Drone-mounted lasers see between the leaves
We recently completed a fascinating project with the National Trust. We flew a drone-mounted laser scanner (LiDAR) over the Iron Age hillfort at Badbury in Oxfordshire. We needed to create a detailed 3D model of the monument’s rampart and interior. This work marked the first application of this low-level LiDAR approach in the UK for a commissioned heritage management project, and we thinks it has great potential.
We typically do drone surveys of this nature with a normal camera and then process the images to produce a 2D orthomosaic and 3D model, as at Penycloddiau in Wales. However the monument at Badbury differs from the Welsh fort in one important aspect, it is completely covered by trees.
A laser scanner works by firing a laser at an object, timing how long it takes the pulse to be reflected back, and then converting that time into a distance. Because it knows its own position and the direction in which it shot the laser, the scanner is able to work out the position of the feature it has struck in three dimensions. A scanner sends out tens or even hundreds of thousands of laser pulses per second to generate a very dense set of 3D points, known as a ‘point cloud’. The extreme rapidity and density of this technique enables enough points to reach the ground, while many millions more will bounce off the leaf canopy. A useful analogy is that of falling snow flakes; many will rest on the tops of the trees but others will fall through the canopy to reach the forest floor. We are able to take those points which reach the ground to create a model of the surface, which can then be examined.
Very clever and traditionally rather expensive, airborne laser scanning (LiDAR) has largely been restricted to full size aircraft, covering many square kilometres in a single flight. In contrast, most archaeologists and heritage managers work at a smaller scale. However advances made in the last five years have seen LiDAR technology come down in price and size, particularly those components that accurately calculate the scanners' precise position throughout a flight, as well as the movement of the aircraft itself in the air, These improvements are making drone-based laser scanning (LiDAR) a viable option for the survey of smaller wooded heritage features and landscapes.
Not only does the drone-based approach make the scanning of smaller areas more cost effective, it also delivers a product that boasts far greater definition than that received from scanners flown in a traditional aircraft. At Badbury a density of 40-50 3D points per square meter was achieved, which may be compared with the c. 8-10 that would be expected from a traditional approach.
Most importantly of course is what the survey revealed about the hill fort. The detailed 3D model allowed examination of the fort’s ramparts, and clearly documented both the differential survival, and the longevity of later, possibly medieval field boundaries that abutted the monument. The scan revealed new information about the shape of the earthwork around the eastern side that suggest the existence of extra defensive components at this point, possibly in-turned or out-turned entrance ways.