Editorial

Abstract

Introduction

Site Details
  Photogrammetry

Methodology

Results

Conclusions

Acknowledgements

References

Case Studies Index

2 Site details and aims of the study

Northern Ireland has a wealth of archaeological sites, which reflect human activity in the region from the earliest Neolithic. The earliest sites comprise individual standing stones and stone circles, and some of the most important and complex of these have been excavated from beneath peat bog. The Copney Stone Circle Complex (CSC), County Tyrone, N. Ireland (H599782, Figure 1) is an important Bronze age site forming part of the Mid-Ulster Stone Circle Complex. The complex is situated on the northern slope of Copney Hill and runs down-slope along a NW-SE axis. The CSC was first noted in 1979 and surveyed in 1981. It became a scheduled monument in March 1993 and was taken into State Care Guardianship in 1990, at which point the site, located in peat bogs, was still un-excavated. The Environment Service: Historic Monuments and Buildings (ESHMB) initiated a program of bog-clearance in August 1994. This work was completed by October 1994 and the excavated site was surveyed in August 1995. Only two of the circles (of a total of nine) and a quarter of a third were excavated at this time. The present visible site comprises these circles (designated circles A, B and C, a stone alignment between circles B and C and a standing stone (Plate 1).

Figure 1. Schematic location map of the Copney Stone Circle Complex, N. Ireland

The complexity of the CSC makes it unique amongst the Mid-Ulster stone circles. As a result, the accurate recording of the details of the site and its components is essential. This requirement became even more important in light of the survey reports following the initial site survey report (MacDonagh, 1995). In this report, the surveyors remarked upon the condition of the stones making up the Complex, particularly their apparently high rate of decay since excavation. The decay of such a site represents a significant loss to both the archaeological record and to the public. For this reason, excavation of the site was paused to allow examination of the stones, surveying and the construction of conservation pathways for the site. The surveyors therefore recommended monitoring of specified keystones within the complex to estimate decay whilst decisions regarding conservation were considered by EHSMB.

At this point, a program of study was initiated in the School of Geosciences, The Queen's University of Belfast to investigate decay features and the effect burial in peat has on the structure and mineralogy of the stones as a means of informing management decisions for this site. The report generated from the first phase of this study (Curran et al, 1997) highlighted the following problems:

• The stones comprising the CSC are significantly altered by hydrothermal processes that have generated individual stones that are cross-cut by extensive fracture networks and veins. The ingress of fluids through these networks has permitted the rapid decay of the stones by weathering processes (freeze-thaw, leaching etc) since their excavation. The penetration of the weathering processes through the body of the stones has resulted in a very weak artifact that is decaying at a very significant rate.
• Present environmental conditions at the site are exacerbating the effects of the weathering processes and deterioration of the site is set to continue at this high rate unless remedial action is taken.
• Perhaps Curran et al.'s most significant observation relates to the processes of chemical weathering of the site. There is a correlation between the rate of decay of the stones and their total surface area exposed to attack. Thus, whilst monitoring of keystones will give some estimation of ongoing decay, once the stones begin decay, the overall rate of stone decay is constantly accelerating as smaller and smaller fragments are produced.

As a result of this preliminary study, a number of remedial actions were suggested to the EHSMB ranging from preservation of the stones using chemical treatment and/or cover to reburial of the site after site preparation to preserve the stones. In choosing how to best preserve the information in the CSC, the EHSMB require an accurate determination of the projected decay rate of the site as a whole and of individual features within the site. Furthermore, development of the site including construction of access routes such as elevated walkways, and covering the site from prevailing winds has been suggested. Thus a rapid method of visualizing the effects of development on the site both from a structural and aesthetic point of view would be beneficial. More importantly, a permanent record of site details through time facilitates examination of changes to the site.

The aims of this study were therefore to evaluate the use of aerial photographic and photogrammetric techniques to the characterization of the Copney Stone Circle Complex and specifically to provide:

• A base-line digital terrain model of the site for calibration of future site surveys and for use as a measuring tool for the efficacy of conservation techniques. For example, if the recommendation to erect shielding structures (such as walls) is followed, it would be possible to measure the change in the stones after the shield has been placed and comment upon its efficacy.
• A rapid method for the acquisition and storage of site measurements and stone dimensions.
• A visually impressive method of presenting large amounts of data to the public.

To highlight the usefulness of this technique in both detailed and more landform-based applications, two sets of images were treated:

• A standard aerial stereo pair purchased from the Ordinance Survey of Northern Ireland (scale 1:10000). The images were commissioned by the OSNI and collected by BKS Surveys of Coleraine in July 1997. This format is typical of commercially available aerial photographs and it was considered beneficial to examine the efficacy of digital photogrammetry within these limitations. It is obviously financially more attractive to purchase images rather than commission lower level flights over archaeological sites. The problems inherent in processing images at this altitude with regards to small-scale features are well known, specifically that at this altitude it is very difficult to resolve small-scale structures. The GSD (Ground Sample Distance) for the 1:10000 scale imagery is 20 cm. For this reason, aerial photography at this altitude is usually confined to larger structures.
• A privately commissioned stereo pair collected by BKS Surveys of Coleraine in April 1998 (scale 1:2000). Whilst a more expensive option, the 1:2000 scale imagery allows the resolution of much more detail on the ground and is essential in the detailed survey of the CSC. The GSD for1:2000 scale imagery is 5cm

2.1 Photogrammetry and the Leica-Helava Digital Photogrammetric Workstation

The Helava Digital Photogrammetric Workstation (DPW) represents a powerful tool in the analysis and generation of data from digitally scanned images. The DPW is equipped with two high resolution monitors and a stereo video card. One monitor is used for viewing the software package and the other (extraction monitor) is used to see the imported images as they are overlapped. Using a pair of stereo glasses, the overlap area may be observed in 3D and features examined for analysis. Furthermore, this capability of the system allows the accurate placement of ground control points onto the surface rather than floating above the image.

The Copney Stone Circle Complex, like many similar sites in Northern Ireland, is surrounded by waterlogged peat bog which makes human access difficult and vehicular access impossible. Therefore, it is not possible to acquire initial photography of the site by conventional means (using scaffolding or crane) but necessitates the use of aerial photographs. The DPW was applied to the calibration and rectification of aerial photographs acquired of the CSC. Aerial photography facilitates the capture of the entire site in a single image (and a considerable amount of the surrounding terrain). Processing of this image allows the rapid and precise recording of data from this complex site and enables the creation of a baseline against which the success or failure of conservation treatments may be compared. Additionally, it allows the graphical presentation of information in formats that are easily examined and presented to researchers and the public.