Examples of city points symbolized based on their importance.
What: Typically on reference maps, atlases, and locator maps, point symbols for cities and towns are based on population and political function. Larger cities have more prominent symbols and labels. On this map the cities are symbolized using three different point symbols that relate to the size of the city's population. Why: Using variation in point symbols for cities on maps provides visual hierarchy that allows readers to distinguish different classes of features. It is important to show villages, towns, small cities, large cities, and capitol cities distinctly to avoid readers drawing incorrect conclusions about the relative importance of the places shown. For more about using point symbols to show different kinds of places see pages 144-149 of Designing Better Maps.
As a rule this works when making a locator map in a given region such as the one above. However, when the map shows an extent where there are densely populated regions and sparsely populated regions, it will be necessary to incorporate more information, such as notoriety or local density of places, to decide which places should be shown in the sparsely or densely populated regions.
How: To symbolize the cities we classified them into three classes by population and applied different symbols to each class. We chose three classes because this was a simple locator map, because we did not want a more complicated portrayal of the cities. Our goal was to show where they are and relatively how important they are. We stored the class assignments in the attribute table so we could use them to make similar maps in the future. Here are the steps we followed:
- Add a new field to store our class assignments; ours was a long integer field called PopCat.
- Open the attribute table for the layer.
- Click the Options button on the table window and then choose Select by Attributes from the menu.
- Our first Query was to create a new selection where [Pop1996] > 40000
- Then we calculated the PopCat field to have a value of 3 &mdash this applies only to the selected features.
- The next query was: [Pop1996] <= 40000 and [Pop1996] > 7500.
- We calculated PopCat field to have a value of 2.
- The last query was: [Pop1996] <= 7500.
- We calculated the PopCat field to have a value 1.
- Then we closed the attribute table and used the symbology tab of the layer properties to set unique values symbology based on the PopCat field.
We symbolized the city classes as follows:
- City (Population greater than 40,000 persons): We used a relatively large (7 points) circle with a gray outline and a small dot in the center. We used a relatively bright orange color as the background of the point symbol making it as easy to see on the map beside the red lines we used for symbols of the most important roads. These roads and largest cities are the most important and therefore the most prominent features on the map in terms of visual hierarchy.
- Small City (Population between 7,500 and 40,000 persons): We used a smaller (5 point) circles with a gray outline, similar to the symbol for a city, but without the dot in the middle. The background was a lighter shade of orange, to create a slightly less prominent symbol than for the larger cities.
- Town or village (Population less than 7,500 persons): We used a small gray square (4 points) with no outline. We used 70% gray instead of black so it would not visually compete with the black text on the map.
We also used our PopCat field values as the basis for label placement which is further described in Labeling cities along coasts.
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