Satellites and aerial laser images are helping guide the rescue effort at the World Trade Center in New York. Scientists are combining them to produce extremely accurate daily maps of the disaster site's topography.
Every day, a twin propeller aircraft passes over the rubble where the World Trade Center towers once stood.
From 1,500 meters up, pulses of an infrared laser beam bounce at the speed of light off the debris back to an instrument aboard the aircraft. The instrument is called lidar, the optical equivalent of radar, sensing the shape of a target some distance away.
Environmental scientist Sean Ahearn of New York's Hunter College says 15,000 laser pulses each second produce data that he transforms with a computer into a three-dimensional topographical image of the terrain below. The less time it takes for the beam to return, the higher the surface is. "If you can measure time," he said, "you can calculate distance. What we end up with is actually thousands and thousands of points throughout the area. We take those points and interpolate a surface from those points."
The result is a map accurate to about 30 centimeters that, when color-coded, reveals the height or depth of any surface feature. Mr. Ahearn says daily updates are an invaluable aid to firefighters and other workers at the wasteland of the September 11 terrorist disaster. "Since it's very difficult to orient yourself," he said, "the shapes of the mounds are often what people understand and see. So the fire guys actually have a big lidar image on the wall and when they discuss logistics, they go over to that image for reference."
Engineers also use the lidar map to see if any ground is sinking. The sensor can detect surface movements as slight as seven to eight centimeters. Bryan Logan is the chief executive of the company that collects the daily images, Earth-Data International. Mr. Logan said, "When we compare day one flight with day two flight with day three flight, we can actually see if there is movement, either on the rubble piles or in any of the surrounding buildings that might detect that a building might be unstable ready to collapse. Another collapse of rubble might take more lives."
While the lidar instrument is shooting its beams to the ground, another instrument communicates with the network of global positioning satellites to determine precisely where the aircraft is. This information is added to the map to give it precise coordinates.
A third tool on the aircraft measures heat radiating from the ground to help rescuers track underground fires. Bryan Logan says this requires the flights to take place at dawn. "That way," he said, "it has had the night to cool off areas that were heated by sun and only the areas that are heated by underground combustion would show up with the sensor."
Knowing the locations of subterranean fires, New York City officials can determine if they endanger gas lines or, indeed, might be fueled by ruptured lines. Mr. Logan says that the data show that the fires are going out.
His company and Hunter College scientist Sean Ahearn were well placed to make such a map. They had already collaborated to produce one of the entire city earlier this year. So New York city and state officials knew where to turn when terrorist airliner attacks caused the World Trade Towers to collapse.
The Earth-Data International aircraft began its daily imaging overflights four days after the disaster. Mr. Ahearn says the next step is to outfit workers at the scene with hand held devices that display the daily rainbow-colored map so they know immediately where in the rubble they are. "We're not creating pretty pictures here," he said. "We're trying to create something that supports the logistics of the people in the field."