NOAA expands geodetic reference network

The National Oceanic and Atmospheric Administration (NOAA) has expanded its international geodetic network with the addition of 43 new Global Positioning System tracking sites.

The sites are part of the Continuously Operating Reference Station (CORS) network maintained by NOAA's National Geodetic Survey (NGS), which helps surveyors and other users determine the 3-D positions of sites or objects to within a few centimeters. The additions bring the number of CORS sites to more than 1,200 in the United States, its territories and several foreign countries.

The Federal Aviation Administration established 13 of the new sites as part of its Wide Area Augmentation System for aircraft navigation. Four WAAS sites are in Alaska, four are in Canada, and five are in Mexico.

The CORS network is part of the National Spatial Reference System, a nationwide array of more than 1 million survey reference points that dates to 1816 and serves as the foundation for all of the mapping, charting and surveying performed in the country. Many of the reference points are passive markers ' brass medallions embedded at known points to establish spatial baselines. They are sometimes embedded in sidewalks or on stone markers; others are on top of broadcast towers, and some are buried a foot or more underground in fields so they won't interfere with farmers' plowing.

When the system was established, surveyors could locate the points within an accuracy of half a mile. Improvements in technology in the past 190 years have allowed NGS to refine those locations, and satellite-based GPS technology is helping to correct errors in the system as large as 5 centimeters, or about 2 inches. Modern technology can now place a point to within 1 centimeter on the Earth's surface.

NGS recently completed a general realignment of the National Spatial Reference System using data gathered during a 15-year survey to more accurately place about 60,000 existing reference points. NGS has also been expanding its CORS network, which broadcasts GPS data. Surveyors and mapmakers can use these known starting points to chart distances and create maps without having to physically set a transit on top of a marker.

Although the million or more passive markers are expected to remain in use for many years, CORS is becoming a more important part of the National Spatial Reference System. Surveyors, users of geographic information systems and others can combine their own GPS data with data from CORS sites to determine 3-D position coordinates to within a few centimeters of accuracy. They can also submit GPS information to NGS' Online Positioning User Service tool to have coordinates computed for them.

About the Author

William Jackson is freelance writer and the author of the CyberEye blog.

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