Having a field day with Lidar in the Surrey HER

February 28, 20207:50 pmLeave a Comment

Since the start of the year, the Surrey HER has been involved with some exciting endeavours in addition to all of the usual public queries and recording tasks, including producing a presentation that can be delivered at public meetings and events, and writing up a landscape survey project based on Lagham Park at South Godstone in Tandridge District. Alongside measurements and observations that were compiled out in the field for the landscape project, one of the useful tools that was used back in the HER office to find out more were Lidar resources. This blog post explains what Lidar is and how the HER uses it to learn more about Surrey’s historic environment.

Lidar and the HER

“Lidar” stands for light detection and ranging. It’s a laser scanning technology that can produce a 3D model of a surface or topography. Its uses are myriad, but archaeologists have adopted it as a remote sensing technique to examine earthworks. Here at the Surrey HER, we use free and open-source Lidar data produced by the Environment Agency. We consult Lidar almost every day when inputting and updating HER records, or answering queries from colleagues and customers, as it frequently helps to inform us about the presence and extent of earthwork features. It is particularly effective when used in conjunction with field survey, and sometimes can reveal further earthworks or additional features that have been obscured by vegetation, or located in places that are difficult to access.

Mickleham and Leatherhead Downs

A really nice example of Lidar images giving insight into the historic environment can be seen across Leatherhead Downs and Mickleham Downs, areas of chalk downland south of Leatherhead (Figure 1). Faint earthwork evidence for probable prehistoric field systems in this region have been known about for a long time. They are “coaxial” field systems, consisting of a series of parallel linear bank features, which are subdivided into smaller sections in a grid-like structure, adhering to a relatively consistent alignment. Field systems of this type were first called “Celtic” field systems by archaeologists OGS Crawford and EC Curwen in the early 20th century, although nowadays this term is considered outdated within archaeology as it is a clear misnomer (there’s nothing really “Celtic” about them), and the name originally only sought to distinguish these field systems from later features, such as medieval ridge and furrow.

Modern Ordnance Survey map annotated with transcription of all surviving early field system earthworks as manifested by Lidar

FIGURE 1: Modern Ordnance Survey map annotated with transcription of all surviving early field system earthworks (in red) as manifested by Lidar.

The field system on Mickleham Downs is first recorded in 1946 in Volume 49 of the Surrey Archaeological Collections, when it was described as “a small group of lynchets, apparently of ‘Celtic’ type, though not very notable specimens”. In the later Volume 50 of 1949 the following comment was made: “Now (1948) the earthworks are wellnigh obliterated… The remains… consist of a small group of typical Celtic fields, laid out regularly on both sides of a fieldway which runs south-west from the north-east corner of the Downs ”. These upstanding banks were first spotted as a result of sections of Mickleham Downs being subjected to ploughing for the first time, wherein the earthworks showed up as embanked lines of large flint nodules interspersed with surface finds of Romano-British pottery. They were mapped and this was reproduced onto a copy of the original monument record (Figure 2) back when the HER was still a card index system!

Image of Surrey HER index card for the 'Celtic' field system on Mickleham Downs

FIGURE 2: Copy of Surrey HER index card for the “Celtic” field system on Mickleham Downs (HER Monument 171)

So-called “Celtic” field systems have well known examples in other parts of the country, but they are unusual in Surrey. That these were discovered following the Downs’ turnover to ploughing in 1943 is not surprising, however – in fact, there are similar instances during the Second World War and the wide-scale agricultural turnover of once-unused land for the “digging for victory” cause (such as on the South Downs, or Salisbury Plain) which facilitated the discovery of archaeological features that had been unnoticed and, until that point, relatively undisturbed. The survival of these field systems may be due in part to their situations within land that is now considered remote – which is, in this case, the chalk downland high above the River Mole. It’s quite probable that similar contemporary field systems existed in many other parts of the county, but most of those that were located on low-lying areas almost certainly have been ploughed flat through later agricultural use or development, leaving only a few precious survivals on areas of less fertile land (e.g. Whitmoor Common, Worplesdon).

The Mickleham Downs area was surveyed in 1997 for the National Trust, and again in 2000 by CKC Archaeology; the Surrey HER holds copies of both of the resultant reports. These works identified further patches of earthworks around the Leatherhead Downs to the north-east, across Mickleham Downs, and further lynchets around Box Hill and the Headley area. The surveys therefore showed that the earthworks expanded further than previously noted – although it was acknowledged that their current state was damaged, and any further earthworks would be too ephemeral to identify under scrub and woodland cover. Indeed, in one case it was stated that “seeing them today is considered an act of faith rather than an objective reality”. Nonetheless, it seemed possible that the smattering of features was once was part of a wider field system that could have covered all of the Downs.

Today, as at the end of the 20th century, much of Mickleham Downs is covered by trees, and very little can be seen on aerial photographs here (as shown in Figure 3A below). This is where a modern tool like Lidar tends to prove particularly useful, as it is capable of identifying topographical details and variation even despite lots of vegetation. It’s a misconception that Lidar can “see through” trees – it can’t – but it’s possible to process the original “DSM” or digital surface model (Figure 3B) to cancel out or smooth out jagged surfaces in the data, where lots of differential high and low points occur together, such as the tops of trees, fences and buildings. This can simulate a model of the ground surface without the “noise” of obscuring vegetation cover, and this is called a DTM or digital terrain model. Sometimes, however, tree cover is just too dense – especially in high summer, or if the area in question is dominated by trees with thicker foliage such as conifers. But luckily in this case, the coverage of points is enough to preserve detail of undulations of the ground surface of the Downs.

An aerial photograph taken in 2012-13 showing the area described on the Surrey HER index card for the 'Celtic' field system on Mickleham Downs

FIGURES 3A and 3B: The same portion of Mickleham Downs seen in an aerial photograph taken in 2012-13 (3A) and Lidar DSM of 2015 (3B).

The Surrey HER has two representations of DTM Lidar, which consists of two sets of images made from the same 3D model that has been lit with an artificial light source from two different directions. Looking at both is important, as the different angles and direction of light can highlight different earthworks, as demonstrated in Figures 4A and 4B below. On the first image at the top, only a few east-west cross divisions of banks can be seen, but the other visualisation shows the faint but continuous north-west to south-east running parallel earthwork banks. Excitingly, they cover a much more expansive area than what was originally seen in the 1940s! Even considering that the Environment Agency’s Lidar data were captured much later in 2015 (and presumably after further weathering and damage to the features), it is possible to see the preservation of earthworks as faint but still upstanding features. Without the limitations of having to navigate rough terrain and undergrowth, Lidar images can greatly expand upon the known area and extent of previously recorded earthworks.

Lidar DTM imagery of Mickleham Downs, illustrating how the 315 degree (4A) and 45 degree (4B) azimuth lighting picks out different component earthworks of the field system here

FIGURES 4A & 4B: Lidar DTM imagery of Mickleham Downs, illustrating how the 315 degree (4A) and 45 degree (4B) azimuth lighting picks out different component earthworks of the field system here.

The previous survey work by CKC Archaeology also lamented the creation of the Tyrell’s Wood golf course to the north-east, as it seemed the site had scrubbed out archaeological features on the other side of Stane Street, the London-Chichester Roman road, around Leatherhead Downs. This loss of archaeology made it difficult at the time to clearly confirm the relationship of the Roman road to the field system. Although landscaping for the construction of golf courses undoubtedly does do a lot of damage to archaeological remains, it seems from Lidar imagery that there are still traces of earthworks across peripheral portions of the golf course, implying that there is still some degree of survival. On the image below (Figure 5), the golf course is located on the right side (east of) the Roman road. The faint parallel banks again share a very similar, cohesive north-west to south-east layout in comparison to Mickleham Downs nearby, separate section of field banks described above (which are located a short distance to the south-west of here), and seems to indicate that the two are related.

In fact, certain banks seen here on the golf course’s side also align with and “match up” to field banks on the other side of Stane Street. This seems to demonstrate that the field system underlies, and therefore likely pre-dates, the Roman road. This is in keeping with the early find spots in this area (e.g. Surrey HER Monuments 129169); the numerous Bronze Age round barrows (e.g. Monuments 2018 and 2019); and indeed with the relative dating of other prehistoric field systems of this type, which are generally thought to be Bronze Age to Iron Age based on finds and other features around them. Archaeological excavations in this area carried out by Archaeology South East in 2013 identified possible structures and features that primarily dated to the Late Bronze Age and Late Iron Age (Surrey HER Monuments 22525 and 22636), which again supports this pre-Roman dating.

Lidar image of the Leatherhead Downs area showing field system earthworks in relation to the line of Stane Street and the Leatherhead-Mickleham-Headley parish boundaries

FIGURE 5: Lidar image of the Leatherhead Downs area showing field system earthworks in relation to the line of Stane Street (in blue) and the Leatherhead-Mickleham-Headley parish boundaries (in red)

Surprisingly, the stop-line for the earthworks shown above is not the golf course specifically but appears to be the red line highlighted in the images above and below (Figures 5 and 6), which is an earthwork boundary that runs parallel with the Roman road and cuts across part of the modern golf course. It is also visible on the 1873 Ordnance Survey First Edition as the parish boundary between Leatherhead, Headley and Mickleham, where it is labelled “centre of furrow”. This may mean that the field system to the east of the parish boundary may have been scrubbed out much earlier, before the construction golf course. Several “tumuli” (another old-fashioned archaeological term, this time for a round barrow) are also marked here on the 1873 OS map extract, but were again apparently lost or destroyed sometime around the start of the 20th century – although two more possible earthwork mounds (both faintly visible in Figure 5 near the bottom right) were uncovered in the 1980s after tree felling (Poulton and O’ Connell 1984).

Ordnance Survey First Edition map of 1873 with field system earthworks added and parish boundary 'furrow' highlighted. Note the several marked 'Tumuli' as well as the provenance of 'Sepulchral Urns' found in 1868, perhaps denoting the site of another, destroyed round barrow. Stane Street is erroneously labelled as 'Ermyn Street'

FIGURE 6: Ordnance Survey First Edition map of 1873 with field system earthworks added and parish boundary “furrow” highlighted. Note the several marked “Tumuli” as well as the provenance of “Sepulchral Urns” found in 1868, perhaps denoting the site of another, destroyed round barrow. Stane Street is erroneously labelled as “Ermyn Street”.

To the west of this area (top left of Figure 7 below) is another region of early field banks, separated from the field system to the east by a curving S-shaped trackway crossing Leatherhead Downs. This western complex has also been known about for a long time as the earthworks are located on open ground, allowing their discovery by aerial photographs (Surrey HER Monument 197). Again, despite early reports stating their poor condition, they are still clearly visible on Lidar images, indicating that more of the system is preserved than previously thought. What’s noticeable is that the former field boundaries have a different alignment in comparison to the field-banks to the east and the Mickleham Downs system to the south (see Figures 1 and 4B), both of which consist of mainly straight, parallel banks with uniform alignment from north-west to south-east. The fields in the left of the image are more sinuous and less regular, aligned seemingly in opposition to the neighbouring system. Perhaps they were constructed at a different period – potential evidence of this could be another faint embankment (highlighted with dashes below) which appears on the other side of the trackway cutting across the eastern coaxial system and shares a closer alignment to the western field system. Could this constitute an overlap of earthworks from different periods of time? Or were they constructed at the same time but to serve different purposes?

Annotated Lidar DTM image of the two contrasting complexes of field system earthworks on Leatherhead Downs, with the possible antecedent boundary line within the eastern complex shown as a dashed line

FIGURE 7: Annotated Lidar DTM image of the two contrasting complexes of field system earthworks on Leatherhead Downs, with the possible antecedent boundary line within the eastern complex shown as a dashed line.

One of the shortcomings of examining Lidar images is that it is not always possible to identify clearly the relationship between many different features. Nor can it tell us anything about the age of earthworks without further evidence backed up by finds, maps, and more intrusive methods. This is where survey on the ground, and archaeological excavation, can provide better insight. Within the HER, Lidar will continue to play its part in adding to our understanding of our constantly growing record of archaeological sites.

Images:

Maps: © Crown Copyright. All rights reserved. Surrey County Council Ordnance Survey License number 100019613, 2010.  

Lidar images: © Environment Agency copyright 2015. Lidar 1 metre DSM & DTM.

Aerial Photo: supplied to Surrey County Council through the APGB agreement by Next Perspectives. TQ1853 TQ1753 2012-3. ©Bluesky International/Getmapping PLC.

References:

Bannister, N. R., 1997. Box Hill and Mickleham Downs: Historic Landscape Survey. North Downs Countryside Management & The National Trust.

Currie, C. K., 2000. An archaeological and historical survey of Mickleham Downs proposed ASHLV, near Leatherhead, Surrey. Volumes 1-3. Report to Surrey County Council & Surrey Archaeological Society.

English, J., 2013. Patterns and Progress: Field Systems of the Second and Early First Millennium BC in Southern Britain. BAR British Series 587.

Hogg, I., 2013. Archaeological Excavations at Cherkley Court, Leatherhead, Surrey: A Post-Excavation Assessment and Updated Project Design Report. Archaeology South-East.

Howe, T., & Egginton, A. 2017. Mole Valley County Sites of Archaeological Importance: Areas of High Archaeological Potential, Revised 2017. Surrey County Council.

McOmish, D.; Field, D., & Brown, G., 2002. The Field Archaeology of the Salisbury Plain Training Area. English Heritage.

Poulton, R.,  & O’Connell, M. G., 1984. “Recent discoveries south of Tyrell’s Wood Golf Course, near Leatherhead” Surrey Archaeological Collections Volume 75, 289-92.

Eleanor Salkeld (HER Assistant)

Written by HER Assistant - Modified by ESP Admin

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