Surface mapping of earthworks in Northern Ireland: Ballyhenry Rath case study
C. Graham1, J. Meneely1, C. McManus2, C. D. Lloyd1, K. D. Lilley1.
1School of Geography, Queen's University, Belfast.
2Archaeological Development Services Ltd., Belfast.
School of Geosciences, Queen's University, BELFAST BT7 1NN
Conventionally, construction of surface models has been carried out using theodolites with an electronic distance measurer (EDM). The current generation of high specification Global Positioning System (GPS) receivers may be used to make measurements with sub-centimetre accuracy. The rationale behind the paper is that, where appropriate GPS equipment is available, it is often possible to carry out a rapid and highly accurate survey with limited time required for setting up the equipment. Conventional survey (for example, using an EDM) may be time consuming. GPS may be considered advantageous if
(i)it can be shown that a GPS-based survey can be completed more quickly than a conventional survey and
(ii) it has a level of accuracy of measurement comparable to conventional survey.
The main potential restriction in the use of GPS is the availability of visible satellites. We demonstrate how to ascertain the optimal time at which the survey may be carried out, given that we know the paths of the satellites in the GPS constellation at any particular time. In this paper, we apply differential GPS technology to the accurate and rapid survey of an early Christian rath (c. 7th � 12th century A.D.) in Ballyhenry, County Antrim, Northern Ireland.
The Ballyhenry Rath comprises a central habitation platform surrounded by a low bank and outer ditch. The site is located in an area being developed by the Northern Ireland Industrial Development Board, and the need to obtain an accurate surface map was increased when it became clear that the site is to be destroyed during construction work. The site was excavated and recorded in the 1970s but recently the opportunity was taken to (i) obtain a detailed surface model and (ii) test the GPS equipment in a range of circumstances, in particular the site is covered in dense vegetation. In this paper, we outline our strategy in terms of the time and spatial coverage of the survey. After establishing a suitable date for the survey we decided to set up a grid with a spacing of approximately one metre and positional measurements were made at each node of the grid using the real-time (that is, no post-processing of the positional data is needed), stop and go principle. The survey was completed over a period of one day. It was estimated that accuracy in the order of 1 cm was achieved. The data were obtained in geographic co-ordinates (WGS84) and converted to the Irish Grid co-ordinate system to facilitate future use of the data. After completion of the survey it was necessary to interpolate from the nodes of the grid to derive a surface model. This was achieved in two ways, through the construction of a Triangulated Irregular Network (TIN) and a Digital Elevation Model (DEM). A range of algorithms for surface generation were used and assessed and the potential implications were outlined. Different approaches to the visualisation of the surface model were explored using a Geographical Information System (GIS). Finally, some possible applications of the surface model, and development of approaches used in this analysis, were discussed.