Airborne Laser Scanning
Airborne laser scanning (ALS) (also known as LiDAR) is an active remote sensing technique that is used to create accurate and precise 3D models and visualisations of landscapes. As with most innovative technologies, ALS has its origins in the military where it was first developed to carry out underwater scanning to identify submarines. In the UK it was widely adopted during the 90s where it was used initially by the Environment Agency to create terrain maps to assess flood risk. However, it was not until the turn of the millennium that the potential of ALS for archaeological survey began to be recognised.
In practice, a ALS survey consists of the transmission of an active laser beam from a fixed-wing aircraft towards the ground. The reflection of the beams transmitted back to the aircraft are then measured to give distance values which are used to create a 3D Digital Elevation Model (DEM) of the landscape below. The intensity of the returning beam can also give an indication of the type of material that the beam was reflected from. This coupled with the height data can be used to identify and remove vegetation from a DEM, which in turn offers a view of hidden features and landscapes that may be obscured by vegetation. Global Navigation Satellite Systems (GNSS), or Global Positioning Systems (GPS) as it is more commonly known is also used during a survey to ensure that the 3D model is geo-located on the earth.
Using ALS on the CHERISH Project
The first CHERISH ALS surveys were flown in 2017 for six islands in Wales (Puffin Island, Skerries, Bardsey Island, St Tudwal’s, Ramsey Island and Grassholm Island). The 0.25cm resolution data has been used to accurately map archaeological features to produce new maps of all upstanding archaeology on each island. In Ireland, an ALS survey has been commissioned for the area surrounding Dublin bay. ALS data has also been used alongside aerial photography from which several cropmarks have been discovered across the islands.