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Cartographers use colors on maps to represent certain features. The use of colors is always consistent within a given map and is often consistent between different types of maps made by different cartographers and publishers. The colors used in the maps are sometimes related to the characteristics of what is represented. For example, blue is almost always chosen to represent water.
Political maps, which show the borders of countries and regions, provinces and cities, tend to use more colors than physical maps, which represent the landscape and nature without taking into account the alterations generated by humans.
Political maps typically make use of four or more colors to represent different countries or their internal divisions, such as states or provinces. Blue is often used to symbolize water, black or red for cities, highways, and railways. Black also shows boundaries, with different types of strokes (dash, dot, solid line) to represent the kind of boundary: international, state, municipal, or any other political subdivision.
Physical maps use colors to show changes in the landscape and in nature, such as changes in the elevation of the terrain. Various shades of green often represent elevations. Dark green generally represents lowlands with lighter shades of green as the elevation of the terrain is higher. For even higher elevations, physical maps frequently use shades of brown, reserving red and white for the highest elevations on the map.
It must be remembered that in these cases the green and brown do not represent the characteristics of the terrain surface. For example, the fact that the Mojave Desert is highlighted in green does not mean that the desert has lush vegetation, but that the color is associated with its low elevation above sea level. In the same way, the white color for the mountain peaks does not indicate that they are covered with snow throughout the year.
On physical maps shades of blue are used for water; darker blues represent deeper water. The colors grey-green, red and blue-gray are used for underwater elevations.
Criteria for color assignment
Road maps and other everyday maps typically use colors according to the following criteria.
- Blue: Lakes, rivers, streams, oceans, reservoirs, roads, and local borders.
- Red: Major roads, highways, urban areas, airports, sites of special interest, military sites, place names, buildings, and borders.
- Yellow: urban or built-up areas.
- Green: parks, reserves, forests and roads.
- Brown: deserts, historic sites, national parks, reserves, military bases, and contour lines (elevation).
- Black: roads, railways, highways, bridges, place names, buildings, and borders.
- Purple: roads, and on USGS (US Geological Survey) topographic maps elements added to the original surveys.
statistical maps
Statistical maps use colors to represent statistical data for a given area. Statistical maps represent a municipality, a state or a country with a color that is associated with the data for that area, according to a scale of tones or a color code.
A statistical map of the United States is the one that shows the electoral results in each state; in red the states in which the Republicans were the most voted party and in blue in which the Democrats were the majority.
Among the data described by statistical maps are educational level, ethnic origin, population density, life expectancy, the prevalence of a certain disease and many other parameters. Cartographers who design statistical maps use different shades of the same color for the value scales that are plotted, producing a pleasing visual effect. An example of representation of population statistical data is the average per capita income in each municipality of a province in shades of green, where the lightest green corresponds to the lowest level of income and the darkest green to the highest.
Fountain
Menno-Jan Kraak et al. Mapping for a Sustainable World International Cartographic Association, 2021.