Laser maps tower rubble

By BBC News Online science editor Dr David Whitehouse

A laser-based sensor flown over the wreckage of the World Trade Center has produced a topographic image of "ground zero".

It shows what remains of the fallen twin towers and is helping rescuers clear the scene.

The aircraft, which uses a lidar - a device that shines a pulsed infrared laser on the rubble and analyses the echo - has special permission to over-fly the WTC site at a height of about 1,520 metres (5,000 feet).

Researchers are producing images each day that assist rescue teams as they pick their way through the rubble, pinpointing unstable and shifting areas, as well as underground fires.

"Ground zero": The red/orange colour shows depressions in the ground

The processed image at the top of the page shows just how little remains of the north and south towers. It also reveals craters reaching below street level and debris on the roofs of nearby buildings.

Echo data

Lidar stands for "light detection and ranging". It is similar to radar, except that it uses light instead of radio waves.

It transmits a laser pulse to the ground and measures the time it takes to be reflected back. The average round-trip takes a hundred-thousandth of a second. Variations in this time give the height of the ground.

The aircraft makes daily flights over ground zero giving out over 15,000 laser pulses a second. The echo data are analysed and converted into a grid of more than 100,000 elevation points, spaced about 1.5 m (5 ft) apart. The resulting map has an accuracy of about 15 centimetres (six inches) in height.

Other sensors carried on the airplane have been taking photographs and looking at thermal radiation to track underground fires that are still burning beneath the rubble.

Large-scale copies of the three-dimensional map have been produced for firefighters and rescue workers. Maps produced on subsequent days allow rescuers to look for indications of movement, but, so far, everything appears to be relatively stable.