NOAA uses GPS to measure monument height

The National Oceanic and Atmospheric Administration last week used a high-tech tape measure to help it determine whether the Washington Monument is going to turn into the Leaning Tower of Washington.

NOAA began a measurement project last week in cooperation with the National Park Service, which is in the midst of rehabilitating the famed obelisk that dominates the National Mall in the heart of Washington, D.C. Using Global Positioning System technology, surveyors hope to determine if the monument, last surveyed in 1934 with manual instruments, is shifting or sinking - and therefore unstable.

Although NOAA is not scheduled to complete its five-day survey of the monument until today, a NOAA spokesman said preliminary results from the GPS survey last week indicated that the monument is 555 feet, 5.9 inches tall, four-tenths of an inch taller than the Park Service determined in 1934.

The NOAA spokesman added that while figures are "subject to fluctuation" during the survey, preliminary figures indicate that the monument is not leaning.

NOAA administrator James Baker said the survey's final results would be added to the National Spatial Reference System maintained by NOAA's National Geodetic Survey (NGS). The system is the basic reference system used for all types of surveys conducted nationwide.

Baker said the monument survey also served as a highly visible example of how civilian users, ranging from hikers to pilots, have embraced the precise position and location information provided by the $10 billion-plus GPS, originally developed by the Pentagon to guide precision munitions and aid in the navigation of warships and fighter and bomber aircraft.

Dave Zilkoski, deputy director of NGS, said surveyors used GPS receivers placed at the apex of the monument and two receivers located in Maryland to help them determine its height with centimeter-level accuracy. Zilkoski said NOAA needs to run the receivers for a period of five days to obtain this precise measurement partly to correct for errors deliberately introduced into the civilian GPS signal by the Defense Department that makes it less accurate than the military signal.

Zilkoski said that data obtained from the survey will have applications beyond determining the monument's height. The survey will help NGS determine the effects of multipath GPS signals - when a satellite signal bounces off the atmosphere or a structure - on system accuracy. The monument's scaffolding is expected to induce more multipath signals than would normally be encountered, Zilkoski said.

Multipath data obtained from the monument survey, Zilkoski said, can be used to help subtract multipath effects from GPS-derived height measurements. This could have great economic benefits for an operator of a container ship, who needs to be able to determine the exact position of the keel of a ship above a channel, Zilkoski said. "Every inch of capacity translates into hundreds of thousands of dollars of cargo," Zilkoski said.