Interpreting Archaeological Topography: 3D Data, Visualisation and Observation

By Oxbow Books

Airborne Laser Scanning (ALS), or lidar, is an enormously important innovation for data collection and interpretation in archaeology. The application of archaeological 3D data deriving from sources including ALS, close-range photogrammetry and terrestrial and photogrammetric scanners has grown exponentially over the last decade. Such data present numerous possibilities and challenges, from ensuring that applications remain archaeologically relevant, to developing practices that integrate the manipulation and interrogation of complex digital datasets with the skills of archaeological observation and interpretation. This volume addresses the implications of multi-scaled topographic data for contemporary archaeological practice in a rapidly developing field, drawing on examples of ongoing projects and reflections on best practice.Twenty papers from across Europe explore the implications of these digital 3D datasets for the recording and interpretation of archaeological topography, whether at the landscape, site or artefact scale. The papers illustrate the variety of ways in which we engage with archaeological topography through 3D data, from discussions of its role in landscape archaeology, to issues of context and integration, and to the methodological challenges of processing, visualisation and manipulation. Critical reflection on developing practice and implications for cultural resource management and research contextualize the case studies and applications, illustrating the diverse and evolving roles played by multi-scalar topographic data in contemporary archaeology.

• Language: English
• Publisher: Oxbow Books
• Release date: Jan 8, 2013
• ISBN: 9781782971085

Book preview

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Interpreting archaeological topography: lasers, 3D data, observation, visualisation and applications

Rachel S. Opitz and David C. Cowley

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The central issues of this volume are introduced, describing laser scanning and 3D data for archaeology. Archaeological interpretations of topography and the skills sets required are discussed in the context of rapidly changing approaches and the need to integrate field experience and computer aided analysis. An outline for understanding and working with ALS ranges across issues of scale, certainty of interpretation, processing, visualisation and integration, concluding with thoughts on the impact of regional research traditions and prospects for the future.

Keywords: 3D data, ALS, experience, archaeological topography, scale, integration, research traditions

Preamble

Airborne Laser Scanning (ALS), or lidar (terms used interchangeably throughout this book), has been described as one of the most important innovations in data collection and interpretation for archaeology (Bewley et al. 2005), and it is the principle focus of the volume. However, the themes, approaches and methods discussed in this volume are broadly applicable to laser scanning, close-range photogrammetry and other 3D data collection methods in use in archaeology, and these too are included.

Laser scanning is a technology which accurately and repeatedly measures distance, based on a precise measurement of time, and combines these measurements into a collection of coordinates. These coordinates are normally stored as a point cloud, from which information on the morphology of the object being scanned may be derived. Photogrammetric approaches also produce 3D point clouds describing the shape of an object based on the triangulation of matched points from multiple images, and many of the applications and processes overlap with those discussed in this volume in the context of laser scanning. Objects recorded with these techniques can range from a small artefact recorded by a triangulation scanner in a laboratory, to vast extents of landscape totalling many 1000s of km² recorded from an airborne platform. The scope of what may be recorded and studied through these techniques, from objects to surface structures to buried archaeology, traversing spatial scales, and the range of questions which may be approached, has led to their adoption across the archaeological community. These specialist techniques are now an integral part of many academic, popular and heritage management projects.

In particular, the last decade has seen an exponential growth in the use and awareness of ALS by archaeologists and cultural resource managers. The ‘magic’ of a new technology with the ground recorded in detail, the ability to ‘see through’ trees and the powerful images produced, all promised a brave new world. And so it is – a world of possibilities and challenges, both in ensuring appropriate, archaeologically reliable applications that inform us about the past, but also in developing practices that integrate the strengths of new possibilities in manipulation and interrogation of vast digital datasets with so-called ‘traditional’ skills of archaeological observation and interpretation (Figure 1.1).

Figure 1.1: A relief shaded digital surface model (DSM) generated from an extract of high resolution Flimap 400 lidar survey flown by helicopter at the deserted medieval settlement of Newtown Jerpoint, Co. Kilkenny, Ireland. Remarkably fine detail is present in the model, the result of a ground sampling distance for the survey of 15 cm. Centre left, tyre marks from vehicles can be seen converging at a break in the field boundary; returns from power lines can be seen running diagonally across the model; and towards the bottom right sheep can be seen grazing in the field. This image records features with minimal surface expression, for example rendering field surfaces in remarkable detail. However, these ephemeral landscape features are mixed in with textures that are a product of data collection and processing such as the slight ‘grain’ that runs from top to bottom across some areas. Interpreting such an image benefits from an ability to manipulate lighting (at the very least), an understanding of how it was created, and experience built on field observation. © The Discovery Programme/Heritage Council

This theme is at the core of this volume, by drawing on 10 years of archaeological engagement with ALS to explore the technical and interpretational challenges of this data, and to address the integration of approaches drawn from direct observation, data manipulation and visualisation, and to move beyond the purely technical or observational to engagements with past lives. The use of archaeological topography in the title of this volume identifies a focus on the topographic expressions of the past in the present, whether the earthworks (humps and bumps) of past settlements and field systems at landscape scale or the micro-topography of tool marks on an artefact. And it is this manifestation of past activities and processes in 3D, whether at micro- or macro-scale, that creates an exciting challenge in combining approaches from a wide variety of archaeological practice. To evoke an obvious generalisation, these approaches range from the muddy-booted fieldworker engrossed in the topography of a hillside to the computer geek sitting in a darkened room surrounded by humming hardware and writing complex software to create a virtual environment! Of course these are polar extremes, but they do highlight the importance of combining ‘field-craft’ and observation with the powerful algorithms and visualisation techniques that dense and/or extensive 3D data demand if we are to do anything more than scratch the surface. This volume draws together expert papers from across a broad spectrum of engagement with archaeological topography as expressions of developing practice in a rapidly evolving field.

This introductory essay provides some background and introduces the main themes of the volume. Principally these are the growing archaeological applications of 3D data, for which laser scanning is now a major source, and how these relate to the interpretation of topography. ALS is not straightforward ‘data’ and its incorporation into routine practice demands a level of understanding and critical thinking, which range across scale of analysis, certainty of interpretation, the roles of processing and visualisation, integration and varying uptake and regional traditions. The essay concludes with some comments on prospects for the future and a brief description of the contents and origins of this volume. The emphasis throughout is on the underlying principles, rather than technical descriptions or issues. For an overview of the technical aspects of laser scanning see Chapter 2 (Opitz) and attention is also drawn to the glossary of key technical terms on p. 266.

3D data in archaeology

Digital 3D data is now entirely embedded in archaeological recording, interpretation and visualisation, within a wide variety of projects and at a variety of scales. ALS has found applications in mapping and prospection surveys in woodland, scrub and open ground, and may provide the only means of survey in difficult to access areas. While basic mapping may draw on relatively low-resolution, usually second-hand, data, the potential of high point densities to provide incredibly detailed recording of small landscapes and individual sites has been demonstrated. ALS has shown its value in providing landscape context, drawing on geomorphological mapping of palaeo-features and landscape characterization, and informing conservation studies focused on the impacts of erosion or modern land use.

On the site scale, terrestrial laser scanners (TLS) are used to collect bespoke data for specific archaeological projects. Such projects may include recording a site or monument before excavation or conservation work takes place, detailed documentation during excavation of complex or easily damaged features, or scanning of highly eroded or abraded surfaces to highlight subtle features. Like ALS, this type of recording has immediate primary applications in erosion and stability monitoring, particularly of monuments exposed to weather, pollution and tourists. With even smaller scale scanners used in laboratories the same processes of documentation and applications apply in equal measure to small objects, and are increasingly part of the study of the manufacture and use of artefacts.

In all cases the integration of 3D data into archaeological practices promotes the use of ever more sophisticated modelling and visualisations, from the creation of virtual replicas for display in a physical or digital museum or dissemination over the internet, to virtual reality and immersive visualization projects. Throughout, while the primary aim of these products may be to communicate and engage with a wide audience, these approaches also have a vital role for the investigating archaeologist in supporting interpretation where the visualization and measurement of very small scale and subtle features is essential (e.g. tool marks or rock art), and to under-pin spatial analyses such as viewsheds and least cost-paths, and inclusion in interactive virtual reality models. Universally, it is the use of 3D data as an articulation of archaeological topography that lies at the heart of the processes.

Archaeological topography

Topographic survey, in the first instance the interpretation and survey of archaeological earthworks, is a long-established tradition with antiquarian origins (e.g. Bowden 1999). Early cartographers experimented with depictions of slopes, developing through military surveys depictive techniques like hachures and shading that were used to denote natural and anthropogenic earthworks. This approach has a rich history as an archaeological technique for documenting whole swathes of landscape, as a means to think through, understand, record and communicate sites like Iron Age earthworks, deserted medieval villages, and, on a grand scale, ancient Rome. Topographic surveys at a range of scales create coherence and aid in understanding the features surveyed, providing a plan view, but also attempting to convey something of the topography of the site/landscape. This latter aspect, of conveying topography, has varied in the degree to which conventions and depiction have successfully allowed the viewer to interpret slope, but is of course inherent in contours and other expressions of height differences such as shaded relief models. For archaeological survey the collection of 3D data was, less than 20 years ago, a time-consuming process, and while for map-making the innovation of photogrammetric pairs of aerial images was a major step-forward for quickly capturing the form of large areas, its uptake by archaeologists has been uneven.

The last ten years have witnessed an increase in pace and intensity in archaeological engagement with 3D data, ranging from data collection by total station and DGPS through the increasing use of terrestrial laser scanning and close range photogrammetry. In tandem, huge improvements in 3D modelling software are rapidly changing how topographic documentation is undertaken on excavations. While these developments encompass the full range of archaeological recording, from close detail in an excavation trench, or within a building, to extensive landscapes even at country-scale, these advances in 3D recording primarily impacted practice on the site to object scale and the links with archaeological topography in the traditional landscape sense were not emphasized.

This is the mix into which ALS has been added, greatly advancing the ability to collect and work with 3D models of large areas. The popularity of ALS for studying forested areas, floodplains and rural areas in general has renewed interest in the topic of topographic survey, and further spurred integration with digital technologies and applications. The potential of these synergies demands critical examination of working practices, especially in the area of generating archaeologically meaningful and stimulating interpretations of topography and in revitalizing the use of topographic data in landscape projects.

To illustrate our views on the intersection of 3D data and the practices of recording, visual depiction and interpretation a brief commentary on the use of hachures and shading in traditional approaches is instructive. These conventions have been used to depict slopes and as a means of recording and communicating the results of an analytical engagement with earthworks – What are the humps and bumps? How do they interrelate? How do they express structures from the past? and so on. Here, the processes of archaeological interpretation and depiction are intertwined. Thus, while the results of such analytical site survey should be metrically accurate, the depiction is a product of the interpretative engagement of the surveyor with the earthworks, and the translation of their interpretation into a drawing. This is a process that is heavily dependent on experience, a sound knowledge-base and a reflexive, self critical approach. The site of Braidwood in southern Scotland is a good example, where the subtle, incredibly complex earthworks are a product of a sequence of construction of timber round houses and palisaded and earthwork enclosures (Gannon 1999). The survey drawing (Figure 1.2) is a result of about three days in the field and an intense engagement with the humps and bumps of the site, what they might mean and how to translate that into a meaningful plan – undertaken by two highly experienced fieldworkers with many years of experience between them. This has produced a plan which expresses their observations and can be ‘read’ – a plan in which the observation and interpretation of the earthwork remains are explicit and completely intermeshed (Figure 1.3).

The plan of Braidwood makes an important point – that the interpretation of archaeological earthworks (or natural topography for that matter) is a skill built on experience and knowledge, where intuition and subjective judgements are very much to the fore. This may seem, at first glance, old-fashioned and irrelevant to the new reality of digital survey data, where height data has often replaced depictive survey and digital drawing packages have taken the place of the draughtsman’s pencil. However, while ALS is providing digital surface models at a scale and level of detail that would have been unimaginable 20 years ago, it also presents considerable methodological and interpretative challenges that relate to the nature of the dataset and how it is processed, manipulated and used to generate archaeological information. Many of these require new approaches rooted in processing algorithms and visualisation software and new skills in digital data manipulation to go along with them, but others take us back to the skill-set that produced the plan of Braidwood or that have been honed examining aerial photographs. We would argue that in engaging with digital 3D data the skills of reading the topography and the employment of experience and knowledge are still very much at the fore.

Figure 1.2: A masterful example of an earthwork survey at Braidwood in southern Scotland produced by Angela Gannon and Strat Halliday. The complex palimpsest of ephemeral earthworks has been examined in detail building an understanding that has been translated into an analytical drawing. This process is a complex interplay of fine-tuned observation, experience and drawing skills. © Angela Gannon, reproduced by kind permission

Figure 1.3: The slight earthworks on Gibbs Hill in southern Scotland are deciphered by a team and translated into an interpretative analytical drawing similar to Figure 1.2. The people are standing between two of the shallow trenches which once held timber palisades. © Strat Halliday, reproduced by kind permission

Understanding ALS

As archaeological use of ALS has developed it has become increasingly clear that it is not an ‘objective’ dataset that can be used uncritically. Indeed terrain models, like any models, are constructs and often riddled with unspoken assumptions. Firstly, the primary data collection and processing parameters have a major impact on output, while the ability to ‘see’ is heavily dependent on software for manipulation and visualisation, and data artefacts may be a trap for the unwary. These factors are a complex mix of objective parameters (e.g. point density) and subjective judgements (‘this visualisation looks better than that’) that are inextricable from the pervasive issue of archaeological interpretation. So, for all the new technology and software, the basic issues of how archaeological information are created remain central.

While fundamentally we see the use of ALS and other 3D digital data sources as a continuation of existing practices, the specifics of how we work with these technologies throw up some important differences of approach which impact on established survey practices and workflows. Firstly, the emphasis of much ALS work in archaeology has been desk-based, with limited engagement in concurrent or subsequent ground observation. A purely desk-based approach carries certain dangers – principally that there may be no or limited feedback between ground observation and ALS interpretation. Thus knowledge of the site types that may be expected in an area and artefacts created by modern landuse, for example, may not inform the desk-based work (processing – manipulation – visualisation – interpretation) as it should. Lack of this basic type of knowledge of a landscape is the main factor in the misinterpretation of aerial photographs (e.g. Wilson 2000) and the same will certainly be true for ALS. A related point concerns the interplay of manipulation, visualisation and interpretation, and at a basic level the role of archaeologist and ALS specialist – two roles which, in current practice, usually do not overlap much. Many ALS data are used by archaeologists who have little understanding of the processes by which it has been generated (e.g. a hillshade model), while ALS data may be processed with little or no consideration of archaeological imperatives (inevitable if ALS data is ‘second-hand’). Such divisions are not desirable and best practice projects have developed a synergy of these different skill-sets where the ability to manipulate data interacts with knowledge of the archaeological landscape.

Addressing scale and certainty

One of the major challenges for archaeological uses of ALS is how to work at an extensive scale, with potentially huge and complex datasets (this also applies to multi/hyper-spectral data (Beck 2011) and to datasets collected over smaller physical areas but with very high spatial or temporal detail). This problem is particularly relevant to areas without good archaeological databases that can support effective heritage management, where lidar may be a key (or the sole) source of new archaeological information and much depends on its interpretation. Archaeologists working in a research context may choose to study relatively small areas for which the lidar and other archaeological and supporting information can be inspected in detail. However, for cultural resource managers using lidar to help set planning priorities ahead of development across large areas, particularly where the overall archaeological record is poor or variable in coverage and quality, a strong dependence on lidar is problematic as a close inspection of all areas of the dataset is impractical and only studying parts of the dataset will not support prioritisation and protection. To address the challenges of fairly assessing large areas given limited resources, and to enable Baden-Württemberg (35,751 km²) to be mapped in six years, Ralf Hesse has developed as many automated data processing stages as possible (Hesse this volume). Thus local relief models (LRM, Hesse 2010) were developed as a technique to extract small-scale ‘local relief’ features from the digital elevation model (i.e. archaeological earthworks usually have low relief relative to the rest of the landscape), and if data with these characteristics can be rapidly extracted there will be a massive timesaving over visual inspection.

This approach may ring alarm-bells with some archaeologists, but it cuts straight to the heart of the problems of dealing with complex, dense or extensive datasets, whether they are aerial/satellite photographs, hyper/multi-spectral data or ALS. If archaeologists and heritage managers are going to make effective use of these data for extensive survey to underpin effective management, then they must engage with techniques that shortcut exclusively ‘manual’ inspection, in the manner of, for example, traditional aerial photograph interpretation – and that means auto-extraction, or more accurately, semi-automated extraction of features (Cowley 2012). No-one really advocates ‘fully automatic’ extraction, and in practice workers mean semi-automatic or supervised feature extraction (i.e. De Laet et al. 2007, 2008, 2009; Trier and Pilø 2012).

This bears directly on the type of information that may be expected and the degree of certainty that will attach to interpretations. In the vast area covered in Baden-Württemberg (above and Ch 14) the approach will generate many potential archaeological features, whose character and even certainty of being ‘real’ are provisional. Within a development control context, where an archaeological flag will identify a location that requires inspection if it is threatened, this is a valuable enhancement of the dataset for cultural resource management. Alternatively, high-point densities, integration with other data sets and ground observation will generate deep information with a high degree of interpretative integrity. Variable scales of reliability or certainty are inevitable, but so long as fitness for purpose is identified from the outset, weaknesses in the survey product should be minimised. An acceptance that some interpretations made relying on semi-automatic techniques will be proved incorrect is a necessary evolution of management practice. In essence, there is a trade-off between a decrease in the confidence of interpretations and an increase in the area studied. Accepting this trade-off is absolutely central to increasing our ability to address management needs and research questions at the regional and supra-regional scale.

Processing and visualisation

Processing and visualization have real and important impacts on interpretations. Anyone who has begun to ‘play’ with different visualizations and terrain model processing techniques quickly realizes that features of potential interest can dramatically change depending on the settings and algorithms used. There are near endless possibilities for the creation of new models and visualizations of these models, and a distinct danger of continually tweaking parameters in the hope of ‘improving’ the appearance.

The many possibilities for manipulating digital models forces consideration of two points. First, there is the question of how much information can be retrieved, which is somewhat like asking how long a piece of string is, but is fundamentally about how to assess cost/benefit. Crutchley (this volume) makes an admittedly entirely subjective observation that if one model gives 90% of the nominal ‘total’ information, then the decision not to chase the other 10% may be taken on cost-effectiveness, depending on the requirements and constraints of a project. At the heart of this assertion is a pragmatic approach to avoiding the dangers of loosing sight of survey objectives in an endless round of data processing and manipulation. It is however, an area that requires structured assessment, perhaps to create benchmarks of cost/benefit that allow users to decide where on a sliding scale they wish to curtail manipulation of data, because a certain approach gives them enough to be fit for purpose.

Second, as noted by Kokalj et al. and Beck (this volume) providing detailed information on how a model and visualization was created is essential for others to understand and evaluate the end product and interpretation. Is a feature really present or is it simply a digital artefact or trick of the virtual light? As noted above, close collaboration and sharing of knowledge between the ALS specialist and the archaeological interpreter (as two people or one person performing two distinct tasks) will enhance the links between manipulation of digital models and the interpretations based on them. Documentation of the process is vital, and linking directly to the first point, including the basis on which ‘when to stop’ is determined – i.e. when ‘enough’ information and an appropriate level of confidence in that information have been achieved. A clear definition of fitness for purpose and survey design are central to this decision-making process.

Context and integration

When integrating a new(-ish) data source into archaeological practice there are two modes of engagement. On one level there is informal integration wherein the data is used in an increasing number of projects in innovative ways to engage with a growing variety of questions, coupled with a growing understanding of how the new data source interacts with existing techniques. On another level we have formal integration where recognized standards and archives are adjusted to accommodate the new data source and best practices are developed and recognized. ALS, TLS and 3D recording from photogrammetry in archaeology have made important strides in both modes.

While early ALS and 3D recording publications often focused on the technology itself, showcasing benefits and pitfalls (e.g. see references throughout the volume), the recent trend is for publications of case studies in which 3D recording is a well-integrated part of a collection of data supporting research into specific archaeological questions (Crutchley, Poirier et al., Bennett et al., Davis et al. this volume). The variety of projects using 3D data has increased, and now ranges from close studies of an object (Evans et al. this volume) or single site to broad regional surveys (Hesse this volume) and from those collecting fundamental basic data for regional surveys (Risbøl this volume) to those using ALS to study the links between manuring patterns and field systems (Poirier et al. this volume). Formal guidelines for ALS and 3D recording have appeared in the last five years, including the ADS Guides to Good Practice and English Heritage Guides to 3D Laser Scanning, which provide important documentation of current practices, advice and standards for these data. These guides and standards are most widely applied in the Anglophone community, primarily the UK where they have been developed, but similar initiatives elsewhere are developing.

Advances in the adoption of ALS, TLS and photogrammetry by archaeologists are closely tied to future developments as well as past ones. The need to efficiently manage large 3D datasets, to more closely tie together technical and interpretational skills, and to link together methodological and regional or local archaeologically driven advances will push forward the agenda in areas like the use of web-based platforms, data sharing and metadata standards, and research in topics like the archaeology of forested environments.

Uptake and regional traditions

Differential uptake of ALS reflects many things, from the history of aerial approaches in a region to restrictions on flying, from the prevalence of non-archaeological agencies collecting and using lidar to the character of archaeological sites/monuments/landscapes and local or regional research agendas. The content of this volume reveals the dominance, both in the number of projects and length of engagement, of Northern and Continental Europe in archaeological lidar. More recently a few Mediterranean European countries and a limited number of North and Central American (but run by North American research consortiums and Universities) projects have emerged. The first Asian archaeological lidar project is planned for 2012 in Cambodia under the aegis of a consortium of international researchers and heritage managers (below). As far as the authors know, currently there are no archaeological lidar projects based in South America, Africa, or the Middle East.

While in places such as Cyprus or much of the Middle East the absence of ALS projects is clearly due to restrictions on airspace and the practical difficulties of finding data collectors who operate locally, the tradition of the use of aerial imagery, especially amongst cultural resource managers, is a major factor in the adoption of ALS. Such data for large areas is often collected and held by a government environmental or land management agency, and other government groups including those responsible for heritage management are typically the first to be made aware of and gain access to the data. This can be seen, for example, in the UK and Netherlands, where data collected by Environmental Agencies for flood risk mapping and monitoring sparked the widespread adoption of ALS by the archaeological community. Whether state-funded archaeologists seek access will depend to some extent on whether they believe that aerial approaches are inherently useful, and the degree to which primary aerial reconnaissance and mapping is an established part of cultural resource management strategies.

Adoption of ALS for research has generally followed once it is embedded in national archaeological management in some way, in part because the cost of purpose-collected survey generally will be beyond most researchers. However, until 2012 EUFAR (European Facility for Airborne Research) and the NERC ARSF (Natural Environment Research Council Airborne Research and Survey Facility) made it possible for a limited number of European and UK research projects with a proven commitment to ALS to acquire such directly-purposed data and these models may in time extend elsewhere. On the other hand, there are examples of archaeologically-commissioned projects which pioneer new uses or techniques for the data in regions without, or preceding, a broad CRM driven initiative for using ALS (e.g. Italy where research-driven applications have preceded widespread release and use of the Il Piano Straordinario di Telerilevamento Ambientale (PST-A), part of the national mapping). In either case the constant interchange between CRM and research communities as they work with ALS and other forms of 3D recording is essential to its continued development and increased use.

This exchange of experience and knowledge can take place across disciplines on many scales. Early adopters of ALS working in Mediterranean Europe (e.g. Italy and Spain), where significant regional surveys are in their early stages and a number of research studies have been undertaken, are broadly part of the same research and CRM tradition as those working in Central and Northern Europe. Consequently there has been extensive communication and collaboration facilitated by organizations like AARG (Aerial Archaeology Research Group), EARSeL (European Association of Remote Sensing Laboratories) and Archaeolandscapes Europe and a level of standardization of data and metadata driven by the Inspire Directive. The rapid development and early work in these areas no doubt benefited from exchanges with colleagues in other areas of Europe, and is now making important contributions to the research community as researchers in these regions confront very different archaeological and land use conditions.

The growing community of archaeologists using ALS in North and Central America, in contrast, has fewer opportunities (and perhaps fewer motivations given the historic separation of New and Old World archaeologies) to collaborate with European colleagues. Real, and in some cases perceived, differences between the state of preservation of surface and buried features, the types of sites dominating the survey record, the traditions of use of aerial imagery, and the dominant types of vegetation between New and Old World archaeologies makes the transfer of relevant knowledge and experience more complicated (though see Burks 2010 for an American approach that will be familiar to many European workers). In spite of differences, it is hoped that exchanges between these communities will increase as ALS becomes a greater part of New World archaeological research and CRM practice, and that similar developments will occur in other regional archaeologies as appropriate.

Prospect

Informal integration of ALS and other 3D digital data sources for archaeological recording has been taking place for ten years, and is spreading to new areas of archaeological practice. These processes can be challenging as they demand development of new approaches and work practices, and the ability to draw together complex data sources and interpretative frameworks. To maximise the added value of these processes for archaeology an open-minded approach is vital – one which is not constrained by demarcation of roles or expertise, but equally recognises specialist skills and archaeologically-based experience.

In looking to the future, the most exciting developments will certainly be those that are not anticipated, but several areas of important further development can be identified. Developing and disseminating best practice is crucial and the ALS community already has an excellent track record in this area. Finding ways to include these usually very large and complex datasets in archives available to the wider research community is important to facilitate re-tasking of datasets, a topic discussed by Corns and Shaw and by Beck (this volume). The transfer of skills and experience can be a valuable short-cut to speed development, but should be progressed without assuming that problems will necessarily be uniform and best practices the same across regional disciplines. While lidar has been established as a basic part of the CRM process in regions where the data is widely available, and bespoke collections are justified in some management situations, now that the first set of technical challenges has been met its place in research projects is less clear and should certainly not be uncritically assumed (‘let’s do lidar’). Finding creative research-driven uses of 3D digital data will be a key part of the future of ALS and of similar data derived from other sources.

Technical developments will likely come from outside archaeology. The sheer size of a typical 3D dataset can put it beyond the resources of many, but the decreasing cost of digital storage and improved efficiency of processing software will make the technology more widely available. Archaeological applications will often demand classification and filtering processes beyond that which is delivered by standard software solutions (above; Opitz this volume), and improvements in these algorithms will also likely come from the wider ALS community. On the other hand, the archaeological community has taken the initiative in developing or adapting visualization techniques and 3D data management strategies and improvements in these areas will probably continue to come from within archaeology. The challenge of developing research agendas where 3D data will be useful and open new avenues will clearly have to come from within the archaeological and cultural heritage fields.

A project underway in Cambodia as this volume is going to press encapsulates many of the issues discussed above and we are grateful to project director Dr Damian Evans (University of Sydney, Australia) for the following information, which is included here as a timely illustration of the many challenges of developing ALS use, the importance of research design and fitting to purpose. A consortium of eight teams has acquired ALS coverage over Angkor and two other areas, in an extension of work carried out over 20 years to map and analyse remains such as rice field walls, occupation mounds, temple sites, ponds, roadways and canals (Evans et al. 2007; Evans and Traviglia 2012). The project has addressed many issues, not least the bureaucracy of obtaining permissions from three different ministries in a process that took four months and went to Prime Ministerial level. Costs are significant and potential suppliers limited, while the constantly cloudy and/or hazy conditions and unpredictable weather are an added difficulty for planning. Fundamentally, however, the major obstacle to the project was the lack of precedent and the fear of gambling on an approach that has never been proven in the region. The project team hope that the Angkor mission will settle many of these issues and provide impetus for further missions in the region as there is enormous potential.

Origins and structure of this volume

Origins

This volume is rooted in discussions and presentations at the annual conference of the Aerial Archaeology Research Group (AARG) in 2010 and an international lidar workshop held at the Bibracte European Research Centre in 2011. The profile of lidar at AARG meetings has built steadily over the last few years, with a few papers in 2006 and 2007, followed by a full session at Ljubljana in 2008 in which the emphasis was very much on potential, and an extended session in Bucharest in 2010, with a strong emphasis on interpretation and a loose link to a session on more general issues of interpreting aerial remote sensed data.

The work presented and issues raised at conferences very much set the tone for TRAIL 2011 (Training and Research in the Archaeological Interpretation of Lidar) held at Bibracte in March 2011. This drew together experts, practitioners and novices in a workshop framework, which ranged across hands-on training, workshops and general presentations. An extended lidar-themed session and keynote address at AARG 2011 in Poznań, Poland, in September 2011 continued this trajectory, with papers mainly concerned with ALS employed in an integrated landscape framework and the issues of interpretation. The change in emphasis from ALS as the new toy to serious applications and critical thinking which can be traced in this timeline represents a maturing understanding of the roles of ALS in archaeological prospection, interpretation and landscape archaeology. As the volume was developed in 2011, further papers were commissioned to expand the scope of the book to include closely related topics in 3D data and terrestrial-based survey. In general terms this timeline tracks an evolution of thinking from early presentations where the stress was partly on ‘look what you could do with this’ to a more recent emphasis on project results and reflections on practice and integration. It is this intellectual context along with the realisation of connections between various trends in 3D data that stimulated this volume.

Structure

The volume is divided into three main sections, following on from this introductory essay and a technical overview of airborne and terrestrial laser scanning in archaeology. Opitz’s overview provides background information for topics covered in the volume in recognition that some understanding of the processes of data collection and processing will help archaeologists to make more informed use of the data. The first group of papers (Towards understanding landscapes – lidar in context) provides a variety of perspectives on interpreting the landscape, from those rooted in ALS datasets (Doneus, Risbøl and Mlekuž) to discursive papers on field experience and observation and interpretation of aerial images (Halliday and Palmer respectively). In all cases there is an emphasis on understanding of, and critical reflection on, the source material and how to make best sense of it, drawing as appropriate on complementary information within integrated, holistic views of the landscape.

In the following section (Working with lidar and 3D data) the emphasis shifts to an examination of processing, visualisation and manipulation of data. With lidar and other 3D datasets so dependent on processing and visualizations Kokalj et al. critically assess options and implications of different approaches for 3D datasets. Remondino’s paper reviews the state of the art of photogrammetry for archaeological survey, identifying advances in hardware and software that are giving this ‘old’ technology a new relevance as a source of 3D data. While multiple scales of analysis are implicit in many papers in this volume, Evans et al. explore micro-topographies of objects in a study that highlights how Laser Scanning Confocal Microscopy of artefact surfaces is driving analyses of biography and identity. The theme of fitting technology and datasets to purpose is continued by Crutchley, who describes a variety of uses of lidar data by English Heritage. The majority of archaeological uses of ALS will, inevitably, be of ‘second hand’ datasets, collected for other purposes such as mapping and environmental modelling. This is likely to remain the case and the ‘formation’ of the rich lidar datasets for Ireland from a variety of sources is discussed by Corns and Shaw. This reflection and lessons learned from Ireland identifies issues such as managing expectations, how collaborations work and the potential for re-use of data. In the concluding paper Challis and Howard discuss the potential of intensity images (i.e. visualisations of the amplitude of returned laser pulses), a component of lidar data that is often ignored.

The final section (Making meaningful landscapes with lidar and being part of something bigger) contains papers that explore the use of ALS in research projects and cultural heritage management. Collectively these highlight the diverse applications of ALS, and the need to consider survey design and intended purpose. Scale is a crucial consideration of Hesse’s paper on large-area prospection in Baden-Württemberg, which addresses head-on one of the major challenges of working efficiently with massive datasets, principally in the development of rapid ways of extracting information (algorithms) and the recognition that ‘certainty’ of interpretation will change with scale of analysis. The role of this data in cultural resource management strategies is discussed in an approach that should have wide-ranging implications. A completely different, heavily cultivated and open, landscape in the Mauguio of Southern France is discussed by Poirier et al. in a study that integrates historical maps, field walking survey and aerial photographs. Integration and identifying complementarities are central to Bennett et al., who discuss developments in airborne multisensor survey with the aim of promoting the full information content of topographic and spectral data to heritage professionals. Defining purpose and making appropriate, informed use of ALS are recurrent themes in papers throughout the volume, and are central to the contribution by Ainsworth et al. describing their experience using ‘ortholidar’ as a ground-based method for rapid survey of earthworks and upstanding remains. A different engagement with complex landscapes is presented by Davis et al. in a discussion of lidar survey in the Brú na Bóinne World Heritage Site, which highlights added value for landscape modelling and understanding. The final two papers look outwards from the ‘narrow’ concerns of archaeology to wider worlds of visualisation and research. Challis and Kincey discuss the potential of immersive visualisation using computer game engines for exploring sense of place, meaning and interpretation in landscape, highlighting the challenges about thinking laterally and imaginatively when trying the get the best from versatile datasets. This too, is a central theme in Beck’s discussion of collaboration and approaches to research, and its communication, explored with reference to heritage applications of ALS.

Collaboration and communication are good themes to conclude on, and are particularly pertinent to ALS and other 3D data and their continuing development within archaeology. The range of skill sets, perspectives and backgrounds represented in this volume highlight how important collaboration and effective communication are in this field. This volume highlights the relevance of multi-scaled topographic data to contemporary archaeological practice, and in a rapidly developing world of possibilities provides stimulating examples of thought and best practice.

Acknowledgments

Our thanks to Kevin Barton, Ant Beck, Ralf Hesse, Billy MacRae and Ole Risbøl for comments on the text, to Rob Shaw for providing Figure 1.1, to Angela Gannon for providing Figure 1.2 and to Strat Halliday for providing Figure 1.3. Parts of the text have been presented at the First International Conference on Virtual Archaeology held at the State Hermitage Museum, St Petersburg, 4–6 June 2012. Attendance at this conference for D Cowley was funded by a grant from The Royal Society of Edinburgh, which is gratefully acknowledged.

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