Map Library
& Data


Mapping &
GIS Quiz

Base Camp


Information & Mapping

Maps are made for many reasons, and as a result, maps are of many kinds. Some made for general purposes may show roads, towns and cities, rivers and lakes, parks, and State and local boundaries. One example of such a versatile map, or base map, is the 1938 topographic map of Oswald Dome, Tennessee. Other maps are much more specific, conveying information primarily on a single topic. The 1989 earthquake map of the United States is a good example of a special-purpose map, or thematic map. Every map is made for a purpose and serves that purpose best.

The history of civilization has been illustrated by maps- battle maps by soldiers, exploration maps by empire builders, thematic maps by scientists. By modern convention, and for no scientific reason, modern maps are usually oriented with north at the top. But Al Idrisi's 1154 world map shows the Arabian Peninsula in the top center of the map, with south at the top. Contrast this map with the 1452 Leardo world map. Different societies in different places literally have different perspectives, which may result from differences in physical geography, language, religion, cultural values and traditions, and history.

Even within a culture, a time, and a geographic realm, maps can vary widely. This is because a map shows the cartographer's bias as well as the purpose. Maps are the result of conscious design decisions. Cartographers decide how to generalize and symbolize what they are trying to show. They select features (or themes) to show and omit other features. They often generalize the data, simplifying the information so that the map is easier to read.

In choosing the scale, mapmakers determine how large an area they can map and how much detail they can show. The selection of symbols (which can include lines, patterns, and colors) also affects the legibility, aesthetics, and utility of the map.

Cartography blends science and art Cartography blends science and art. A beautiful map may become popular, even though it may be less accurate than a plainer version. Details of cartographic style affect how a map is perceived, and perception varies with perspective. In short, people understand the world differently, have different modes of expressing this understanding in maps, and gain different understanding from maps.

Geographic features can be shown at different sizes and levels of detail by using scale. Maps include selected basic geographic information to provide context. Every map has a purpose or theme. The map design, which includes artistic aspects such as composition and balance, affects the success of the map ‹that is, its ability to communicate.

Scale is the relationship between the size of a feature on the map and its actual size on the ground. Scale can be indicated three ways. The bar scale is a line or bar that has tick marks for units of distance. The bar scale is especially important because it remains accurate when a map is enlarged or reduced. A verbal scale explains scale in words: "one inch represents 2,000 feet.'' The representative fraction is a ratio such as 1:24,000, in which the numerator(l) represents units on the map and the denominator (24,000) represents units on the ground; in the example of 1:24,000 scale, one unit (any unit‹feet, millimeters, miles, etc.) on the map represents 24,000 of the same units on the ground.

Scale controls the amount of detail and the extent of area that can be shown. Scales can be described in relative terms as large scale, intermediate scale, and small scale. A large scale map (for example, the 1886 Sanborn map, originally at 1:600 scale) shows detail of a small area; a small scale map (for example, the 1877 geologic map of north-central Colorado, originally at 1:253,440 scale) shows less detail, but a larger area. (A comparison of representative fractions shows that 1/600 is larger than 1/253,440.)

Context is information that serves to orient the map reader to the mapped place. As you look at the maps on the poster, you may look for familiar features (such as the "boot" of Italy) to Identify the area shown. Geographic information that provides context can include coastlines, boundaries, roads, rivers and lakes, cities and towns, topographic features, place names, and latitude and longitude.

Distortion is another important aspect of context; every flat map of a curved surface is distorted. The choice of map projection determines how, where, and how much a map is distorted. It is important to understand the kind and amount of distortion on the map sheet. The typical mapping project now plots information on a base map, which shows where the place is and establishes the scale, orientation, context, and spatial distortion of the information to be mapped. The type and scale of the mapping project affect the choice of base map. Digital, or computerized, mapping frees the cartographer from some constraints imposed by a base map, because features can be readily selected or deleted, and the projection and scale can be changed easily.

A map's purpose is usually clear from its title and explanation, but other information (author, date, publisher, source of funding, etc.) hints at why and for whom the map was made. A knowledgeable map reader, recognizing that a map is both a simplification and a distortion of reality, will look for clues to the cartographer's purposes and biases.

The information collected for a mapping project is called spatial data. Any object or characteristic that can be assigned a geographic location can be considered spatial data. Spatial data always include location, but many also include values to be represented.

These two kinds of information are qualitative data (for example, schools, roads, rivers, States) and quantitative data (for example, altitudes, amount of precipitation, per capita income, population density). Qualitative data, while not numeric values, may be ranked, as in categories of roads or schools.

Quantitative data can be treated in many ways. The cartographer may first decide to generalize data. Several closely spaced points may be generalized to one symbol; features may be eliminated as map scale is reduced; questionable data may be eliminated where other data are sufficient.

Likewise, grouping of data can be done in different ways. Large ranges of numbers may be grouped with breaks at round numbers (for example, 10, 20, 30) or at statistical mean and standard deviation values; in this case, the individual points may be mapped in various colors or sizes to correspond with group values. Another way to group data is within geographic areas, using colors or symbols for areas, rather than symbols at each data location. Generalization and grouping dramatically affect the message the map presents by simplifying the data.

The success of a thematic map depends on map design. Scientific maps like Edmund Halley's 1701 map of compass variations usually show only enough geographic data to orient the user, while emphasizing the content. Halley, for whom the comet is named, pioneered several cartographic techniques. The 1701 map introduced isolines, lines of equal value, a technique now used on topographic and other kinds of maps. The 1886 Sanborn fire insurance map includes as much as its business purpose requires, but nothing more. Triangulation maps, such as the 1744 map of France, show the network of points and lines, in this case colorfully framed within national boundaries. The 1989 earthquake map of the United States indicates the relative hazard by a contoured and colored surface, which also shows State boundaries.