Thematic maps use keys to decode what may seem strange to the untrained eye.
In geography class, students learn to read maps. They are taught to use the legend to find places and identify features on the map. Scientists and researchers in many fields use more in-depth maps for their studies that go beyond simple country names and borders. These maps are called thematic maps, because they generally are designed for a specific purpose or theme such as locating water resources, identifying an area's terrain, or land-use planning.
Topographic and Terrain
Maps printed on paper are two-dimensional, which makes it difficult for those reading them to get an idea of the terrain. Topographic maps use the restraints of two-dimensional physicals and turn them into a type of three-dimensional construct. In the book "Topographic Maps," Ian Mahaney writes that topographic maps show length, width and height so that the user can get an idea how steep a mountain is or the flatness of plains. The map is often colored in shades of brown to illustrate elevation. The map key serves as a guide to the user to decode those shades.
Photoimage and Satellite
Aerial photographs are used to create photoimage maps. The Eastern Geographic Center breaks down photoimage maps into four categories: satellite image produced maps, orthophotomaps (multicolored aerial photos), orthophotoquads (black and white aerial photos) and border maps (aerial photos taken above a country's borders.) They can be used to show images for marshlands and coastal zones for a more accurate view of topographic data.
Geological and Subterranean
These maps reveal natural resources, such as limestone and granite. They can also help predict hazards and how those hazards will impact nearby ecosystems. In the book "Printed Sources," Kory Leland Meyerink writes, "These maps show the composition and structure of earth materials and their distribution across and beneath the earth's surface." Geological maps are also used for planning strategies for using the land.
Hydrologic Maps
Hydrologic maps measure water levels. They also identify water sources from groundwater and surface water. In the book "Subsurface Hydrology," George Francis Pinder writes, "A hydrologic map refers to a graphic representation of groundwater flow based on equipotential lines and streamlines applied to an aquifer within which vertical averaging has been applied." It can also be used to determine water temperature and availability.
Astronomical
Maps of distant stars and planets are possibly those most associated with science. These maps are constructed from satellite images, telescopic images, sound waves and radio waves. Astronomers are able to calculate distances between objects in space, identify galaxies similar to the Milky Way, predict the trajectory of comets and asteroids, and create theories of historical events in the universe.
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