Reference Guide to Emerging Technologies to Implement NEPA

1. What are geographic information systems (GIS)?

Geographic information systems (GIS) are defined as: "computer systems capable of assembling, storing, manipulating, and displaying geographically referenced information." (ESRI). GIS store information about the world as a collection of thematic layers that can be linked together by geography.

Defining features of GIS

For a complete introduction to GIS, please visit ESRI's GIS Tutorial Web site, http://www.gis.com.

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2. What are the key features of geographic information systems?
There are two defining features of GIS: the ability to overlay spatial data and the ability to be changed as new data becomes available.

The first key feature of GIS programs is the capability to overlay multiple sets of databases into a final map that graphically explains the relationships between the data. Spatial data - points, boundaries, and lines - comprise the base of the map and can be supplemented with tabular data - tables linked to the maps with further information - and image data, such as that from satellites. This powerful and versatile concept has proven invaluable for solving many real-world problems, from recording details of planning applications to modeling global atmospheric circulation.
The second key feature of GIS is that they can be thought of as "dynamic maps" which can be updated and altered as needed. These maps may also be manipulated for scientific analyses and to create models of different environments.

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3. What is the relationship between remotely sensed data and geographic information systems (GIS)?
Remote sensing data applications and geographic information systems (GIS) have an established history of interdependency. GIS provides a format to distribute of remote sensing data and to create valuable information from the data. Remotely sensed data is also a critical means to create base GIS maps and updating many data layers in a GIS. The integration of remotely sensed data and GIS is particularly attractive because 1) the conversion of remotely sensed raster-format data to GIS vector-format data is inexpensive and 2) remote sensing data offers a cost-effective way to visualize large geographic areas in a digital format.

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4. What is the Global Positioning System (GPS)?

The United States' Global Positioning System (GPS) is "an all weather, worldwide, continuous coverage, satellite-based, radio navigation system." The GPS can be described as operating in three segments: Space, Ground Control, and Users.

Space Segment: The Space segment consists of 24 satellites, uniformly distributed in orbit around the Earth. The satellites are divided into 6 orbital planes, and a user located anywhere on the globe has access to at least four satellites all the time, which is the number needed for accurate positioning.
Ground Control Segment: The ground control segment is comprised of one master control station, six monitoring stations, and four ground antennas. The stations determine the orbital mode and clock correction parameters for each satellite and relay this information to the ground antennas for transmission to the satellites for broadcast to the users in real-time or store the information to be entered as a data layer into a GIS.
Users Segment: The users are antennas and receivers that measure and decode the satellite transmission to provide positioning, velocity, and precise timing in real-time. The receivers can be handheld, personal data devices. Many receivers are capable of data collection and maintenance, and in the field, would be supported by a GPS antennae carried in a backpack. Data collected with machines such as this one can be easily transferred into a GIS for further data processing.

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5. How does the GPS collect information?
GPS receivers work via a process called "triangulation." Triangulation involves a receiver computing distances by measuring the travel time of signals from multiple satellites. The satellites are very carefully timed using atomic clocks timed to thousandths of a second, because a timing error of one-thousandth of a second would set the measurement off by nearly 200 miles. Three satellites are used to accurately triangulate the distance, and a fourth satellite is in orbit to correct for the timing inaccuracy of the receivers.

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6. How accurate is GPS information?
GIS data that is collected and referenced using a GPS receiver is subject to the error of the GPS satellites. Currently, an average GPS receiver has an "autonomous accuracy" range of +/- 5-10 meters depending on the sensor quality, the environment in which the recording was taken, and the latitude at which the recording was taken. The errors affecting a receiver with autonomous accuracy are: distortions of the signal by the atmosphere, distortions of the signal by ground interference, error caused by gravitational pull, timing errors from the atomic clocks aboard the satellites, and basic geometric error stemming from the receiver. Scientists using a receiver with Differential GPS (DGPS) capabilities can have real-time accuracies in the +/- 1-5 meter range, and even in the sub-meter and sub-centimeter accuracy range depending on the quality of the receiver.

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The materials on this Web site were developed by Ken Markowitz as part of a presentation at the ALI-ABA Course of Study: Environmental Impact Assessment: NEPA (National Environmental Policy Act) and Related Requirements, December, 2001, Washington, DC.