Visual Grouping

Visual grouping, in Gestalt psychology, is the tendency for human viewers, when seeing large numbers of individual objects, to organize them into groups. As an aspect of map composition, visual grouping can be used to help viewers organize the symbols on a map into groups representing different classes of geographic features, and focus on the pattern in one group while ignoring the others.

Laws of Grouping
In gestalt psychology, a number of laws or tendencies have been discovered to explain why viewers group objects in certain ways. An understanding of these laws helps cartographers design map symbology that encourages map readers to organize the information on a map in a way that serves its intended purpose.

Proximity
Objects that are closer to one another are perceived to be more related than objects that are spaced farther apart. This allows the mind to group neighboring elements into clusters, which are easier to process than a larger number of small elements, allowing the viewer to understand and conceptualize information more quickly.

In maps, readers often use this principle to identify concentrations and regions, and recognize the co-location of possibly related phenomena.

Similarity
Objects that are similar in nature (such as size, shape, or color) tend to be grouped together. In maps, readers use this principle to isolate one theme from the rest, such as isolating the distribution of dark-shaded counties in a choropleth map from counties with other shades.

Continuity
Objects that appear to be connected are grouped together. This is especially important on maps in which topology is useful, such as a street map, in which readers should be able to "see" a particular route. Good continuity also encourages readers to follow from one object to another.

Closure
The human visual system is tuned to recognize objects in the visual field, so we tend to look for distinct, bounded shapes by grouping visual stimuli that seem to form boundaries. This even occurs when the edge of an object is incomplete or obscured, because people can perceive the whole by filling in the missing information.

Symmetry
Objects that are symmetrical are grouped to form figures that include all of the objects between them.

Common fate
When objects are moving, those moving in the same direction tend to be grouped together. This is especially useful in animated maps, or other maps attempting to show change over time. For example, a military map showing several distinct blue arrows pointing in generally the same direction would be perceived as a single army making an assault.

Prägnanzstufen, or the Law of Good Gestalt
Objects tend to be perceptually grouped together if they result in a pattern that is regular, simple, and orderly. It implies that as people perceive the world they eliminate elements that are complex and unfamiliar (basically what they see as un-useful detail) in order to see a more simplistic form of reality. Features can be perceptually grouped based on discrete occurrences "characterized by regions of figural stability".

Past Experience
Once a group of objects is formed in the viewer's mind, he or she may later identify the same group even if the above laws no longer justify it. For example, in the first of a series of maps, a viewer may recognize a spatial cluster of animals. In the next map representing a later period of time, the viewer may still perceive them as a group, even if they have moved apart from one another.