The U.S. Geological Survey published a new and extraordinarily detailed global geologic map of Mars on July 14, 2104, based on remotely sensed image and topographic data gathered over 16 years by four orbiting spacecraft: Mars Global Surveyor (NASA), Mars Odyssey (NASA), Mars Express (ESA) and Mars Reconnaissance Orbiter (NASA). It is the first Mars map drafted entirely by digital means, using geographic information system (GIS) software to store, integrate, analyze, manipulate, and display the data sets. It also shows Mars geologic history in greater detail than ever seen before. See a link to the map in Additional Readings below. See also: Digital geological mapping; Geographic information systems; Geology; Mars; Space probe; Topographic surveying and mapping

Geologic maps use signs, symbols, and colors to indicate surface or subsurface features. The Mars map, for example, uses 44 colors to indicate geologic units, which describe Mars’ geologic history in terms of the ages of surfaces, stratigraphic relations, and other metrics. Additionally, 14 symbols are used to indicate landforms (caldera rims, ridges, etc.) and processes (tectonic, erosional, etc.) as well as one symbol (nongeologic) to indicate spacecraft landing sites. See also: Caldera; Cartography; Geologic mapping; Geomorphology

Digital geologic maps are essentially GIS databases. One example is the topographic data set collected on board the Mars Global Surveyor by the Mars Orbiter Laser Altimeter (MOLA) and stored as a digital elevation model—that is, an electronic file that relates elevation data to Cartesian coordinates. The MOLA data were used as the base layer of the Mars map over which other data sets (for example, cratering) were registered. Geologic map databases are useful because researchers can produce many different types of maps and can incorporate new information as it becomes available. See also: Altimeter; Database management system; Digital elevation models