Lidar

An exciting new technology using lasers mounted beneath aircraft is revolutionising how we map and visualise our landscapes and cities. This is known as ALS (Airborne Laser Scanning) or lidar (Light Detection And Ranging). It is used to precisely capture the shape of the earth’s surface and opens new and fascinating insights into entire cultural landscapes.

A lidar system is based on sending out thousands of laser pulses per second. The laser pulses are reflected by vegetation or buildings or soil. By measuring the time that passes while the light travels from the airplane to the ground and back, the distance between airplane and ground can be measured. The current position of the airplane and the direction into which each laser pulse was sent are measured using GPS and inertial navigation units.

By combining millions of such measurements, a very detailed three-dimensional model of the earth surface can be computed. But what about archaeological sites hidden under trees and bushes? This is where we begin to see the true power of lidar mapping systems. Some laser light is reflected from the vegetation canopy, but some reaches the ground – the scanner onboard the airplane receives multiple reflected signals. To visualise the ground surface beneath the canopy the data needs to be filtered. For every pulse that was sent out, only the last reflected signal - the one that has travelled the longest way - is chosen. Some laser pulses do not reach the ground at all. They can be identified and discarded by comparing neighbouring points. The result is a three-dimensional model of the ground surface from which the bushes, hedges and trees have disappeared. Barrows, hollow ways, mining pits or former field structures become visible in this virtually de-forested landscape and can be mapped by archaeologists.

The articles have been written by Arcland members about Lidar.