A Lidar sensor uses a laser to measure the distance between the aircraft and the ground. By recording millions of these measurements we can create a record of the relative heights of the ground surface. The laser also reflects off trees and buildings, cars, cows and cursus monuments – anything present in the landscape at the time of the survey is recorded. This means that the models must be interpreted by humans to make sense of the information they contain.
The Lidar models have been processed into a series of images that help us to make sense of the topography. The images used in the portal are described below.
Multi-direction hillshades of the surface model (containing all vegetation and buildings) and the terrain model (just the ground surface with all vegetation and buildings digitally removed). A multi-direction hillshade combines the modelled illumination from three sun directions to make a colour image to assess both big changes in the landscape like hills and valleys ( known as macro-topography) and small changes that can represent human actions over millennia (known as microtopography).
Local Relief Model (LRM) of the terrain data accentuates small scale changes in a clear image where positive features (banks, boundaries) are white and negative features (ditches, pits) are dark. This image is particularly suited to identifying subtle archaeological features.
Lidar technology
Lidar (Light Detection and Ranging) is based on the principle of measuring distance through the speed and intensity recorded for a pulse of light to be fired from sensor equipment and reaching a target, before sending a return signal.
Airborne scanning
Airborne Lidar systems rely on lasers to measure distance from the sensor to a target, in this case the ground and any features upon it. Lidar sensors commonly operate by scanning a laser beam from side to side as an aircraft flies over the survey area and recording the reflections. A cloud of height points is created from these reflections which can then be turned into 3D models of the landscape.
Global Positioning Systems
Although Lidar systems were developed in the 1960s, it was the widespread developments in Global Positioning Systems in the late 1980s that allowed for the creation of the highly accurate land surface models which we are familiar with today.
In the UK, it was The Environment Agency who instigated the widespread use topographic Lidar from 1996 onwards for the production of terrain maps to assess flood risk. In 2024, Natural Resources Wales completed their first complete coverage of Wales at 1m resolution, providing an incredible resource for understanding the historic environment in Wales. Although it is particularly well-known and useful in wooded areas, Lidar can also assist archaeologists in recognising earthworks and other features otherwise difficult to detect in open ground.
In order to visualise the data easily, a surface is created which can be modelled and shaded as a raster image. The most common raster images differ based on the point at which the pulses are returned to the sensor and the resulting data is processed. The Digital Surface Model (DSM) is the first return or highest point of pulse return, usually showing the whole landscape, including the ground surface in open areas, vegetation, woodland canopy and buildings. The Digital Terrain Model (DTM) is created by filtering the lidar dataset to remove all non-ground returns such as those that are reflected from vegetation and buildings. This filtering allows features under the vegetation canopy to be visualised.
Find out more!
To find out more about how lidar can be used to understand the landscape and identify archaeological features join the Arfordir Penfro portal volunteers to take our online training course.
Get involved
Become a ‘citizen scientist’ and help to discover, understand, and preserve the cultural heritage of the Pembrokeshire Coast National Park, by exploring the Lidar mapping and identifying new and exciting archaeological sites.