Visual variable

Although variations in the appearance of symbols are only limited by the imagination, there are a few categories into which they can be grouped. French cartographer Bertin distinguished six categories, which he called the visual variables, which may be applied to point, line and area symbols. As illustrated in Figure, the variables are:

• size;

• value (lightness);

• texture;

• colour;

• orientation; and

• shape.

Mentioned visual variables can be used to make one symbol different from another. In doing this, map-makers have, in principle, freedom of choice, provided they do not violate the rules of cartographic grammar. They do not have any such choice when deciding where to locate the symbol in the map: the symbol should be located where the feature belongs. Visual variables influence the map user’s perception in different ways. What is perceived depends on the human capacity to see:

• what is of equal importance (e.g. all red symbols represent danger), saturation differences;

• order (e.g. the population density varies from low to high—represented by light and dark colour tints, respectively);

• quantities (e.g. symbols changing in size with small symbols for small amounts); and

• instant overview of the mapped theme.

There is an obvious relationship between the nature of the data to be mapped and the “perception properties” of visual variables. In Table below, the measurement scales as defined in the Table are linked to the visual variables displayed in the Figure above. “Dimensions of the plane” is added to the list of visual variables; it is the basis, used for the proper location of symbols on the plane (map). The perception properties of the remaining visual variables have been added. In the next subsection we discuss some typical mapping problems and demonstrate the use of the principles that have been outlined.