Further Reading

Turcotte, Donald L., and Gerald Schubert. Geodynamics. 2nd ed. Cambridge: Cambridge University Press, 2002.

geographic information systems (GiS)

Geographic information systems (Gis) are computer application programs that organize and link information to enable users to manipulate that information constructively. They typically integrate a database management system with a graphics display that shows links between different types of data. For instance, a Gis may show relationships among geological units, ore deposits, and transportation networks. Gis allows users to layer information over other information already in the database. A Gis database allows the storage of information in a particular geographical area no matter what that information may be.

Gis is a powerful tool for environmental data analysis and planning, allowing for better viewing and modeling of changing environmental conditions and the relationships that influence a given critical environmental setting. Many fields have come to rely on Gis for data collection and analysis, including environmental and health science studies concerned with risk assessment and mitigation, environmental modeling, resources exploration, sustainable development, natural resource management, transportation, air pollution and control, and forest fire management. GIS is used widely in science, industry, business, and government to sort out pertinent information for the particular user from the GIs database.

For a GIS to be accurate the data must be entered with precise knowledge of the location of features on the ground, referenced to a map grid or reference frame. Benchmarks are well-defined, uniformly fixed points on the land's surface used for reference points from which other measurements can be made. They are generally marked by circular bronze disks with a diameter of 3.75 inches (10 cm) and are embedded firmly in bedrock or another permanent structure. In the United States benchmarks are installed and maintained by the U.S. Coast and Geodetic Survey and the u.S. Geological Survey. The elevations of many benchmarks were in the past established by the surveying technique of differential leveling. Now it is more common to determine elevations with satellite-based differential global positioning systems. Benchmarks, often identified on topographic maps by the abbreviation B.M., are used for determining elevation and for surveying and construction.

Geographic information systems are typically used in conjunction with global positioning systems to collect spatially accurate location data in the field. Global positioning systems, commonly referred to by their acronym GPS, were developed by the U.S. Department of Defense to provide the U.S. military with a superior tool for navigation, viable at any arbitrary point around the world. The Defense Department pays billions of dollars for the development and maintenance of the GPS program, which has matured a great deal since its conception in the 1960s.

The configuration of the global positioning system includes three main components: the GPS satellites, the control segment, and the GPS receivers. Working together these components provide users of GPS devices with their precise location on the Earth's surface, along with other basic information of substantial use such as time, altitude, and direction. Key to understanding how GPS functions is the understanding of these components and how they interrelate with one another.

GPS satellites, named Navstar Satellites, form the core of the global positioning system. Navstar satellites are equipped with an atomic clock and radio equipment to broadcast a unique signal, called a "pseudorandom code," as well as ephemeris data about the exact position of the satellite relative to Earth and astromical reference frames. This signal distinguishes one satellite from the other and provides GPS receiv

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