Visual Variable

Any graphical object, including point, line and area symbols on maps, is drawn using a collection of visual devices that could potentially be drawn in different ways (hence "variable") and chosen by the designer. Although cartographers have been using these devices since the invention of maps, this concept was first developed during the advent of academic cartography in the mid-20th Century and systemized by Jacques Bertin in his 1967 book, Sémiologie Graphique.

In Sémiologie Graphique, Bertin recognized a core set of graphical variables that can be chosen and controlled to create symbols on maps and other graphic design media. This set has since become canonical and has been extended. The most commonly accepted visual variables include:

• Shape
• Size
• Orientation
• Color, including hue, value/lightness, saturation/chroma/intensity, and transparency/opacity
• Texture or pattern in a composite symbol, including the grain/spacing and arrangement of its constituent symbols

These variables can be used to establish contrast between different geographic features, build a visual hierarchy, and represent quantitative or qualitative attributes of the features.

• A road (line symbol) drawn as purple (color) and 1mm thick (size).
• Cities (point symbols) drawn as black (color) circles (shape), with their diameter (size) varying by total population.
• A lake (area symbol) drawn with a light blue fill (color), with a slightly darker blue "wave" pattern, with individual waves being semi-randomly arranged and generally 10mm apart (grain)