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Lasers map Ground Zero

17 Jun 2002

Laser scanning helps US scientists map the devastation at the World Trade Center and the Pentagon.

US scientists are using airborne laser swath mapping (ALSM) and ground-based laser scanning to map regions around the World Trade Center and the Pentagon. By collecting millions of laser measurements from the ground, rubble and buildings at these areas, the team will build three-dimensional models of the disaster sites.

The collaboration involves the US Department of Defense's Joint Precision Strike Demonstration, the University of Florida, the National Oceanic and Atmospheric Administration, the National Geodetic Survey and Canadian imaging equipment maker Optech.

Airborne imaging provides broad coverage of the mapping areas while ground-based laser scanning gives higher resolutions. By combining the two, the researchers obtain the information they need to make topological maps of the regions.

"These maps will be more detailed and accurate than any other maps used by recovery workers and planners," said team member Dave Bloomquist of the University of Florida.

Canada-based Optech manufactured both laser technologies. Based on laser radar, the systems operate in a similar way to ordinary radar. Narrow light pulses reflect off an object and return to a sensor or counter. The sensor then measures the time travelled by the pulse and converts it into distance.

The intensity of a laser pulse depends on an object's reflectivity, for example vegetation has a high reflectivity, while wet, dark surfaces like mud have low reflectivities. An object's elevation, density and orientation to the sensor also affect the reflectivity. These differences allow data processing software in the mapping systems to classify different objects.

To map an area from the air, the mapping system and a digital camera are attached to a laser sensor unit. These are mounted on a light aircraft, which flies between 600 and 1000 m above ground level, at 200 km/h. Real time GPS navigation tracks the measurements and aircraft locations.

Bloomquist says that the ALSM system emits 10 000 pulses per second and collects data every 2 m. By flying over an area several times, the team can increase the density at which they collect data. "[Soon] we will have an updated 33 kHz system," he added. "This will reduce the spacing to around 0.5 m."

The team expects to return to the World Trade Center within the next two months to complete their mapping of the area.

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