LiDAR and the Antonine Wall

Not all archaeology is done with a trowel and bucket, and digging a hole isn’t the only way we can learn about our past. A growing number of archaeological projects are undertaken using remote sensing techniques and some can even be done without leaving the house. LiDAR is one technique which is widely used to investigate archaeological landscapes and can yield fascinating results.

The Antonine Wall is a Roman Frontier stretching for 62km between the Firth of Clyde (northwest of Glasgow) to the Firth of Forth (west of Edinburgh) and was occupied by three Roman legions for a relatively short period between 142 AD and 162 AD.  The monument is comprised of a large ditch with an earthen rampart to the south, this rampart has a number of forts and fortlets (small forts) attached to its rear and forms a linear defensive system crossing central Scotland.

The Hidden Landscape of the Roman Frontier project was a 3-year project jointly funded by Historic Environment Scotland and Canterbury Christ Church University, it used LiDAR data covering the Antonine Wall World Heritage Site to investigate the monument.

What is LiDAR?

LiDAR stands for Light Detection And Ranging. It is a laser scanning technique which has become a standard tool in the kit of the landscape archaeologist. The technique uses a laser sensor mounted on an aircraft to digitally survey the land beneath during the aircraft’s flight. The sensor emits tens of thousands of pulses of light each second, these pulses of light hit the ground and are reflected back to the sensor; the length of time this takes is recorded and using the speed of light the distance to the ground is calculated.

From these measurements a detailed 3-dimensional model of the ground (a digital elevation model or DEM) can be generated and investigating this model can reveal traces of archaeological earthworks.  For the DEM to become useful to archaeologists the data needs to be visualised. This is achieved by loading the data into a computer program and in effect shining a digital sun across the model’s surface.  This creates digital shadows which highlight banks and ditches just like the sun does in late afternoon, allowing these earthworks to be easily identified.

Why Do Archaeologists Use LiDAR?

An additional benefit of using LiDAR as a survey technique is that it allows archaeologists to investigate the ground surface beneath areas of tree cover, as some of the beams of light can pass through the tree canopy and reach the ground beneath and are reflected back to the aircraft; these are called ‘the last returns’.  If the last returns are identified and isolated from those generated by the tree canopy, it allows the canopy to be removed from the DEM therefore revealing any surface hidden beneath and allowing it to be searched for past remains.

Aerial photograph of Rough Castle Roman fort showing the wooden areas to the east of the fort, and interpretation of the LiDAR data for Rough Castle Roman fort, showing the fort and an earlier field system (© HES & CCCU 2020)

In the aerial photograph above we can see the area to the east of Rough Castle fort is under dense tree coverage, preventing any archaeological features beneath from being identified.  If the LiDAR data for the area is processed to remove the tree canopy, the ground surface beneath is revealed. When the area to the southeast of the fort is examined, traces of a field system (the thin red lines) pre-dating the construction of the fort are revealed.

Using this technique, the Antonine Wall World Heritage Site was investigated, and previously unidentified archaeological features were mapped and over 400 new sites were identified in total (two examples of which are shown above and below).

Discover more about the Antonine Wall by visiting the monument’s website or follow the @AntonineWall on Twitter for information on the “Rediscovering the Antonine Wall” project.

By Dr Nick Hannon, an archaeologist specialising in the use of remote sensing techniques to investigate archaeological landscapes. The LiDAR analysis of the Antonine Wall World Heritage Site was undertaken by Dr Hannon whilst a doctoral student at Canterbury Christ Church University, under the supervision of Dr Lyn Wilson (Historic Environment Scotland) and Dr Darrell Rohl (Canterbury Christ Church University).  Nick’s thesis title was “The Hidden Landscape of a Roman Frontier” and involved the detailed analysis of a range of remotely sensed data covering the World Heritage Site.

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Header Image: Rough Castle ramparts, part of the Antonine Wall built by the Romans in AD 140 (© VisitScotland / Kenny Lam)

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This article was produced as part of Scotland Digs Digital. In the summer of 2020, we shone a spotlight on Scottish archaeology with the Scotland Digs Digital campaign which brought together online and offline events, as well as live updates from across the country for everyone to enjoy.

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