Symbols are used to represent geographic phenomena, which exist in, and are represented by, a variety of spatial forms. Different kinds of symbols are used to portray different spatial forms. Phenomena can be categorized a number of ways, but two are most relevant to symbology: ontological form and dimensionality. When a symbol is representing a property of the phenomenon as well as its location, the choice of symbol also depends on the nature of that property, usually classified as a Level of measurement.
Geographic phenomena can be categorized into objects, which are recognizable as a unified whole with a relevant boundary and shape; and masses, in which the notion of boundary and wholeness are not relevant to their identity. Features such as buildings, cities, roads, lakes, and countries are geographic objects that are often portrayed on maps using symbols. Mass phenomena include air, water, vegetation, and rock. These are rarely represented directly on maps; instead, map symbols portray their properties, which usually take the form of geographic fields, such as temperature, moisture content, density, and composition.
The number of spatial dimensions needed to represent a phenomenon determine a choice of Geometric primitive; each type of geometric primitive is drawn with a different type of visual symbol.

The dimensionality of a map symbol representing a feature may or may not be the same as the dimensionality of the feature in the real world; discrepancies are the result of cartographic generalization to simplify features based on purpose and scale. For example, a three-dimensional road is often represented as a one-dimensional line symbol, while two-dimensional cities are frequently represented by zero-dimensional points.
Many map symbols visualize not just the location and shape of a geographic phenomenon, but also one or more of its properties or attributes. Geographers and cartographers usually categorize properties according to the classification system of Stanley Smith Stevens, or some revision thereof, such as that of Chrisman. Different kinds of symbols and visual variables are better at intuitively representing some levels than others, especially when the visual variable portrays the same kind of differences as the represented attribute.

In cartography, the principles of cognition are important since they explain why certain map symbols work. In the past, mapmakers did not care why the symbols worked. This behaviorist view treats the human brain like a black box. Modern cartographers are curious why certain symbols are the most effective. This should help develop a theoretical basis for how brains recognize symbols and, in turn, provide a platform for creating new symbols.

According to semiotics, specifically the Semiotic theory of Charles Sanders Peirce, map symbols are "read" by map users when they make a connection between the graphic mark on the map (the sign), a general or specific concept (the interpretant), and a particular feature of the real world (the object or referent). Map symbols can thus be categorized by how they suggest this connection:
• Iconic symbols (also "image", "pictorial", or "replicative") have a similar appearance to the real-world feature, although it is often in a generalized manner; e.g. a tree icon to represent a forest, brown denoting desert, or green denoting vegetation.
• Functional symbols (also "representational") directly represent the activity that takes place at the represented feature; e.g. a picture of a skier to represent a ski resort or a tent to represent a campground.
• Conceptual symbols directly represent a concept related to the represented feature; e.g. a dollar sign to represent an ATM, or a Star of David to represent a Jewish synagogue.
• Conventional symbols (also "associative") do not have any intuitive relationship but are so commonly used that map readers eventually learn to recognize them; e.g. a red line to represent a highway or a Swiss cross to represent a hospital.
• Ad hoc symbols (also "abstract") are arbitrary symbols chosen by the cartographer to represent a feature, with no intuitive connection to the interpretant or referent. These can only be interpreted with a legend. An example would be using various colors to represent geologic layers.

An important factor in map symbols is the order in which they are ranked according to their relative importance. This is known as intellectual hierarchy. The most important hierarchy is the thematic symbols and type labels that are directly related to the theme. Next comes the title, subtitle, and legend. The map must also contain base information, such as boundaries, roads, and place names. Data source and notes should be on all maps. Lastly, the scale, neat lines, and north arrow are the least important of the hierarchy of the map. From this we see that the symbols are the single most important thing to build a good visual hierarchy that shows proper graphical representation. When producing a map with good visual hierarchy, thematic symbols should be graphically emphasized. A map with a visual hierarchy that is effective attracts the map user's eyes to the symbols with the most important aspects of the map first and to the symbols with the lesser importance later.