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Choosing map symbols and objects

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The practice of cartography, or mapmaking, exists for a long time. Although a computer makes the process easier and offers the ability to explore and edit designs, as a cartographer you must still understand the basic principles behind portraying spatial data and the aesthetic challenges in designing an effective and attractive way to communicate ideas.

Map designers may choose a variety of strategies for symbolizing features in a map. A data layer may simply be portrayed using one symbol, or different features can be assigned different symbols depending on the value of an attribute field. Point data are shown using marker symbols, line features with linear symbols, and polygon features with shaded area symbols.

Cartographers have many ways to signify differences between features in a map: shape, size, thickness, line type, color, pattern, and font. Traditionally these variations are used to show either changes in category (the type of thing) or in quantity. Shape, line type, pattern, and font are typically used to show changes in category, such as different types of wells (points), different classes of roads (lines), or different geologic units (polygons). Size and thickness are generally used to indicate increases in quantity, such as the population of a city (points) or the discharge of a river (lines). Text symbol variations usually indicate categories (towns versus rivers), although font size can indicate qualitative differences in value, such as town size, as long as not too many different sizes are employed.

Color may be used to indicate either category or quantity. Colors may be designated using one of several common methods. The RGB method is based on mixing red, green, and blue light on a computer screen. The CMYK method is often used for printing and specifies mixtures of inks used in printers or plates (cyan, magenta, yellow, black). The HSV method is instructive for discussing the use of color in portraying features on a map.

HSV stands for hue, saturation, value
Hue refers to the shade of color, such as red, blue, or yellow,  and is established by the wavelength of the light observed. Typically it is portrayed as a color wheel so that the color values range in degrees from 0 to 360.

Saturation corresponds to the intensity of the color and is measured as a percentage. Imagine mixing a can of paint, starting with a white base and adding a single pigment - a small amount of pigment yields a low saturation but a large amount of pigment results in high saturation.

Value refers to how light or dark the color is, transitioning from the full color to black. Value is also measured as a percentage. Any color can be defined using a combination of the three properties.

The set of colors chosen for a map layer should follow certain guidelines. A set based on variations in hue should be used to depict changes in category, such as different soil units. For categories, the saturation and value of the colors should be similar. Quantities are generally indicated using differences in saturation and/or value, with light or unsaturated colors indicating lower quantities and darker or more intense colors indicating higher quantities. Sometimes a divergent color set  is helpful in showing variation around a significant middle value; for example, a climate change map showing increases or decreases of temperature from current values. Areas with no significant change are shown in the middle neutral color, colder temperatures are shown in blue, and warmer temperatures are shown in orange.

The importance of black and white symbol schemes cannot be neglected. In commercial printing or copying, color still costs more than black and white. Publishing figures in professional journals or reports may also cost significantly more in color. One should consider, too, how most viewers will see the map. Figures in a master’s thesis may look wonderful in color, but some of the people reading it may receive a black and white copy version from interlibrary loan. There are many reasons why one might wish to design a map in black and white at the outset. In a black and white map, only four or five different shades of gray are typically discernible, so selecting symbols for such maps relies heavily on variations in shape, size, thickness, line type, or pattern rather than value.

Combinations of these factors may also be used when specifying symbols, such as using points that change in both shape and color, polygons that change color and pattern, or text that has differences in size and font and style. However, combinations should be used sparingly, as the more complex and varied the symbols become, the more difficult it may be to interpret them.

Color choice is complicated by the fact that about 10% of men and 1% of women are color blind. Different types of color blindness are possible, although red-green color blindness is the most common. If layers are being symbolized for data exploration by one person, then the user may employ any set of colors that aids interpretation of the data. However, if the maps are intended for use by many people, it is better to avoid red-green color sets and rely on saturation, value, pattern, and shape variations instead. If divergent colors or multiple hues are needed, combinations such as brown-purple or orange-blue can be interpreted by most color-blind viewers.

Map designers also need to be sensitive to conventions and connotations associated with different colors or symbols. A convention refers to the use of a particular color or symbol in a commonly understood way; for example, using blue for displaying water, blue-red for contrasting cold-heat, or a blue cross to indicate a hospital. Using conventional symbols aids interpretation of the map.

Connotation is an emotional or psychological impact associated with a particular symbol, such as the color red indicating danger, or red-white-blue evoking feelings of patriotism. However, connotations are often culturally specific. Red may indicate danger in the United States, but is associated with joy in Thailand, or national pride in China. Making maps for an international audience demands particular care and knowledge in the choice of colors and symbols. Context can matter as well; even in the United States, a red-green map at Christmas might engender more joy than fear.

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