Global Positioning Tutorial

In a field of study that is thousands of years old, GPS represents a quantum leap in geodesy. As advanced as GPS technology is, most commercially available GPS receivers are only accurate within several meters. Considering that the Earth is almost 25,000 miles in circumference, the difference of a few meters may not seem important. This level of accuracy may be adequate for a hiker in the woods or someone driving a car. But there are many scientific, military, and engineering activities that require much higher levels of positioning accuracy — often to within a few centimeters or less!

To provide measurements at this level of accuracy, NGS developed the NOAA Continuously Operating Reference Stations (CORS) Network (NCN). The NCN is a network of a couple thousand stationary, permanently operating GPS receivers operated by NGS and its partners, which are located throughout the United States and its territories. Working 24 hours a day, seven days a week, CORS continuously receive GPS radio signals and integrate their positional data into the National Spatial Reference System. This data is then distributed over the Internet for download and archived for future use. Using CORS data, users can post-process their GPS receiver data and provide coordinates that are accurate within a couple of centimeters. NGS provides a free service that does this post-processing with the CORS data which is called the Online Positioning User Service (OPUS). Monitoring CORS positions helps track where the ground is moving, especially in areas that are geologically active, sinking, or have been impacted by natural disasters such as hurricanes or floods.

Map of CORS locations around United States

The Continuously Operating Reference Stations (CORS) system is a network of stations throughout the United States and its territories that continually record GPS signals, and then provide the data to GPS users over the Internet.

Another powerful tool that continues to evolve along with GPS technology is the Geographic Information System (GIS). A GIS is a system designed to capture, store, manipulate, analyze, manage, and present spatial or geographic data. GIS applications are tools that allow users to create interactive queries (user-created searches), analyze spatial information, edit data in maps, and present the results of all these operations.

A GIS allows users to analyze data from different sources, including ground surveys, existing maps, aerial photos, and satellite imagery. In a GIS, specific information about a place, such as the locations of utility lines, roads, streams, buildings, and even trees and animal populations, can be overlayed and analyzed correctly when referenced to geodetic data called datums and reference frames. Using consistent datums and reference frames with many different data sets allows users to perform analysis on data that are all consistently aligned to each other, assuring that decisions made while performing spatial analysis are correct.

Using special software, regional planners and scientists can examine the layers of data and imagery individually or in various combinations to perform many tasks like:

  • Improving traffic flow.
  • Merging construction with utility systems.
  • Planning around environmentally sensitive areas.
  • Protecting the public from potential natural disasters.

Because a GIS stores data digitally, information can be quickly and economically updated, easily reproduced, and made widely available. In fact, because of its power and speed, GIS technology is doing most of the cartographic (mapmaking) work that, in the past, was laboriously done by hand on paper charts and maps.

The most important element needed to reconstruct geographic reality in a GIS is good spatial information and metadata (data about the data) documenting how it was created. If the spatial information provided to a GIS is sparse or of poor resolution, then the world created by the computer will be a lifeless digital shell — a sharp contrast to the complexity of our living Earth.

GIS Layered map

This example shows how GIS information about a place is stored in several separate but overlapping layers. In this illustration, the layers represent utility lines, buildings, roads, aerial photography, and topography.

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Author: NOAA

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