Map symbol

Map symbols are the graphical tools cartographers use to represent real-world features on a map, functioning like other types of symbols. They can take the form of point markers, lines, regions, continuous fields, or text, and their visual design—including shape, size, color, pattern, and other graphic variables—conveys information about the phenomena they represent. These symbols serve multiple purposes at once: they indicate that a geographic feature exists, show its location and extent, display attribute data, affect the map's aesthetic appeal (for better or worse), and create an overall visual order that makes the map more or less useful, including establishing a clear visual hierarchy.

To represent spatial phenomena, symbols must match the spatial form of what is being depicted. Phenomena are categorized in two key ways for symbology: ontological form and dimensionality. When a symbol also represents a property of the phenomenon, the choice depends on the nature of that property, typically classified by its level of measurement.

In terms of ontological form, geographic phenomena fall into objects—things with a recognizable boundary and shape, like buildings, cities, roads, lakes, and countries—and masses, where boundaries and wholeness are not central to identity, such as air, water, vegetation, and rock. Mass phenomena are rarely shown directly; instead, symbols represent their properties, which usually appear as geographic fields like temperature, moisture, density, or composition.

Dimensionality determines the geometric primitive used for a symbol. The number of dimensions a symbol has may differ from the real-world feature due to cartographic generalization, which simplifies features based on the map's purpose and scale. For instance, a three-dimensional road is often drawn as a one-dimensional line, and a two-dimensional city is frequently shown as a zero-dimensional point.

When map symbols visualize a property or attribute of a feature, geographers and cartographers typically categorize these properties using Stanley Smith Stevens's classification system (or a revision like Chrisman's). Different symbol types and visual variables are better suited to intuitively represent certain levels of measurement, especially when the visual variable mirrors the kind of differences found in the attribute.

Cognition and semiotics explain why certain map symbols work. Early mapmakers often didn't care why symbols were effective, treating the human brain as a black box. Modern cartographers, however, seek to understand why some symbols are more effective, aiming to build a theoretical basis for how brains recognize symbols and to guide the creation of new ones. According to Charles Sanders Peirce's semiotic theory, map users "read" symbols by connecting the graphic mark (the sign) to a concept (the interpretant) and a real-world feature (the object or referent). Symbols are categorized by how they suggest this connection: iconic symbols resemble the real-world feature (e.g., a tree icon for a forest); functional symbols represent an activity (e.g., a skier for a ski resort); conceptual symbols represent an idea (e.g., a dollar sign for an ATM); conventional symbols have no intuitive link but are widely learned (e.g., a red line for a highway); and ad hoc symbols are arbitrary choices by the cartographer that require a legend to interpret (e.g., colors for geologic layers).

A map symbol is created by altering the visual appearance of a point, line, or region using one or more visual variables. French cartographer Jacques Bertin introduced this concept in his 1967 book *Sémiologie Graphique*, identifying seven main categories: position, size, shape, value, color, orientation, and texture/grain. Cartographers have since modified and expanded this set. Each variable can convey information, provide contrast between features and layers, establish figure-ground contrast and a clear visual hierarchy, or enhance the map's aesthetic appeal.