NOAA measuring Washington Monument using GPS

The National Oceanic and Atmospheric Administration kicked off a survey today to determine within centimeters the exact height of the Washington Monument and whether it has sunk or listed to one side.

NOAA Administrator James Baker said the agency would use a "satellite tape measure"—referring to Global Positioning System satellites—to obtain an exact measurement of the monument.

The Pentagon developed the $10 billion-plus GPS system to guide precision weapons, but it has evolved into a worldwide navigation and measurement tool embraced by a variety of users, including hikers, pilots and surveyors.

Baker spoke to reporters at the Meridian Stone, installed as the result of surveys started by Thomas Jefferson on the Ellipse between the White House and the monument, which is covered in scaffolding as it undergoes rehabilitation.

Dave Zilkoski, deputy director of NOAA's National Geodetic Survey (NGS), said surveyors will use GPS receivers located at the apex of the monument and two others located in Maryland to help them determine the monument's exact height. Zilkoski said NOAA needs to run these receivers for five days to obtain the centimeter-level measurements of the monument. Such accuracy will be achieved by, among other things, eliminating the errors deliberately introduced by the Defense Department into the civilian GPS signal. DOD purposefully skews the civilian signal to make it less accurate than the military signal.

Baker said the space-age survey of the monument—which was last surveyed in 1934 with handheld optical instruments called theodolites, spirit levels and measuring rods—will help the National Park Service determine whether the 555-foot-tall obelisk has sunk or listed to one side.

Zilkoski said that data obtained from the five-day GPS survey of the Washington Monument will have real-world applications far from the Capitol City. He said the survey will help NGS fine-tune the use of GPS receivers to determine the height of a variety of structures by helping the agency determine the effects of multipath GPS signals—when the signal from a satellite bounces off the atmosphere or a structure—on system accuracy.

Zilkoski said this data can be used to help subtract multipath effects from GPS-derived height measurements. For example, it could have great economic benefits for container ship operators, who need to determine the exact position of the keel of a ship above a channel, he said. "Every inch of capacity translates into hundreds of thousands of dollars of cargo,'' Zilkoski said.