Visual hierarchy

Visual hierarchy is the apparent order of importance of phenomena perceived by the human eye. As the viewer organizes the visual field, some objects naturally appear more important than others. In cartography, this principle is a fundamental part of map composition; since the goal of map composition is to clearly convey a desired purpose, the attention of readers should be focused on the map elements that are most relevant to the purpose. Not all map elements are given the same visual importance; if they are (i.e., there is a lack of Contrast), the map becomes difficult to read. Visual hierachy can be achieved by manipulating the symbology of the objects on the map using a combination of visual variables such as size, color, and texture.

Visual Hierarchy in the Gestalt Process
Visual hierarchy is related to Gestalt psychological theory, which proposes that the brain has inherent organizing characteristics. The theory suggests that people subconsciously notice certain objects before others. It states that the human mind tends to "structure individual elements, shapes or forms into a coherent, organized whole." When presented with a picture, map, or other visual stimuli, the brain will take individual elements of the scene and organize them into a whole.

Visual hierarchy is established as the viewer notices exceptions to the whole; elements that most sharply contrast from the pattern of the whole draw attention. The greater the disconnect of an object from its surroundings (the whole), the more the brain highlights it as important to take note of. For example, the fluorescent color of tennis balls contrast with their surroundings, making it easier for a tennis player to focus on during high-speed play. An example in cartography is that a bold title in a large font will draw the attention of the viewer first before a smaller, unbolded subtitle.

Visual Weight
The relative visual importance of an element in a map, also known as its visual weight, is the tendency of the element to attract the attention of the map reader away from other elements. This weight is not only relevant to visual hierarchy--it also is the fundamental principle for establishing Balance in a design work. In his consideration of the psychological underpinnings of visual balance, Arnheim states that the following properties contribute to visual weight:


 * Size: larger objects attract attention and are perceived as heavier. This can be difficult to manage in maps, as the size of geographic phenomena cannot always be controlled; for example, a very large but unimportant feature such as a lake can be difficult to subdue.
 * Location: the element at the center is generally perceived to be the most important. However, heavy objects away from the center attract increased attention due to the perceived imbalance, as they would in a lever. Essentially, the reader's attention is attracted by the question "what is this doing way over here?"
 * Spatial Depth: because the viewer knows that perspective means that distant objects are larger than they appear, an object perceived as far away will carry more weight than its actual size would dictate. This is more important in three-dimensional scenes than two-dimensional maps.
 * Color: Strong colors (dark, bright) tend to stand out more than weaker colors (pale, subdued). However, the actual weight of a color is not universal, but is partially determined by its contrast with the rest of the visual field. On a dark background, a light color will stand out more than a dark color.
 *  Value: objects with a lightness that contrasts most with the background (black on white, white on black) stand out. For this reason, symbology designed on a computer screen (with its inherently dark background) often has a different visual hierarchy when it is printed on paper (with its inherently white background).
 * Hue: complimentary colors will stand out most, such as a blue object on a generally yellow, tan, or beige field. Heavy colors, such as red or black, will emphasize important areas.
 *  Saturation: bright colors will stand out on a muted or gray background. The converse (a gray object on a brightly colored background) rarely works as well.
 * Intrinsic Interest: Some elements attract more attention because they are interesting. On maps, this is often the result of complexity in a shape, pattern or texture. For this reason, shaded relief naturally stands out, and a river with many meanders will stand out over one that is straight. Therefore, cartographic generalization can be a useful tool in strengthening the visual hierarchy by simplifying less important features.
 * Isolation: Even a simple shape can stand out if it is surrounded by Negative space, such as a small town in the middle of a sparsely populated desert.
 * Shape: Some geometric shapes tend to attract attention; compact shapes such a circles and squares are more easily recognizable as distinct objects than sprawling, meandering shapes (such as the extent of a geologic formation). This helps establish Figure-ground contrast, which contributes to visual hierarchy.
 * Knowledge: Elements that are perceived as novel or surprising will carry more weight than those seen as normal or familiar. For example, in a Cartogram, it is not only the artificial size that builds the visual hierarchy, but the degree to which each region has been distorted from its familiar shape.

Many of these properties are closely aligned with the visual variables listed by Bertin and others, and can be directly controlled through the design of map symbols to establish a desired hierarchy. However, some properties are inherent to the geographic characteristic of the phenomena being represented (e.g., the meandering shape of a river, the size of a lake, the ruggedness of the terrain), and are more difficult for the cartographer to manipulate. In these cases, other means will need to be used to adjust the inherent weight to that desired. For example, a large lake or a complex shaded relief could be subdued through the use of color (lighter value).

Map Layout
Visual Hierarchy applies to the whole map layout, and not just the map image. Well-designed maps will have titles, legends, scale bars, inset maps, texts, and so on that fit within a hierarchy of their own relative to the map image, preferably matching an intended order of importance. An example of this is the map title, which is generally very important to map readers understanding the purpose of the map; therefore, it should be one of the first things the reader notices. The map legend is very useful for making sense of the map, but is of minor importance compared to the map itself. Factors such as size, color, position, transparency, font, and Negative space all apply to map layout and how it affects the visual hierarchy of the map.

Principles of Visual Hierarchy
Maps are products; understanding the principles of visual hierarchy are important to designing a map that is understandable and useful. There are 5 basic principles of visual hierarchy that explain how the human mind works and takes in information and are as follows:

1. The F' and 'Z' principle, which studies have shown to be the patterns eyes follow when scanning a page to decide if it is worth reading or not. Making sure the title is readable and interesting is important to ensuring the map is used and choosing the pattern of the map itself plays a big role in whether or not it will be successful in broadcasting statistics and facts.

2. Bigger objects signify greater importance, they also attract the attention of the user. Important symbols and objects should be larger in size while symbols used for reference should be smaller.

3. Negative space. Negative space also known as “white space,” negative space is the empty space on a map that is available for placing map elements such as a legend, North arrow, scale bar, etc. The contrast created by the negative space against the map elements help to establish a visual hierarchy for the features. The more an element contrasts with its surrounding negative space, the more attention it will attract. An element that is surrounded by a large white area will stand out more than the same element on a complex background. However, using too much negative space can cause the opposite effect.

Maps that use negative space unwisely become cluttered and may therefore fail to convey their desired message.

4. Symbology. Just as bigger objects signify greater importance, the type of symbol used ranks its importance as well as its relevance. Symbols that relate to the theme of the map are enhancing while symbols that appear to be random can be confusing and disorienting.

5. Colors: bright colors stand out, muted colors do not. Using bright colors and muted colors together can create an appealing visual effect while using one or the other can make the symbols difficult to see and the map difficult to read.

6. Understanding how to use direction. Maps are made of horizontal and vertical grids, understanding how the eye follows those grids and designing the map that stays on those grids can help make the map readable to all users.