New Light on an Ancient Landscape: Lidar Survey in the Stonehenge World Heritage Site

Article excerpt

Introduction

With the singular exception of Libby's invention of radiocarbon dating, archaeology has always sought to take advantage of emerging technologies developed for other purposes. For nearly 100 years aeroplanes have provided archaeologists with stable platforms for aerial reconnaissance and photography, which has produced invaluable information (St Joseph 1966). Rapid advances in remote sensing have also had a significant impact on the way in which we observe the earth, not just from aircraft but also other airborne platforms, as well as satellites. Recent developments of airborne digital survey for environmental mapping are opening a new chapter in the discovery and recording of archaeological sites from the air using multispectral scanners and lidar (Holden et al. 2002). Aerial survey for archaeology, in the twentieth century, can be characterised as having been focused on data capture until, in the 1980s, archaeological aerial survey developed its interpretative and analytical skills, taking evidence from millions of aerial photographs (Wilson 2000). Air photo interpretations and syntheses have transformed our understanding of past human settlement and land use (Bewley 2001; RCHME 1984; Stoertz 1997), and have contributed directly to the development of archaeological investigation (Crutchley 2001; McOmish et al. 2002). The development of these interpretative skills means that new and powerful techniques can be more easily assessed and a measure of their significance can be made.

Airborne lidar measures the height of the ground surface and other features in large areas of landscape with a resolution and accuracy hitherto unavailable, except through labour-intensive field survey or photogrammetry. It provides, for the first time, highly detailed and accurate models of the land surface at metre and sub-metre resolution.

The Stonehenge lidar survey had its origin in the requirement for English Heritage to develop new approaches for investigating the historic environment. This applies particularly in such projects as the ongoing development of a management plan for the World Heritage Site at Stonehenge. As well as contributing to the archaeological record itself, there was also a need to provide an archaeological context to the proposed improvements to the roads around Stonehenge, and to the design and decision on a location for a new visitor centre. These demands mean that all known investigative techniques were used in order to maximise our knowledge of this important landscape. A comprehensive landscape model, nevertheless, would also provide a major resource for future research, and for the presentation of the site to the public. The Stonehenge landscape is one of the most studied in Europe. If the new lidar technique proved useful here it is likely to have similar or greater impact for other historic landscapes (Batchelor 1997; RCHME 1979; Cleal et al. 1995; Crutchley 2002).

Accordingly, English Heritage commissioned the Environment Agency to apply the new technique in the survey of the Stonehenge World Heritage Site, with the objective of testing the value of airborne lidar against the known information derived from conventional aerial survey (Crutchley 2002).

Methodology

Lidar survey is based on the principle, now familiar in terrestrial survey with a total station instrument, of measuring distance through the time taken for a pulse of light to reach the target and return. Airborne lidar does this with a pulsed laser beam which is scanned from side to side as the aircraft flies over the survey area, measuring between 20 and 100 thousand points per second to build an accurate, high resolution model of the ground and the features upon it. The width of the scanned area is limited by the need to avoid obscuration of the ground by the laser beam encountering buildings and trees. The locations of the points are computed with reference to a geographical positioning system, and the three-dimensional co-ordinates are used to generate an accurate digital model of the surface of the ground (a digital surface model or DSM) (Figure 1). …