Cartography

No single map can accurately show every feature on Earth's surface. There is simply too much spatial information at any particular point on Earth's surface for all of the information to be presented in a comprehensible, usable format. In addition, the process of reduction has certain visual effects on geographic features and spatial relationships. Because every feature is reduced by the ratio of the reduction, the distance between features is reduced, crowding them closer together and lessening the clarity of the image. The width and length of individual features are also reduced.

When designing a map, cartographers strive for clarity and effective communication. They use the technique of selection to determine which pieces of information to include and what kinds of symbols will most effectively portray that information.

A wide array of geographical information is available to mapmakers. When preparing a map, cartographers must choose only those pieces of information that are pertinent to the purpose of the map and then display those pieces of information in a way that effectively communicates their significance. Only information deemed significant or useful is selected for inclusion in the map.

Once cartographers have selected the information that will be portrayed on the map, the information must be displayed in an effective manner. Cartographers deal with this problem by applying the techniques of cartographic generalization. Both map geometry and map content are generalized.

Geometric generalization techniques change the placement and appearance of various map features in order to make the map easier to interpret and more pleasing to the eye. For example, not every twist and turn of a 15 mi stretch of river can be accurately portrayed at a 1:500,000 scale, where 1 in equals 7.89 mi. The path of the river is simplified, reducing excessive detail and angularity. A railroad running 50 ft from the river would appear to run in the riverbed when shown at a 1:500,000 scale. Using cartographic generalization, the cartographer displaces the railroad, showing it next to the river, avoiding graphic interference and increasing the readability of the map. The numerous right-angle bends in a highway following rural property boundaries along the river would be smoothed by the cartographer, reducing their angularity and thereby making the line of the highway easier for the eye to follow.

Linear and areal features can be generalized using the techniques of simplification, displacement, smoothing, and enhancement. Additionally, the techniques of dissolution, segmentation, and aggregation are applied to areal features. Point features are generalized by displacement, graphic association, and abbreviation.

Map content is generalized using the technique of classification, in which similar features to be grouped together are represented by a single symbol. Campgrounds, for example, are often represented by a tent-shaped symbol, even when the facilities can accommodate trailers or large recreational vehicles. Categorization is another form of classification. Many maps, for example, use one point symbol for population centers of 1,000–10,000. Another point symbol for population centers of more than 10,000 but less than 100,000, and a third point symbol for population centers of more than 100,000 but less than 500,000. Cartographers must carefully consider the implications of such classification schemes. The system described above implies that towns of 1,000 and towns of 9,000 have more in common than towns of 9,500 and towns of 10,500.