Contrast (map design)
Contrast is the degree of difference between items. As a Cartographic Design technique, contrast refers to making different map symbols visually distinguishable from one another (and their background) in order to facilitate the association of each symbol type with a distinct class of geographic feature. The effective use of contrast produces a map that is clear, organized, and professional. It is also a crucial component of establishing a Visual hierarchy.
To visualize this principle, think about how difficult it is to see at night or in darkly lit places. There is not much light to reflect off of objects making it hard to distinguish where one object ends and another begins. 
Contrast and Visual Variables
Contrast can be created by manipulating any of the Visual variables: light vs. dark, red vs. blue, thick vs. thin, large vs. small, etc. Contrast as a design technique is different than the contrast of an image, which typically refers only to differences in value (light vs. dark). A lack of contrast, such as using black lines as the borders of counties, cities, and vegetation zones, or labeling all features with the same size and style of text, can make it difficult to distinguish the different themes of the map, and decide what is more important. Contrast thus aids in Visual hierarchy as it makes some features look significantly more important than others.
- Lines: As lines can function as labels, borders, routes, a contrast can be made through character (nature of the line; Ex. dashed vs. solid) and weight (line thickness).
- Texture: Can be applied to direct the reader's attention.
- Value: Areas can be distinguished by applying light and dark colors on map.
- Detail: Considerable amounts of detail added to features on a map will stand out more than less detailed features.
- Color: Creates contrast by differentiating areas on a map with several purposes. Using colors on opposite sides of the color wheel (such as red and green) will create more contrast than using colors that are close together (such as yellow and orange).
Color Contrast Ratio
Color contrast ratio is a formula that can assist the cartographer ensure the readability of the map. This formula quantifies the amount of contrast between visual elements (such as text and background). A ratio of 1:1 would describe the contrast of an image with black text on a black background- in other words, no contrast. On the other hand, a ratio of 21:1 would describe the contrast of an image with black text on a white background- such as the text on this page.  This is the most contrast possible, and is the easiest for our eyes to distinguish. Consequently, it is little wonder that books, webpages, and other media generally contain black text on white background. Using the color contrast ratio formula, the contrast values for text colors contained in your map can be compared against their backgrounds and it can be determined whether they meet specific color contrast requirements, such as the following:
- Contrast (Minimum): The visual presentation of text and images of text has a contrast ratio of at least 4.5:1, except for the following: (Level AA)
- Large Text: Large-scale text and images of large-scale text have a contrast ratio of at least 3:1.
- Incidental: Text or images of text that are part of an inactive user interface component, that are pure decoration, that are not visible to anyone, or that are part of a picture that contains significant other visual content, have no contrast requirement.
- Logotypes: Text that is part of a logo or brand name has no minimum contrast requirement.
- Contrast (vision), the perception and manipulation of visual contrast, especially useful in remote sensing.
- Aileen Buckley, "Design principles for cartography" Esri: ArcGIS Blog, October 28, 2011
- MSF&W Color Contrast Calculator: http://www.msfw.com/Services/ContrastRatioCalculator