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Analysis of the Effectiveness of Airborne LIDAR Backscattered Laser Intensity for Predicting Organic Preservation Potential of Waterlogged Deposits

Using archive airborne LIDAR data, this project creates new research material which is confidently expected to provide substantial benefits to the field of archaeology as well as wider environmental management themes, for example, consideration of catchment management with respect to changing flood frequency and magnitude associated with future climate change. It also may provide substantial benefits to the aggregates industry. As an emerging and not fully understood technology LIDAR fully merits a focused exploration of this kind. The project is led by the University of Birmingham and the University of Exeter and other contributing organizations include: CAPITA SYMONDS, English Heritage, the Environment Agency and Network Mapping Ltd. The project is funded by Aggregates Levy Sustainability Fund (administered by English Heritage) and runs from 1 April 2006 to 31March 2007

The Project

Airborne remote sensing techniques have traditionally been employed to great effect in mapping cultural archaeology and to a lesser extent the geomorphology of valley floor landscapes. Archaeologists have largely focused their attention on the comprehensive mapping of cropmarks and other features of the archaeological landscape revealed from aerial photographs and large areas of England have been comprehensively mapped as part of the National Mapping Programme undertaken by English Heritage. Aerial photographs have also been employed in mapping geomorphology in alluvial landscapes, for example in extensive studies of the valleys of the Rivers Trent and Thames. Such mapping of fluvial geomorphology provides a context for past cultural landscapes and assists in identifying topographical features of high archaeological potential (for example relict river channels) and isolating areas of past river erosion where little in the way of archaeological material might be expected to survive. The systematic reconnaissance, mapping and classification of aggregate-bearing valley floor landscapes in this way has played a significant role in the strategic management of the geoarchaeological resource and intimately associated archaeological remains (e.g. fish weirs, bridges, platforms, trackways, etc.) in the face of growing impacts from aggregate extraction and other development pressures. More recently, such information relating to former fluvial regimes is also being increasingly used in understanding wider issues of floodplain management, particularly with respect to assessing the impacts of changing flood frequency and magnitude with respect to future climate. While the two-dimensional record of aerial photography provides an approach to mapping the quantity of archaeological material (both cultural and geoarchaeological) within aggregate-bearing landscapes, it provides no indication of the state of preservation of that material or the associated cultural evidence. In particular, aerial photography provides no indication of the moisture level and the potential for the preservation of organic sediments. This is a significant shortfall in the usefulness of the data, since the presence of moist, organic-rich sediments may greatly increase the archaeological value of deposits. Airborne laser altimetry provides access to high-resolution, high-accuracy terrain information and as a secondary output a laser 'image' of the land surface derived from measurements of the intensity of reflection of each backscattered laser pulse.

Archaeological applications of LIDAR have focused largely on its ability to provide a high-resolution record of terrain variation, allowing the detection and mapping of subtle archaeological features, mapping of fluvial geomorphology and its unique ability to penetrate vegetation cover to map underlying archaeological earthworks. Backscattered laser intensity measurements have largely escaped attention, and indeed do not form a part of the standard data product supplied by EA (although intensity data is collected on each EA flight and can be accessed by reprocessing original flight data). The LIDAR system used by EA, NERC and many UK-based commercial LIDAR providers (an Optech Airborne Laser Terrain Mapper) operates in the near infra-red (NIR: 1047nm) and so backscattered intensity is in-effect a record of the reflectance of earth surface materials at this wavelength (Figure 1).

While it is recognized that other airborne remote sensing techniques (ATM, CASI, etc. cf. Figure 2) may be equal to or more effective than LIDAR at detecting geoarchaeological and anthropogenic features, this study focuses on LIDAR for a number of reasons. Principally LIDAR has been chosen as the basis of this study because it is an emerging technology, still poorly understood within the archaeological community. There is a growing demand for and supply of commercial LIDAR providers. National agencies such as EA already operate LIDAR, while Ordnance Survey are actively investigating a move into ownership or a LIDAR system for data acquisition to assist national mapping. Some good work has been done of understanding the parameters of LIDAR terrain mapping required for archaeological purposes and so archaeologists are equipped to specify such work when commissioning survey. However, little or no work has been undertaken to investigate intensity, which until now has been largely ignored either by archaeologists or by the LIDAR industry as a whole. The generic potential of this research may be summed up as follows:

• It will create a tool to allow the rapid assessment of valley floor corridors to provide baseline information on the environmental potential of organic sediments, usually preserved within palaeochannels. Such information may prevent the need for costly environmental investigations of potential extraction areas prior to the submission of planning proposals.
• It may provide information on the hydrological condition of valley floor sediments prior to aggregate extraction, an important issue in the consideration of the effects of artificial pumping and draw-down on the water table.

Primary Aims

The project aims to produce the following materials as project outcomes:

Good Practice Guide

A good practice document, aimed at stakeholders and written in a non-technical style will be produced at the end of the project. It will outline:

• how to obtain LIDAR data;
• basic information on processing;
• how this is applicable to the stakeholder community in general;
• specific information related to intensity and its applicability to the stakeholder community.

Once approved by English Heritage the document will be widely and freely distributed, initially to members of the Association of Local Government Archaeological Officers (ALAGO) and to selected stakeholders (eg EH Science Advisors) via a mail shot. A summary of the document will be offered for publication in The Field Archaeologists and Quarrying Today the magazine of the Quarry Products Association.

Website

Information about the project will be disseminated via the Trent Valley GeoArcheology website (www.tvg.org.uk) which is maintained by the project principal investigators and is located on a University of Birmingham server. This portal is already used for the dissemination of a number of ALSF projects being undertaken at Birmingham. Digital versions of the good practice guide will be made available for download in Adobe portable document format (PDF), as well as a copy of the PowerPoint presentation and accompanying notes used in the workshops (see below). Users will be required to complete a short registration process before downloading reports. This process will follow the model successfully implemented for TVG2002 reports. By November 2004, 42 registered users had downloaded TVG2002 reports including users from the UK, USA and continental Europe.

Workshop

A series of workshops will be held, initially at the University of Birmingham and subsequently at two regional venues (York and Southampton) inviting key members of the stakeholder community (aggregates representatives, heritage managers, other academics etc) to view the results of this work and its applicability to the minerals and archaeological communities. In essence, the workshop will take the stakeholders through the Good Practice Guide and offer the opportunity for comment and question.

Academic Journal Paper

The project team will produce an academic paper summarizing the research undertaken as part of the project for publication in a suitable peer-reviewed journal. The paper will reflect both the archaeological and remote sensing aspects of the project. Briefly, it will consider:

• Use of terrestrial LIDAR as an analogue for airborne LIDAR
• Analysis of terrestrial and airborne intensity
• Relationship of intensity to soil and vegetation properties
• Relationship of intensity to archaeological deposit preservation.

Archive of Project Data

A digital archive of data generated by the project ream will be prepared. The archive will be accompanied by full metadata using the standards recommended by ADS; it will be deposited with ADS at the completion of project work

Publications/Further Reading

Challis, K., 'Airborne laser altimetry in alluviated landscapes', Archaeological Prospection, DOI: 10.1002/arp.272. <http://dx.doi.org/10.1002/arp.272> ( in press)

Challis, K.,'Airborne LIDAR: A Tool for Geoarchaeological Prospection in Riverine Landscapes'. in Stoepker, H. (ed) Archaeological Heritage Management in Riverine Landscapes, Rapporten Archeologische Monumentenzorg, 2005, 126: 11-24

Conference papers anticipated at Geoarchaeology 2006 and EAA 2006; publication of these in peer-reviewed journals to follow.

Tools and Methods

Tools

MDL Laser Ace Terrestrial Laser Scanner; ESRI ArcGIS 9.1; ERDAS Imagine 8.7; Surfer 8; Voxler 1.

Method Categories

Data Structuring and Enhancement; Data Analysis; Data Publishing and Dissemination; Practice Led Research; Communication and Collaboration Strategy; Project Management.

Specific Methods

3D Laser scanning; digital remote sensing; use of existing digital data; image feature measurement; image enhancement; GPS survey; geophysical survey; geochemical survey; electronic data distribution.

Data Formats

ESRI shape; ESRI grid; ERDAS Imagine IMG; GeoTIFF; JPG; Surfer Grid; Voxler voxb; ASCII text; Excel xls; MS Word; Adobe PDF; HTML.

Metadata Standards

Dublin Core.

Project Website

<http://www.tvg.bham.ac.uk/research.html>

Staff and Advisors

Principal Staff Members:

• Keith Challis, HP Vista Centre, Birmingham Archaeology, University of Birmingham.
• Dr Andy J Howard, Institute of Archaeology and Antiquity, University of Birmingham.

Other Staff Members:

• Dr Ben Gearey, Senior Environmental Research Fellow, Birmingham Archaeology, University of Birmingham.
• Dr C Carey, Department of Geography School of Geography, Archaeology and Earth Resources, University of Exeter.
• Dr David Smith, Institute of Archaeology and Antiquity, University of Birmingham.

External Expertise:

• Dr Alan Thompson, Head of Earth Science, Capita Symonds Ltd.