What is LiDAR? What can it show us?

What is LiDAR survey?

LiDAR stands for ‘Light Detection and Ranging’, and is a means of surveying large areas of terrain from the air, using sophisticated laser and positioning equipment. Essentially, LiDAR works in the same way as laser scanning does on land, by firing pulses of light from a laser and recording the time it takes for each pulse to return to the instrument, and accurate distance can be measured. Modern scanners can fire thousands of laser pulses per second, and by mounting the instrument on an aircraft (either an aeroplane or helicopter), very large areas can be covered in high resolution in short spaces of time. Each distance measurement is positioned in geographic coordinates provided by an associated GPS and the results plotted in a computer, producing a very large 3D model of the area surveyed. Nearly a billion points were collected in the LiDAR survey presented on this website.

How is LiDAR used?

LiDAR has the advantage over aerial photographs that the surface texture and colour can be removed, allowing you to inspect terrain without the distraction of vegetation and other colours. By lighting the terrain from different directions, slight features like walls and earthworks can be highlighted in ways that would be impossible with a photograph. Unlike traditional 2D maps, LiDAR data can also be rotated and inspected in 3D, allowing archaeologists to inspect new features and analyse how they relate to one another within the landscape.
Additionally, the survey data can be sampled and ‘cleaned’, to remove vegetation and buildings, providing what is known as a ‘bare earth’ model of the survey area. This can be used to help identify archaeological sites without the distraction of bushes and trees growing around archaeological sites.

The image above shows the bare earth model on the right. When compared with the ‘uncleaned’ LiDAR data
(left) the enclosure in the centre of the image can be seen in its entireity.

This technique of survey allows us to produce as detailed a record as is currently possible of important archaeological landscapes such as that around Cnoc Freiceadain in Caithness.

 

Displaying LiDAR data

As might be expected, LiDAR elevation models can be displayed in a range of ways. Most typically, the terrain models are displayed as hillshaded relief, with computer-simulated shadows cast by artificial light thrown over the terrain. This helps to produce a realistic representation of the elevation model, shading features like walls, ditches and banks with modelled shadows. This provides an easy way for non-technical users to inspect the data and identify archaeological features easily.
The data on this website is presented using a Swiss-style hillshading technique, using several superimposed layers of elevation data displayed according to slope and variation in illumination, to provide a good representation of the terrain in a single image.