By Aileen Buckley, PhD, Esri Research Cartographer
I recently got a very good question from one of our users—”Is there a cartographic convention or guideline for legend item labels being singular or plural? Do plural labels imply something different if there are both singular and plural entries?”
Let’s start with the first question. As a rule, legend items labels should be singular nouns. Think of reading the map legend this way—”This black line symbol represents a road” or “This red circle represents a city“. For example, the symbol at the top of Figure 1 indicates that there is a campground at the location of the symbol on the map.
Figure 1. Correct use of singular nouns for legend item labels. (Source: https://www.nps.gov/hfc/carto/SYMBOLS/map_symbols.pdf, updated May 28, 2016.)
Don’t make the mistake of thinking of the map legend this way—”This black line symbol is used to show my roads layer” or “This red circle is used to show my cities layer.” This will not only lead you to incorrectly use plural nouns, further it is an interpretation of how to read the table of contents for a map document, not the legend for a map.
Now let’s turn to the second question―”Do plural labels imply something different?” Yes, they do. The most common interpretation of legend item labels that are plural is that there are multiple things of the same type in a single location. For example, the symbol at the top of Figure 2 indicates that there are multiple restrooms at the location of the symbol on the map—most likely they include a men’s restroom and a women’s restroom, at the least.
Figure 2. Correct use of plural nouns for legend item labels. (Source: https://www.nps.gov/hfc/carto/SYMBOLS/map_symbols.pdf, updated May 28, 2016.)
A second, less common, reason to use plural nouns in a legend item label is when multiple features are shown in the legend item. For example, in Figure 3, two interchanges are included in the legend item to illustrate the consecutive numbering system for the exits—something that would be impossible to show with a single interchange.
Figure 3. Correct use of plural nouns for legend item labels that show multiple features. (Source: Benchmark Maps, California Road & Recreation Atlas, 2010.)
To conclude, here are two hard and fast rules for legend items (from Robinson et al. 1995, Elements of Cartography, p. 336):
- “Any map symbol that is not self-explanatory should be explained in a legend.”
- “When symbols are used in the legend, they should appear exactly as they look on the map, drawn in precisely the same size and manner.”
There are many other guidelines for legends, depending on the type of map the legend is created for, but the guidelines above relate to any type of legend you are making.