GIS and GPS: Principles, Components, and Applications

Geographic Information Systems (GIS) are computer application programs designed to organize and link spatial information, enabling users to manipulate data constructively. These systems typically integrate a database management system with a graphics display that reveals connections between different data types, such as geological units, ore deposits, and transportation networks. A key feature of GIS is the ability to layer new information over existing database content, allowing storage of any type of data pertaining to a specific geographical area. This functionality supports complex spatial analysis and decision-making across multiple disciplines.

GIS serves as a powerful tool for environmental data analysis and planning, facilitating improved visualization and modeling of changing environmental conditions and their interrelationships. Numerous fields rely on GIS for data collection and analysis, including environmental and health science studies focused on risk assessment, environmental modeling, resource exploration, sustainable development, natural resource management, transportation, air pollution control, and forest fire management. In science, industry, business, and government, GIS helps extract pertinent information from databases tailored to specific user needs.

For a GIS to maintain accuracy, data must be entered with precise knowledge of feature locations on the ground, referenced to a map grid or reference frame. Benchmarks are well-defined, uniformly fixed points on the land’s surface used as reference for subsequent measurements; they are typically marked by circular bronze disks measuring 3.75 inches (10 cm) in diameter, embedded in bedrock or permanent structures. In the United States, benchmarks are installed and maintained by the U.S. Coast and Geodetic Survey and the U.S. Geological Survey. Historically, benchmark elevations were established using differential leveling; today, satellite-based differential global positioning systems are more common. Benchmarks, often identified on topographic maps by the abbreviation B.M., are essential for elevation determination, surveying, and construction.

Geographic information systems are typically used alongside Global Positioning Systems (GPS) to collect spatially accurate location data in the field. GPS was developed by the U.S. Department of Defense to provide the military with superior navigation capabilities usable at any point worldwide. The Department of Defense invests billions of dollars in the development and maintenance of the GPS program, which has significantly matured since its conception in the 1960s.

The configuration of the global positioning system comprises three main components: GPS satellites, the control segment, and GPS receivers. Working together, these components provide GPS device users with precise location on Earth’s surface, along with additional useful information such as time, altitude, and direction. Understanding how GPS functions requires knowledge of these components and their interrelationships.

GPS satellites, named Navstar Satellites, form the core of the global positioning system. Each Navstar satellite is equipped with an atomic clock and radio equipment to broadcast a unique signal called a pseudorandom code, as well as ephemeris data regarding the satellite’s exact position relative to Earth and astronomical reference frames. This unique signal distinguishes one satellite from another and provides GPS receivers with accurate information about the satellite’s location. The satellites follow specific orbits around Earth, and their collective arrangement is termed a constellation. The GPS Navstar constellation is configured so that at any point on Earth’s surface, a GPS receiver can detect signals from at least six Navstar satellites, making precise constellation maintenance essential for proper GPS function.

Geosynchronous satellite orbit maintenance is performed by the control segment (or satellite control centers) with stations located in Hawaii, Ascension Island, Diego Garcia, Kwajalein, and Colorado Springs. If any satellite deviates slightly in altitude or from its correct path, the control segment initiates corrective actions to restore precise constellation integrity.

The component most visible to GPS device users is the GPS receiver. GPS receivers are single-direction, asynchronous communication devices that do not broadcast any information to Navstar satellites but only receive signals from them. Recent years have seen the miniaturization and mass proliferation of GPS receivers, which are now small enough to be integrated into hybrid devices such as cellular phones, radios, and personal desk accessories. GPS receivers are also standard equipment on many vehicles for land, sea, and air travel. The accuracy of consumer GPS receivers is typically no better than nine feet (2.7 m), whereas advanced GPS receivers can measure location to within less than one centimeter.

GPS receivers determine a location on Earth’s surface through trilateration—a process that calculates distances between the Navstar satellites and the receiver. For this to work, two conditions must hold: the locations of the Navstar satellites must be known, and a mechanism for precision time measurement must exist. For very precise measurements, an additional system is needed to reconcile errors caused by various phenomena.

GPS receivers are programmed to calculate the location of all Navstar satellites at any given time. A combination of the receiver’s internal clock and trilateration signal reconciliation (performed by the receiver itself) establishes a precise timing mechanism. When a GPS receiver attempts to locate itself, it receives signals from Navstar satellites; by measuring the time offset between the receiver’s internal pseudorandom code generator and the received pseudorandom code signal, the receiver applies the simple-distance equation to compute the distance to each satellite.

To accurately locate a point on Earth’s surface, at least three distance measurements are required. One measurement places the receiver within a three-dimensional arc; two measurements narrow it to a circle; three measurements identify one of two possible points. One of these points is typically floating in space or moving at an absurd velocity, so the GPS receiver eliminates it, thus resolving the receiver’s actual location. A fourth measurement enables correct point location and provides necessary geometric data to synchronize the receiver’s internal clock with the Navstar satellite’s clock.

Depending on the GPS device’s quality, error correction may also be performed during location calculation. Errors arise from multiple sources: atmospheric conditions in the ionosphere and troposphere alter the speed of light, introducing impurities into the simple-distance equation. Weather modeling can help calculate the difference between the ideal speed of light and its likely speed through the atmosphere, and calculations based on the corrected speed yield more accurate results. However, because weather conditions rarely fit models, other techniques such as dual frequency measurements (comparing two different signals to calculate the actual speed of the pseudocode signal) are used to reduce atmospheric error.

Ground interference, such as multipath error (caused by signals bouncing off objects on Earth’s surface), can be detected and rejected in favor of direct signals via complex signal-selection algorithms. Another source of error arises when Navstar satellites are slightly out of position; even a deviation of a few meters from the calculated position can affect high-precision measurements. Geometric error can be reduced by using satellites that are far apart rather than close together, as larger inter-satellite distances ease certain geometric constraints.

When precision down to the centimeter is needed, only advanced GPS receivers can produce such accurate measurements. These receivers utilize one of several techniques to pinpoint locations more precisely, primarily by reducing error or employing comparative signal methods. One such technique is differential GPS, which involves two GPS receivers: one monitors variations in satellite signals and relays this information to the second receiver, which then determines its location more accurately through improved error correction. Another method uses the signal carrier-phase as a timing mechanism; because the signal carrier operates at a higher frequency than the pseudorandom code it carries, carrier signals can synchronize timers more precisely.

Finally, a geostationary satellite can serve as a relay station for transmitting differential corrections and GPS satellite data. This approach, called augmented GPS, underlies the new WAAS system (Wide Area Augmentation System) installed in North America. The WAAS encompasses 25 ground-monitoring stations and two geostationary WAAS satellites, enabling superior error correction. This type of GPS is essential for aviation, particularly during landing sequences.

FURTHER READING: Demers, Michael N. Fundamentals of Geographic Information Systems. 4th ed. New York: John Wiley & Sons, 2007.
Gorr, Wilpen L., and Kristen S. Kurland. GIS Tutorial: Workbook for ArcView 9. 3rd ed. Redlands, Calif.: ESRI Press, 2008.
Longley, Paul A. Geographic Information Systems and Sciences. 2nd ed. New York: John Wiley & Sons, 2005.
Yeung, Albert K. W. Concepts and Techniques of Geographic Information Systems. 2nd ed. Upper Saddle River, N.J.: Prentice Hall, 2007.