WHAT IS LIDAR PICKING UP?

A recent addition to our Roman Road Abstracts submitted a new Roman Road in Dorset running from Bridport to the Roman fort at Axminster, which was largely identified by LIDAR (Ref 1). The author claims this new technique removes the need for extensive fieldwork and proof of a Roman Road by excavation. This claim may be false, as we have found problems of how LIDAR works, which is reflected in this Ref 1 report.

The author admits field work without the LIDAR would not have established this new Road, and this is borne out by the contents of Ref 1. The Environment Agency LIDAR has a resolution c 1m in on low ground, but only 2 or 3 m on high ground. You ought to be able to see a 1m structure on the ground with little difficulty. So there is a problem with what it is picking up - no excavations were undertaken. Much of the route was not on low ground, and structures on the ground were not seen at many LIDAR sites.

We used LIDAR to find how the 40' lane of the Chichester Roman Road crossed the r Meon flood plain - its a 3 lane Roman Road, with the 40' lane taking a different route from the 25' lanes. We did find evidence of patterns of light and dark lines over the expected line of the stonework www.nehhas.org.uk/rrch10.htm, (Ref 2). But this was over the flood plain where no optical evidence showed the Roadway. This is shown on the map in Ref 2, going off the west side of the map just below Exton. On a flood plain you can expect a meter or more of washed down material before you reach any Roman levels.

I did my Ph D using RADAR, and know the radiation penetrates the medium in the reflection process - and it seemed the LIDAR must penetrate the ground to give these effects. But distance penetrated is related to the wave length. LIDAR are lasar pulses of visible, UV or IR light - which I would not expect to penetrate the ground to such an extent as the wave lengths are millionths of a mm. Thus we do not know what process cause LIDAR effects being observed.

The map in Ref 2 shows the LIDAR image of a Grid system which we are interpreting as the street grid of a Roman settlement. The text describes the colour scheme used on the map relating to the various prints of the LIDAR, and if they are also followed by air photo evidence. As can be seen some of the LIDAR is over the flood plain. We are excavating on one of these Grid Lines, and believe we have found what they are made of: rammed chalk with Roman or earlier finds in it at 80cms below the ground surface, resting on the natural chalk some 108 cms below ground surface. The natural chalk has also been rammed to give a foundation layer. Above the rammed chalk there are a lot crushed flints in the subsoil, with Roman finds - this may be the Running Surface of the Street or the Roman Road lane crossing it. The crushed flints started at 26 cms below ground surface. There was no optical evidence visible at the excavation area for either the Grid or Roman Road lane.

Thus we know there is clearly Roman engineering going on, but cannot see what physical processes are causing LIDAR to react to them. We can understand that the greater stone content of the ground can cause air photography to react - but you still need excavation to determine what and who put it there. There have been a lot of modern cuts in the ground for pipe laying and drainage, and clearly we don't know how LIDAR reacts to these - which is relevant to the claims in Ref 1 to dispense with excavation and fieldwork.

LIDAR is available where buildings and vegetation, including trees and woods, are removed - to give the ground profile. LIDAR works by timing the lasar pulse from it being sent to its reflection from the ground being recorded in the aeroplane - this is the general principle of RADAR - but timing devises are now much more accurate that in WW2. In my Ph D work I was limited to a micro-second counter - but light travels 327 yards in a micro-second. Today LIDAR processes the lasar pulses taking the longest time to get back to the aircraft to form the ground profile. One can understand in vegetation or trees some of the lasar pulses get back from the ground to the aircraft (though it often does not work completely in woods). For buildings, are we to understand that some of the lasar pulses manage to go through the roofs and floors, and return through them after being reflected from the ground? If this is so, then its understood that LIDAR will react to different things below the ground surface as the lasar pulse penetrate significantly into the ground during the reflecting process. However its not understood how any significant amount of visible light will penetrate a roof and several floors, and be detectable on reflection in an aircraft.

With these problems an enquiry was undertaken (Ref 3). LIDAR has developed a lot more applications than simply giving a profile of the ground. By trial and error and changing the colours of the light pulses it is possible to detect a range of materials - rock, water and chemicals present in the ground. In sending pulses upwards into the air it's possible to detect clouds and different type of molecules. Light can be returned from the air or ground by various forms of back scattering (Raleigh, Ramen, fluoresce). These can also enable different forms of vegetation to be detected, including growth of crops and problems with them. For archaeology it is recognised that the intensity of the returned light is affected by features buried under the ground - as it effects plant growth - especially as seen in IR light.

We are using an available source of LIDAR - so we are not in a position to experiment with different colours of light pulses. It means that LIDAR must also be regarded as partly an air photo, where the light illumination is also provided. We are thus not able to predict how underground features will show up on the LIDAR prints. We can be confident in the example above of the 40' lane crossing the flood plain that the LIDAR is reacting to the Road's stone work through series of light and dark lines, by the effect on vegetation growth. It is however more important to excavate to learn exactly what is giving rise to this LIDAR result. We are on stronger ground on excavating over the Street Grid, finding that chalk and stone work is much nearer the ground surface. The LIDAR result of light lines bounded by dark lines is similar to the air photo result for Roman Roads - and quite a lot of this Grid is showing as light lines on air photos also.

There is not an explanation on how buildings are removed. The method is to filter out the earlier reflected pulses, assuming the later returning pulse is the ground surface. Like trees, buildings often are not removed.

Richard Whaley

REFERENCES

1. Hugh Toller, The Roman Road from Dorchester to Exeter, Devon Archaeological Society Proceedings 72 2014, p103.

2. Exton towards Old Winchester Hill, NEHHAS Journal 8 No. 10. 2017 www.nehhas.org.uk/rrch10.htm,

3. Wikipedia/Lidar https://en.wikipedia.org/wiki/Lidar