in order to allow the apparent mixing of colors in a dither pattern.
The production processes must be kept in mind when choosing color schemes on a color monitor that will be printed on a paper output device. Colors on a computer display may not appear the same on a printed sheet, because of the change in creation method. In addition, on a computer monitor, a point can take on any of the possible colors generatable by the graphics system, and resolution is independent of color. On paper output, the dither patterns that are used to create the appearance of many colors may cause a change in apparent color and, more significantly, output resolution. ARC/INFO allows the use of color lookup tables in commands such as HPGL2 (ESRI ARC Command References, 1991) to enable redefining screen colors to pretested printer colors to help compensate for changes in color; changes in resolution must be accounted for by the cartographer.
A final consideration for color (particularly hue) is its social interpretation. Colors can have meanings that must be taken in to consideration when designing maps and graphics, for example: red as stop; yellow as caution; green as go. For environmental maps, use of red (particularly, intense red) can indicate imminent danger, and with dark shades of other colors (blues, grays and browns, for example) can evoke a sense of foreboding or futility (this is demonstrated in The Nuclear War Atlas and movies such as Blade Runner). Other color schemes can invoke other reaction: pastels (colors of low intensity and high value) can indicate serenity (unclassified maps by the Central Intelligence Agency often make use of pastels); intense colors such as yellow and cyan grab attention, and can indicate happiness--they are often used in maps for children.
The final three variables that Bertin identified are orientation, shape and texture. Orientation is readily discriminated by the human visual system and is therefore good for indication of nominal data, although by using common symbols such as clock faces, ordinal and interval/ratio data can be displayed with orientation. Shape should be used for nominal data categories, because shapes in general do not have an apparent order and are not as readily distinguished as other visual variables. Texture is the relative coarseness of an area fill, and because rough textures appear closer than fine textures, texture is good for establishing visual hierarchies. Texture should generally be used for nominal data, but it can also be used for ordinal and interval/ratio data.
Unlike orientation and texture, shape has a continuum of possible representations: from mimetic to abstract (see See Classifying Space). Mimetic symbols convey the appearance of what they represent (an animal's outline represents sightings of that animal). Abstract symbols must be defined (a legend indicates that triangles represent sightings). ARC/INFO provides many abstract symbols, a few that are less abstract (for example, an anchor that might be used to represent a marina), and the ability to define new symbols (see Map Display and Query). Selection of appropriate levels of abstractness must be considered in designing a display--use of mimetic symbols may allow less dependence on the legend, but too many mimetic symbols may give the appearance of a map oriented toward children.
Two additional visual variables that have been identified since Bertin proposed his list are arrangement and focus. Arrangement is the order of symbols in an area fill (from regular to random to clustered), and nominal and ordinal data can be represented by changes in arrangement. Focus is the last visual variable; it is the crispness of the edges of symbols. Because this has an apparent order, focus can be used to display ordinal data and could be used for interval/ratio data, although too much variability of focus may make a map hard to read.