- Roadmap to Resources
- Tutorial PDF
- Subject Review
- Geodesy (audio podcast)
- Is the Earth round? (Ocean Facts)
- What is Geodesy?
- The History of Geodesy
- Figure of the Earth
- The Horizontal Datum
- The Vertical Datum
- The National Spatial Reference System
- The Global Positioning System
- CORS and GIS
The Elements of Geodesy
Taking it to the next level: CORS and GIS
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 Continuously Operating Reference Stations (CORS) network. CORS is a network of hundreds of stationary, permanently operating GPS receivers throughout the United States. Working 24 hours a day, seven days a week, CORS stations continuously receive GPS radio signals and integrate their positional data into the National Spatial Reference System. This data is then distributed over the Internet. After logging onto the CORS Web site, users can determine the accuracy of their coordinates to the centimeter. This system has been especially useful in assessing the integrity of buildings and bridges in areas that are geologically active or have been impacted by natural disasters such as hurricanes or floods.
Another powerful tool that has evolved along with GPS technology is the Geographic Information System (GIS). A GIS is comprised of three parts: spatial information, special software, and a computer. These components work together to provide a digital platform for viewing and processing layers of spatial information.
A GIS assembles information from a several of 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, is layered over a set of geodetic data. Using special software, regional planners and scientists can examine the layers individually or in various combinations to improve traffic flow, merge construction with utility systems, develop around environmentally sensitive areas, and protect 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. 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.