Geodetic team looks to the skies for new map survey
- By William Jackson
- May 20, 2005
Those party animals at the National Geodetic Survey plan to celebrate the agency's 200th birthday with a general realignment of the National Spatial Reference System.
What is the National Spatial Reference System?
'NSRS is the most important element of our national infrastructure,' said chief geodetic surveyor David Doyle.
Doyle said he was being only slightly facetious. NSRS is a nationwide array of more than a million survey reference points that serve as the foundation for all of the mapping, charting and surveying in the country.
'Everything we do is related in some way to mapping something,' Doyle said. NSRS is used to help determine zoning law compliance, to keep track of shifting coastlines and waterways, and to help guide aircraft to safe landings at airports.
The NSRS dates back to 1816, but more precise data from today's Global Positioning System will be used to update information for the reference points in a 20-month process scheduled to be completed by Feb. 10, 2007. NGS now is a part of the National Oceanic and Atmospheric Administration, and NSRS data is available on the Web at www.ngs.noaa.gov
You might have seen some of the NSRS survey reference points. They are brass medallions you sometimes find embedded in sidewalks or on stone markers. Others are the top of broadcast towers and some are buried a foot or more underground in fields where they won't interfere with a farmer's plowing.
These are passive markers. NGS also uses a network of about 600 Continuously Operating Reference Stations that broadcast GPS data. Surveyors and mapmakers use the known starting points provided by the markers to chart the nation.
A handful of NSRS markers date back to the 1820s and 1830s, although 'they're getting scarce,' Doyle said. The positions originally were determined through astronomical observations, and in the early 19th century surveyors were able to place a marker anywhere on the Earth's surface with an accuracy of within half a mile. This is because they were working on the assumption that the Earth was a sphere.
'The Earth isn't round,' Doyle said. 'It ain't even close to round.'
But half-a-mile on a global scale was close enough for early surveyors, especially since most survey work is relative. That is, you don't particularly care where your starting point is in relation to the rest of the world. What matters is that the points you survey are accurately laid out relative to each other.
The basic techniques for establishing location are essentially the same today as 1816, but the technology has improved and along with it the accuracy of the observations. Today, GPS satellites have replaced observations of stars.
'GPS is the most phenomenal tool for expanding the NSRS,' Doyle said, so NGS is using that data to correct errors in the system as large as 5 centimeters, or 2 inches. 'In this day and age, 2 inches is something surveyors will get apoplectic about.'
The best accuracy today can place a point to within 1 cm on the Earth's surface.
The last general adjustment to the NSRS was in 1986. In 1987, NGS began a comprehensive survey of the system to gather more accurate data from what was then the new GPS. Over the next 15 years surveyors visited about 60,000 of the survey points to make observations.
'The survey has been done,' Doyle said. Beginning in June, the staff will begin the 20-month process of putting more accurate information in the database and making adjustments to bring the entire system into a better alignment.Built to last
The 600 Continuously Operating Reference Stations in the NSRS let surveyors get positioning data without having to physically set a transit on top of a marker.
Doyle said CORS is becoming a more important component of NSRS. But the million or so passive markers are expected to be in use for a long time, and improving their accuracy seems an appropriate way to celebrate the agency's bicentennial.
William Jackson is a Maryland-based freelance writer.