Airborne lidar (light detection and ranging) measures the height of the ground surface and other features in large areas of landscape with a resolution and accuracy hitherto unavailable, except through labour-intensive field survey or photogrammetry. It provides, for the first time, highly detailed and accurate models of the land surface at metre and sub-metre resolution. This provides archaeologists with the capability to recognise and record otherwise hard to detect features.
Lidar operates by using a pulsed laser beam which is scanned from side to side as the aircraft flies over the survey area, measuring between 20,000 to 100,000 points per second to build an accurate, high resolution model of the ground and the features upon it. For further details of the technology see the Unit for Landscape Modelling - Cambridge University (ULM) or the Environment Agency Geomatics Group.