Cartography

There are many different types of maps. So many, in fact, that it can be difficult to classify them into groups. A common classification system divides maps into two categories: general and thematic. General maps are maps that show spatial relationships between a variety of geographic features and phenomena, emphasizing their location relative to each other. Thematic maps illustrate the spatial variation of a single phenomenon or variable, or the spatial relationship between two particular phenomena or variables, emphasizing the pattern of the distribution.

Maps can be either general or thematic, depending on the intent of the cartographer. For example, a cartographer may produce a vegetation map showing the distribution of various plant communities. If the cartographer shows the location of various plant communities in relation to a number of other geographic features, the map is properly considered a general map. The map is more likely to be considered thematic if the cartographer uses it to focus on something about the relationship of the plant communities to each other, or to another particular phenomenon or feature, such as the differences in plant communities associated with changes in elevation or changes in soil type.

Some examples of general maps include topographic maps, planimetric maps, and charts. Topographic maps depict the form of the earth's surface, most commonly expressed as elevation above sea level, and are general maps if they also include features such as cities, rivers, and roads. Bathymetric maps depict underwater topography. Planimetric maps show features such as cities and roads without depicting elevations. Charts are used by the navigators of aircraft and seagoing vessels to plot positions and courses. World maps on a small or medium scale showing physical and cultural features, such as those in atlases, are also considered general maps.

Although the subject matter of thematic maps is nearly infinite, cartographers use common techniques involving points, lines, and areal units to illustrate the structure of spatial distribution. Isarithmic maps use lines to connect points of equal value; these lines are called isopleths, or isolines. Isopleths are used to show how certain quantities change with location and those used for a particular purpose may have a particular name. For example, isotherms connect points of equal temperature, isobars connect points of equal air pressure, and isohyets connect points of equal precipitation. Isopleths indicating differences in elevation are called topographic contour lines, and a topographic map that does not depict general features such as cities and roads would be a thematic map.

A topographic map is a good example of how isopleths are used to present information. Topographic maps use isopleths called elevation contour lines to indicate the topographic relief. Each contour line connects points of the same elevation, and the difference in elevation between each contour line is known as the contour interval. A contour interval of 20 ft (6.1 m) means that there is a 20 ft (6.1 m) difference in elevation between the points connected by one contour line and the points connected by an adjacent contour line. Closely spaced contour lines represent steep slopes and widely spaced contour lines represent gentle slopes. Closed contour lines, for example in the general shape of a circle or ellipse, represent hills.

Chloropleth maps are another type of thematic map. They use areas of graduated gray tones or colors to show spatial variations in the magnitude of a phenomenon.