Visual hierarchy is content organized in a manner that visually communicates order and importance. Visual cues, such as size, position, arrangement and color, can be used to create visual hierarchy in a map (as well as in a website or an interface). Visual hierarchy creates a sense of depth on a flat map and enables quicker reading of the map for meaning and importance. For example, when you view the figure below, do some elements look like they are on top of the other elements?
Your map’s visual hierarchy should be somehow related to the map’s intellectual hierarchy (Krygier 2003). To create an intellectual hierarchy for your map you will need to consider what the necessary map elements are and how important each of these elements is. This intellectual hierarchy is often closely related to the map’s purpose (recall our discussion of map purpose in Lesson 1, Part III: Map Purpose and Audience).
We can think about visual hierarchy in maps at two different scales: within the map frame itself and within the map layout. Within the map frame, visual hierarchy helps map readers differentiate between different kinds of data that are represented in the map (see the figure below). Once map readers can ‘see’ that there are multiple data themes represented in the map, each layer’s place in the visual hierarchy helps the map reader understand what data layers are most important. A layer that is higher in the visual hierarchy will stand out. In Figure 2.cg.2 below, the map on the left is an example of a map that does not have a clearly defined sense of visual hierarchy inside the map frame. If you were trying to use this map to figure out what was going on with the weather in the south central U.S., that task would be very difficult as it would be hard to determine which line belonged to which data set. The map on the right depicts the same data, but uses symbolization differences to differentiate between data layers and create a visual hierarchy in the map frame.
We can create visual hierarchy within the map frame by making design decisions that promote figure-ground relationships. Objects that stand out in the visual field are called figures, while the rest of the display makes up the ground. Our eyes will automatically separate parts of an image into figure and ground. In other words, it is a perceptual mechanism – one that we do not have to consciously think about. Although there are many different variables that can affect what the map reader sees as figure or ground, generally, design decisions that make the visual field more homogenous make it harder for the map reader to differentiate between figure and ground, and those that make the visual field more heterogeneous promote the map reader’s ability to see figures. In the Figure 2.cg.3, below, notice that the river always stands out because of a high degree of contrast between it and the other map features. However, the lack of contrast between the figure(s) and ground in the image at the left would make it quite hard for the map reader to discern where the city blocks are located.
I recommend that you watch this mini-lecture by Sara Fabrikant: http://www.csiss.org/streaming_video/csiss/fabrikant_perceptual.htm to learn more about factors that promote figure-ground relationships. You will need RealPlayer installed on your machine to watch this lecture.
We can also think about an intellectual and visual hierarchy of elements within the map layout. Visual hierarchy within the layout is driven by two main factors: size and position. Large map elements will generally appear higher in the visual hierarchy than small map elements. Elements that are placed near the top and towards the center of the page will appear higher in the visual hierarchy than those that are towards the bottom or are at the edges of the map. Visual hierarchy in the layout helps direct map readers to the most important information first. Consider two scenarios. In the first scenario, imagine you are creating a thematic map of median household income by state for the United States. For this map, you probably don’t even need to include a north arrow. Most map readers will already know which direction north is simply by seeing how the shape of the U.S. is oriented on the map. However, if you are creating a street map of Tokyo, it will probably be very important to include a north arrow on the map to help map readers navigate from one location to another. In the first scenario, a north arrow would be very low on the map’s intellectual hierarchy (i.e., this element is not very important) if it is even included on the map at all. In the second scenario, the north arrow should be higher on the map’s intellectual hierarchy as it is more important for that map’s purpose.