NOAA team uses GPS to size up monumental task

NOAA team uses GPS to size up monumental task

Geodesists take on a tall order'figuring exact height of the Washington Monument'and get results

By Frank Tiboni

GCN Staff

NIST scientist David Ward inspects a bracket the agency built to hold a GPS antenna atop the Washington Monument.

New York's World Trade Center buildings: 1,758 feet. The Empire State Building: 1,250 feet. Chicago's Sears Tower: also 1,377 feet.

But how tall is the Washington Monument'perhaps the most recognizable structure in the nation's capital?

That's a frequent question on the National Mall when tourists look up at the landmark honoring the father of the country and its first president, George Washington.

A team of National Oceanic and Atmospheric Administration geodesists, whose job it is measure the size and shape of the Earth and precisely locate points and coordinates, used Global Positioning System calculations this month to get figures on the exact height of the white granite obelisk.

'Engineers will also use this information to monitor the monument's stability, measuring any shifting, settling or other movement of the structure,' NOAA administrator D. James Baker said.

The last official geodetic measurements from atop the monument were made in November 1934 by the Coast and Geodetic Survey, the predecessor agency of NOAA's National Geodetic Survey. The earlier survey crew used a metal chain hung from the apex of the structure to the ground to get the height, said Charles W. Challstrom, the survey's current director.

The crew measured the monument'which was dedicated by President Chester A. Arthur in 1885'at 555 feet, 5.5 inches.

Sixty-five years later, a NOAA team traded in a chain for 24 Defense Department navigational satellites and portable GPS receivers.

Using GPS, a special antenna and a receiver that costs $20,000 to $25,000, the team got a precise 3-D position survey. The team also had to have something more'nerve.

The team members rode a caged elevator up the scaffolding on the outside of the monument'which is in the midst of repairs'to the 490-foot level. Then they climbed to the apex on a 60-foot ladder, which had a catwalk under it.

'It is an awesome experience to touch the top of the monument,' said Dave Zilkovski, the National Geodetic Survey's deputy director.

Zilkovski tested some of the equipment at the apex. 'It's a feat that only a few people will experience.'

Challstrom said he could not wait to touch the aluminum that blankets the top of the monument. In the late 1800s, aluminum was considered a precious metal.

Once the team members had climbed to the top of the ladder, waving at sightseers inside the monument on their way up, they placed on the apex a mounting bracket designed and built by the National Institute of Standards and Technology. Then they attached a GPS antenna the size of a Frisbee to the bracket.'The antenna had a microstrip that received radio signals from GPS satellites on two frequencies.

Via a cable, the antenna was attached to a GPS receiver inside the monument. The receiver captured and corrected for the time delay between the time the signal left the satellite and when it hit the antenna, Challstrom said.

Time to read

The team used five to seven satellite readings for the survey. NOAA took the data from the receiver along with other data received from GPS stations at NIST and NASA's Goddard Space Flight Center in Greenbelt, Md., to NOAA headquarters in Silver Spring, Md., for processing.

In their offices, the team members hooked up the receivers to Pentium PCs running Microsoft Windows NT 4.0 to analyze the data with a trio of GPS analysis programs.

How tall is the Washington Monument today? GPS data found it is 555 feet, 5.9 inches. The 1930s-era metal chain was only 0.4 inches off.

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