Map symbol

A map symbol, also called a cartographic symbol, is a graphic mark that stands for a real-world feature on a map, much like symbols used in other contexts. These symbols can take the form of points, lines, areas, continuous fields, or text. Their visual appearance—including shape, size, color, and pattern—can be adjusted to convey different types of information about the feature they represent.

Map symbols serve several functions at once: they indicate that a geographic feature exists, show where it is and how much space it covers, communicate details about its attributes, affect the map’s visual appeal (either improving or harming it), and help create an overall visual order—such as a clear hierarchy—that makes the map more or less useful.

**Representing spatial phenomena** Symbols represent geographic phenomena, which come in various spatial forms. Different symbol types are used for different forms. Phenomena can be classified in several ways, but two are especially important for symbology: ontological form and dimensionality. When a symbol also represents a property of the phenomenon, the choice of symbol depends on the nature of that property, typically categorized by its level of measurement.

**Ontological form** Geographic phenomena can be divided into objects—things with a clear boundary and identity, like buildings, cities, roads, lakes, and countries—and masses, where boundaries and wholeness are not essential, such as air, water, vegetation, and rock. Masses are rarely shown directly on maps; instead, symbols portray their properties, which usually take the form of geographic fields like temperature, moisture, density, or composition.

**Dimensionality** The number of spatial dimensions needed to represent a phenomenon determines the geometric primitive used, and each primitive is drawn with a different type of symbol. The dimensionality of a map symbol may not match the real-world feature’s dimensionality; this difference results from cartographic generalization, which simplifies features based on the map’s purpose and scale. For example, a three-dimensional road is often shown as a one-dimensional line, and two-dimensional cities are frequently represented by zero-dimensional points.

**Level of measurement of property** Many map symbols show not only where a feature is and its shape, but also one or more of its attributes. Geographers and cartographers typically classify these attributes using Stanley Smith Stevens’ system (or a revision like Chrisman’s). Different symbols and visual variables are better at intuitively representing some levels of measurement than others, especially when the visual variable matches the kind of differences in the attribute.

**Cognition and semiotics** Cognitive principles are important in cartography because they explain why certain map symbols work. In the past, mapmakers did not care why symbols worked—a behaviorist view that treated the human brain as a black box. Modern cartographers, however, want to know why some symbols are more effective, aiming to build a theoretical basis for how brains recognize symbols and to create new ones. According to Charles Sanders Peirce’s semiotic theory, map users “read” symbols by connecting the graphic mark (the sign), a concept (the interpretant), and a real-world feature (the object or referent). Map symbols can be categorized by how they suggest this connection:

- **Iconic symbols** (also called image, pictorial, or replicative) look like the real-world feature, often in a simplified way—for example, a tree icon for a forest, brown for desert, or green for vegetation. - **Functional symbols** (or representational) directly show the activity at the feature, like a skier for a ski resort or a tent for a campground. - **Conceptual symbols** directly represent an idea linked to the feature, such as a dollar sign for an ATM or a Star of David for a synagogue. - **Conventional symbols** (or associative) have no intuitive connection but are so widely used that map readers learn them—for instance, a red line for a highway or a Swiss cross for a hospital. - **Ad hoc symbols** (or abstract) are arbitrary choices by the cartographer, with no intuitive link to the concept or referent; they can only be understood with a legend, like using various colors for geologic layers.

**Visual variables** A map symbol is made by altering the visual appearance of a point, line, or region. This appearance is controlled by 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. Since then, cartographers have modified and expanded this set. Each variable can be used to convey information, create contrast between features and layers, establish figure-ground contrast and a clear visual hierarchy, or enhance the map’s aesthetic appeal.