Map symbol

Geographic phenomena can be categorized into objects, which are recognizable as a unified whole with a relevant boundary and shape; and masses, in which the notion of boundary and wholeness are not relevant to their identity. Features such as buildings, cities, roads, lakes, and countries are geographic objects that are often portrayed on maps using symbols. Mass phenomena include air, water, vegetation, and rock. These are rarely represented directly on maps; instead, map symbols portray their properties, which usually take the form of geographic fields, such as temperature, moisture content, density, and composition.

There are basically five types of spatial dimensions that are used to classify phenomena for map symbolization. Point phenomena are assumed to have no spatial extent and are said to be zero-dimensional. These use point symbols on a map to indicate their location. An example of these would be fire hydrants or trees in a park. Linear phenomena are one-dimensional and have a length. This would include any line feature on a map like roads or sidewalks. Areal phenomena are 2-D that has both a length and a width. The best example of this would be a lake or other body of water. When volume comes into consideration, it is broken down into two types, 2 ½ dimensions and 3-D. A good example of 2 ½ D would be the elevation of a place above sea level, while 3-D being any three-dimensional objects.

The dimensionality of a map symbol representing a feature may or may not be the same as the dimensionality of the feature in the real world; discrepancies are the result of cartographic generalization to simplify features based on purpose and scale. For example, a three-dimensional road is often represented as a one-dimensional line symbol, while two-dimensional cities are frequently represented by zero-dimensional points.

While the absolute location of a feature on a map is usually unalterable, the position of labels and other information can be controlled.

The size of a label or symbol is how much space it occupies on a map.[8] Size differences are relatively easy to recognize, making it a useful variable to convey information, such as a quantitative amount of something, or relative importance. Such value estimations are the most accurate from bars and squares. From circles, especially larger ones, value estimations are underestimated, on average - there is a large variation between people in abilities to estimate values from circles.[9]

Because geographical features have an actual size on the Earth, this cannot always be controlled, and sometimes works against the wishes of a cartographer; for example, it can be difficult to make a world map in which Russia does not stand out. In a cartogram the size of features is purposefully distorted to represent a variable other than area.

A shape is a simple design that is used to symbolize an attribute on a map.[10] Shape is most useful for representing nominal categories of features, especially for point features.[11] Some shapes are simple in nature and thus are more abstract, while other shapes are more pictorial and are easy for the reader to comprehend what is trying to be conveyed.[12]

Some aspects of shape are inherent to the phenomenon and may not be easily manipulable, especially in line and region symbols, such as the shape of a road or a country. However, shape can still play a role in line and region symbols, such as a region filled with tree symbols or an arrowhead on a line. Also, the shape of a feature may be purposefully distorted by Cartographic generalization, especially when creating schematic representations such as many transit maps, although this distortion is rarely used to convey information, only to reduce emphasis on shape and location.

Hue is the visual perceptual property corresponding in humans to the categories called red, green, blue, and others. Maps often use hue to differentiate categories of nominal variables, such as land cover types or geologic layers.[13] Hue is also often used for its psychological connotations, such as red implying heat or danger and blue implying cold or water.

As an aspect of color, value refers to how light or dark an object appears. Value effectively connotes "more" and "less," an ordinal measure; this makes it a very useful form of symbology in thematic maps, especially choropleth maps. Value contributes strongly to Visual hierarchy; elements that contrast most with the value of the background tend to stand out most (e.g., black on a white sheet of paper, white on a black computer screen).

The saturation of a color is its purity or intensity, created by the variety of light composing it; a single wavelength of light is of the highest saturation, while white, black, or gray has no saturation (being an even mixture of all visible wavelengths). Of the three psychological aspects of color, this is the least effective at conveying specific information, but it is effective at establishing visual hierarchy, with bright colors generally standing out more than muted tones or shades of gray.

Orientation refers to the direction labels and symbols are facing on a map. Although it is not used as often as many of the other visual variables, it can be useful for communicating information about the real-world orientation of features. Common examples include wind direction and the direction in which a spring flows.

Texture refers to the aggregate pattern made up of many individual symbols. For example, a dense network of street lines could collectively convey the concept urban area. An evenly spaced lattice of green dots could mean Orchard, while a random distribution of the same green dots could mean Forest.

A fairly recent addition, the control of opacity has become common in digital cartography. While it is rarely used to convey specific information, it is common to make features translucent to reduce contrast or to retain underlying information.

Following on the widespread usefulness of Bertin's variables, cartographers have proposed analogous sets of controllable variables for animated maps,[14][15] haptic (touch) maps,[16] and even the use of sound in digital maps.[17]