Airborne lidar and historic environment records.

The authors assess the potential contribution of lidar surveys to national inventories of archaeological resources ('Historic Environment Records), and compare the relative costs and sensitivity of lidar and aerial photography.

Keywords: England, aerial survey, lidar, aerial photography

Introduction

The new technique of lidar survey (Light Detection and Ranging) has already been applied to landscape mapping (Barnes 2003; Bewley et al. 2005; Bofinger et al. 2006; Harmon et al. 2006; Powlesland et al. 2006), geoarchaeological prospection and evaluation (Brunning & Far-Cox 2005; Challis 2005; 2006; Challis et al. 2006; Carey et al. 2006; Jones et al. 2007) and the detection of upstanding archaeological remains beneath the vegetation canopy (Devereux et al. 2005; Doneus & Briese 2006; Risbol et al. 2006; Sittler & Schellberg 2006; Crow et al. 2007). Lidar only detects features that rise above ground level (however slightly), so cropmark and soilmark sites are generally beyond its reach, although variations in crop height--one aspect of the cropmark phenomenon--may be detectable (Challis et al. 2006). The utility of lidar for finding new sites and defining historic environments has been broadly recognised (Holden et al. 2002; Bewley 2003; Crutchley 2006) but systematic application and evaluation is still in its infancy. It will be important to keep under review its growing utility within the armoury of prospection procedures applied in the research and management of archaeological resources.

This paper assesses the potential of lidar to enhance existing records of the historic environment, using a case study from a river valley in the agricultural midlands of England. Lidar elevation data at a spatial resolution of 2m was collected over a 25km stretch of the valley of the River Dove on the Derbyshire/Staffordshire border. These data were compared with the existing inventory of sites for the study area and with a selected sample of vertical aerial photography. While this assessment is based on a British example, we feel that the results have implications for cultural resource management internationally.

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Study area

The study area comprised a 25km segment of the valley floor of the River Dove between Rocester and its confluence with the River Trent at Newton Solney, a total land area of 10 703ha (Figure 1). The study area is dominated by pasture, with limited arable cultivation and woodland. The landscape is a patchwork of fields and farmsteads of post-medieval origin, with extensive areas of earthwork ridge and furrow betraying traces of an earlier, medieval, agricultural regime. Other upstanding earthworks relate to water meadows, deer parks, a moat and a miscellany of others, also largely of medieval and post-medieval origin. Belowground archaeology includes infrequent prehistoric round barrows, and occasional surface finds of prehistoric and Romano-British origin. The relative scarcity of arable agricultural land means that sites which depend for their detection on cropmarks or artefacts from the plough zone are poorly represented (8 per cent in Staffordshire and 18 per cent in Derbyshire). Sites evidenced by earthworks, and so visible to airborne lidar, dominate the existing record of the study area (18 per cent in Staffordshire and 38 per cent in Derbyshire).

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Method

Lidar data collected by the UK Environment Agency currently covers approximately 16 million hectares, 66 per cent of the land surface of the United Kingdom (Brown et al. 2003). Although the usual resolution (2m) is low, the broad coverage means that its assessment for archaeological usage is imperative. The present study used data from the survey covering 107km2 of the Dove valley. The data were imported into a geographical information system (GIS) developed using ESRI ArcGIS 9.1 and viewed as shaded relief images at a scale of 1:10 000. Earthwork features of archaeological interest were grouped into a number of categories including agricultural remains (such as ridge and furrow), settlement remains, quarries and other features considered as evidence of past human activity. Each feature was listed and digitised as a polygon within the GIS.

The inventory of features drawn from the lidar survey was then compared with pre-existing records of the historic environment held by the local authorities. First, the number of sites found by lidar was compared with the number of sites in the existing Historic Environment Records (HER, the current UK term for an inventory of archaeological resources; also known as Sites and Monuments Records (SMR), compiled at county level over several decades). Then a selected number of aerial photographs were used to assess the extent to which lidar might improve on the record collected from a more conventional survey, using only aerial photography. Two epochs of aerial photography were selected as being representative of the total body of aerial imagery available; these comprised vertical photographs from the late 1940s (Early AP) and vertical photographs from the 1970s (Late AP). In this way it was also possible to assess the degree to which the non-appearance of sites on the lidar might be due to their destruction.

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Results

Analysis of the lidar data identified 915 archaeological features in total (Figure 2; Table 1) the combined extent of which covered 1471ha of the study area (13.7 per cent of the total land area examined). Figure 3 shows examples of the predominant type of site defined by lidar: ridge and furrow patterns of low elevation indicating medieval agriculture.

Of the sites captured by the lidar survey, 84.4 per cent had not been previously noted in the Historic Environment Record for the area (Table 2; Figure 4). These figures reflect the tendency of HERs to exclude large-scale landscape features such as fields and field systems which together make up some 85.5 per cent of the record collated from lidar (a total land area of 1418ha). However, some important non-agricultural sites had also eluded the HER. For example, in addition to extensive ridge and furrow, Figure 5 shows a rectilinear ditched enclosure/platform measuring c. 80 x 80m. Lidar can also enhance features barely recorded by aerial photography, as the example in Figure 6 shows.

At the same time some features recorded in the local HER were not recognised by lidar (Table 3). The majority of these are cropmarks (43), artefacts (21) or documentary records (52), while many are standing buildings (184); these are all forms of evidence not easily detected or described using airborne lidar. In addition, 76 earthwork sites documented by the SMRs across the study area were not recognised in the lidar data. An example given in Figure 7 shows a set of ridge and furrow earthworks recorded as upstanding in 1948, but undetectable in the lidar survey made 50 years later, probably because they have been erased in the interim by continued ploughing.

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Costs and benefits

Given the different success of photographic and lidar survey, it is interesting to compare the cost of each method, and thus its availability to archaeologists. The acquisition of aerial photographs is the cheaper of the two: 43 photographs were acquired from existing government databases for 1659 [pounds sterling], while the lidar data from the same area was charged at 11 873 [pounds sterling] by the government's Environment Agency. By contrast, scanning, locating and entering the 43 photographs into the GIS required 35 days of experienced and efficient researchers; while entering, transforming and examining the 43 tiles of lidar data required only 15 days.

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An added advantage of lidar over photography is that it may be manipulated to show three dimensional images, contour maps and profiles (Figure 8). Further manipulation can produce animated 'flythrough' visualisation. These visually rich records of cultural heritage are invaluable, both for research and for the management of cultural resources, providing interactive and high impact images well suited to public consultation and non-specialist users (Hill & Veitch 2002; Challis & Howard 2006).

Conclusions

The present study suggests that airborne lidar offers considerable potential for the enhancement of historic environment records in landscapes dominated by upstanding earthwork remains. Clearly, aerial photography continues to provide a reliable method for the identification of features in landscapes such as that of the middle Dove valley. However, many of the features recorded through analysis of lidar data would have escaped a traditional air-photographic transcription. No ridge and furrow are recorded on the Derbyshire or Staffordshire SMRs as having been transcribed from APs. It is not likely that such gross features were missed; more likely that the monument-focused bias of traditional HERs has excluded their transcription and recording. Furthermore, lidar, through the three-dimensional nature of its records, provides a tool to assess the preservation of sites through determination of the extent and character of upstanding remains, in a manner difficult to achieve through use of aerial photography alone.

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Working within a purely digital environment, such as a regional government corporate GIS, lidar may significantly assist in the rapid compilation of a digital archive as well as providing a three-dimensional record of archaeological earthworks. The use of lidar is thus to be commended to researchers for use in enhancing HERs. However in the UK, the present high cost of the data makes its selection for purely archaeological research prohibitively expensive; up to seven times more expensive than conventional vertical aerial photography. However, this may be overcome where lidar data are already held as part of a corporate GIS, or where cost of ownership might be shared across government departments.

Lidar data is increasingly available across the developed world where it is routinely collected by government agencies (e.g. Petzold et al. 1999; Brown et al. 2003). In the developing world, survey is limited as yet, but the rapid, wide-area character of lidar survey is recognised as of great economic benefit for infrastructure and environmental project applications (e.g. Sanchez et al. 2007). With an ever-increasing need to map and understand the physical remains of natural and cultural assets, particularly in the light of newly perceived and global threats such as climate change (Chapman 2002; IPCC 2007) the requirement to produce rapid, cost effective maps at a landscape scale compels consideration of airborne lidar as a routine data source.

Acknowledgements

This research was supported by the Aggregates Levy Sustainability Fund, administered for DEFRA by English Heritage, grant number 3850. The authors are grateful to the UK Environment Agency who facilitated the supply of lidar data for the study area. In addition Gill Stroud of Derbyshire County Council and Suzy Blake of Staffordshire County Council assisted with the supply of digital SMR data, while the staff of the National Monuments Record Centre at Swindon provided access to, and digital copies of, aerial photographs from the NMR collection. The research contribution of Ziga Kokalj was generously funded by a grant from European Commission Socrates Erasmus programme. The authors are grateful to Simon Crutchley and Dr Bernard Devereux for their helpful comments on a draft of this paper.

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Keith Challis (1) *, Ziga Kokalj (2), Mark Kincey (1), Derek Moscrop (1) & Andy J. Howard (3)

* Author for correspondence

(1) IBM Visual and Spatial Technology Centre, Birmingham Archaeology University of Birmingham, Edgbaston, Birmingham, B15 2 TT, UK (Email: K [e-mail])

(2) Institute of Anthropological and Spatial Studies, Scientific Research Centre of the Slovenian Academy of Sciences and Arts, Novi trg 2, SI-1000 Ljubljana, Slovenia

(3) Institute of Archaeology and Antiquity, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK

Table 1. Feature types recorded from lidar.

Site type              No. of records  % of total

Barrow                  11                1.2

Feature complex         10                1.1

Quarry                 102               11.1

Agricultural remains   782               85.5

Settlement remains      10                1.1

Total                  915                100

Table 2. Lidar-derived features previously recorded in HER databases.

                  No. of features recorded   % of total

Derbyshire         64                           7.0

Staffordshire      79                           8.6

Not in database   772                          84.4

Total             915                          100

Table 3. SMR features not recorded by lidar.

Evidence                                   Staffordshire   Derbyshire

Null                                        53              18

Cropmark                                    16              27

Documentary evidence                        48               4

Documentary evidence and Cropmark            1               -

Documentary evidence and Earthwork           3               -

Documentary evidence and Extant Building     7               -

Earthwork                                   43              33

Earthwork and Extant Building                1               -

Extant Building                            156              28

Find                                        10              11

Find and Documentary Evidence                1               -

Sub Surface Deposit                          4               -