Mapping from coast to coast, two centimeters at a time
Survey USA Day contributes precise GPS data to national database
- By William Jackson
- Mar 22, 2011
Hundreds of surveyors using Global Positioning System technology hit the fields across the country Saturday to provide detailed positioning information for the online database maintained by the National Geodetic Survey (NGS).
Survey USA Day was, first of all, a public relations event for the National Society of Professional Surveyors, said Joe Evjen, a geodesist with the NGS. But the data submitted to the Online Positioning User Service (OPUS) will contribute to the reference system that is the basis for most mapping and surveying in the country.
The National Spatial Reference System consists of about 1.5 million passive markers installed by the NGS over the last 200 years, as well as about 1,700 Continuously Operating Reference Stations that provide streams of real-time GPS data.
Fixing the flaws in North American maps
Geodesy is a specialized area of surveying that involves accurately measuring factors such as the shape of the Earth and its gravitational fields, which can affect positioning information. For the first 200 years of its history, field workers with NGS, now a part of the National Oceanic and Atmospheric Administration, surveyed and placed the markers for the spatial reference system.
“We don’t really do anything anymore,” Evjen said. “All of this work is being cloudsourced now.”
Since the 1970s, individual professional surveyors have placed the markers and submitted survey information to NGS. With the advent of professional-grade GPS equipment, this survey data has become more accurate, and OPUS makes it easier for surveyors to use and submit data.
GPS satellites orbit around the center of the Earth’s mass, which now has been located to within less than a centimeter. These precisely measured orbits enable the current generation of consumer GPS devices to be accurate to within a few meters on the Earth’s surface instantaneously. But more sophisticated equipment used by surveyors to take measurements over a matter of hours can provide data on horizontal location to within about two centimeters, and within three to eight centimeters in height.
The surveyor’s equipment receives precise timing signals from GPS satellites every 30 seconds. With 15 minutes of data, a surveyor can submit it to OPUS and receive an e-mail response detailing the location of the spot surveyed. If the surveyor collects four hours of data, it can be submitted for inclusion in the database, to become part of the National Spatial Reference System and used by other surveyors.
Evjen said OPUS has about 1,200 users a day and about 1 percent of them choose to submit data. These submissions not only establish new points in the spatial reference system, but they also can track changes in previously charted points.
With the accuracy of GPS data, “we’re seeing geophysical phenomena we’ve never seen before,” Evjen said. “Plate tectonics was a theory,” but the NGS now can observe the movement of continental plates, even detect them buckling upward when the moon passes over them.
Results from Saturday’s Survey USA Day have not yet been submitted, but the event's chairwoman, Debi Anderson, said surveyors were taking part in all but one or two states. She said the event was intended in part as a morale booster for the profession, “since the economy has devastated so many of us.”
Evjen said surveyors in Pennsylvania were charting the position of a number of original markers along the Mason-Dixon Line, the boundary between Pennsylvania and Maryland and the traditional dividing line between North and South. In Washington, D.C., a stone now on the National Mall near the Washington Monument, placed by Thomas Jefferson to mark the prime meridian for the United States, was surveyed. This was only the second time the position of the stone has been officially checked by NGS, Evjen said.
“We ignored it for most of 200 years,” he said, before positioning it about 10 years ago. Saturday’s survey was a revisit to check for accuracy.
Evjen himself surveyed a point in the District of Columbia’s Fort Reno, designated unofficially by the National Park Service as the highest point in the District, at 409 feet above sea level. He said he thought it was a shame the point had not been officially surveyed and added that he has wanted to do the job for years.
“This gave me the impetus to get out of bed on a Saturday and do it,” he said. He has not yet gotten the results of his measurements on the official height of the point.
William Jackson is a senior writer of GCN and the author of the CyberEye blog.