Legend

The role of the map legend is to explain the symbols on the map. Symbols are point, line, area, volume, and pixel graphic marks that represent real-world geographic phenomena. Legends explain symbols that represent natural and human-made features, such as roads, coastlines, buildings, and cities. Legends also explain symbols that represent thematic data. Examples of thematic data representations include light-gray to dark-gray shades and their associated values (ranks) showing population density, and various circle sizes representing average tuition costs at state universities and their associated values.

There are two schools of thought on which map symbols should be placed in a legend. One is to include every symbol present on the map. This often results in a complex and sometimes visually cumbersome legend if not effectively designed. The other school of thought represents a more minimalist view, which is to include only those symbols that are not self-explanatory. For instance, blue lines that represent rivers on a map are often labeled with the river name (e.g., Mississippi River, Colorado River). A cartographer might decide that this blue-line symbol has been sufficiently identified on the map and therefore not include it in the legend. Deciding which school of thought to follow can be a matter of design preference on the part of the cartographer or the preference of the agent or organization for whom the map is being produced.

It is critical that the thematic symbols in the legend look exactly like the thematic symbols on the map. This means that the symbols in the legend must be the same color, size, and orientation as the thematic symbols they are referring to.

The legend is set apart from the mapped area using several methods. One method is to place the legend outside of the mapped area, a style common on U.S. Geological Survey (USGS) topographic maps. Another method is to place the legend in the mapped area. In this instance, it is common practice to highlight the legend with a “legend box,” so that it is visually separate from the mapped area. The legend box can take the shape of a square, a rectangle, or even a less standard shape, such as an oval. A similar, but more visually subtle method to visually separate the legend from the mapped area is to create a partially opaque legend box that defines the area of the legend. This method partially masks the mapped area underneath the legend so that the mapped area is somewhat visible but at the same time does not interfere with the thematic symbols in the legend.

When designing the legend layout, it is common to group together symbols showing base map information and to group together symbols showing thematic [Page 253]data. The design of legends for various thematic maps, such as choropleth or proportional symbol maps, requires information specific to that thematic data.