Graphic marks are considered to be symbols when they are agreed to have a certain meaning. This could be a picnic table to identify a park site, or a blue cross to identify a hospital. Others are more specific to maps—the use of a circle to represent a city, for example. The mapmaker assigns a symbol that represents the information he or she wants to bring across to the map user. Good, intuitive symbols can often be presented without defining them in a legend, but for most purposes, we add a legend to ensure clarity.
Many organizations that produce volumes of maps have rigid symbol systems that are the same on every map they produce. The topographic maps of the United States Geological Survey (USGS) have a multiple-page booklet for all of their symbols, and every major travel map company has their own version of the expected roadside symbols. If we are familiar with an area, we may attach memories and personal meaning...