The Stour Valley Project, England: a cropmark landscape in three dimensions
The river Stour forms the boundary between counties of Essex and Suffolk in south-east England (Fig. 1). It flows along a band of alluvium, terraced valley gravels and glacial sands and gravels, in an area that is predominately Boulder Clay with pockets of underlying London Clay. The dense concentration of cropmark sites along the river valley was regularly flown from the 1950’s on by the Cambridge University Committee for Aerial Photography (CUCAP); the Royal Commission on the Historical Monuments of England (RCHME); and the Archaeology Section of Essex County Council. The latter included survey carried out in the exceptional conditions in 1995-6, which not only discovered new sites, but also afforded important additional detail at a number of important sites (Strachan 1996 and 1997).
The area was re-mapped between 1997-8 (Strachan and Ingle 1998 and 1999) by the Essex Mapping Project (Ingle and Strachan 1994) as part of the National Mapping Programme (Bewley 1995 and 1998), revising the existing 1:10,560 cropmark plots maintained by the SMRS. This process illustrated the extent and diversity of cropmark sites along the flood-plain, and in particular, the prehistoric “Monument Complexes” consisting of ring-ditch cemeteries (including large dual concentric examples); elongated enclosures (interpreted as long mortuary enclosures and long barrows); and the two cursus monuments at Wormingford and Stratford St. Mary. This current project arises from the preparation of a regional research framework for the eastern counties (Glazebrook 1997) and has been funded by English Heritage (EH) as part of the implementation of the Monuments at Risk Survey. The first stage builds on the work described above and involves the large-scale mapping of the cropmark landscape, and the use of a digital terrain model (DTM) within a GIS environment, as the basis for study and interpretation. This summary describes the methodology employed by the project, and it is hoped that results will appear in a forthcoming AARGnews.
Air Photographic Sources
Air photographic collections held by the Essex Heritage Conservation Record (EHCR) and the SMR at Suffolk County Council were the primary sources used for image rectification. These collections contain a variety of sources including images by CUCAP, local flyers, RCHME survey and Essex County Council (Fig. 2). The CUCAP specialist oblique collection was also consulted, in addition to black and white Aerofilms vertical runs, at a scale of 1:12,000 (dating from June 1960 and September 1990) which are held by the Essex County Council Information Resource Centre.
Image rectification, interpretation and mapping
Selected images were scanned at a resolution of 300 dpi and imported into Aerial 5.5 software (Haigh 1996) for rectification based on OS land-line data accessed via the Essex County Council Arcview GIS. The residual error for rectification was kept below 5m, allowing future work, such as geophysics or excavation to be added to the GIS and confidently correlated with identified cropmark features. When insufficient control data existed on photography for a site, the NMP plot (accurate to less than 10m) was imported into GIS and used. The resulting rectified images were then geo-referenced and imported into the GIS, to be viewed with OS land-line data and allowing mapping of identified features (Fig. 3).
The cropmark landscape consists of three main elements: circular, sub-circular and elongated enclosures (both rectilinear and curvilinear); the rectilinear landscape; and areas of local drift geology and palaeo-channels. These features were mapped on screen at a scale of 1:500, producing three polygonal vector layers (“Monument Complexes”, the “Rectilinear Landscape” and “Geology”) which can be viewed independently or in combination. In addition to these, a number of other data sets, from a variety of sources, have been added to the GIS. These include 1:25,000 drift geology data (partly made available by Wessex Archaeology) and find-spot information from the SMRs (divided by period: Mesolithic, Neolithic, Bronze Age, and Iron Age). A total of 334 individual sites were recorded in the “Monuments Complex” class.
Viewing sites in the landscape and view-shed analysis
The resulting plots and associated data sets can be viewed at a variety of scales (ranging from individual sites at e.g. 1:1,250 to distributions at around 1:200,000). A site, or group of sites, can therefore be viewed against geology and find-spots, and in relation to other sites and topography (in the form of the 5m contours). In addition, however, the 5m contour data can be used to create a Triangulated Irregular Network (TIN), or surfaced elevation model, on which the data can be draped. The 3D Analyst and Spatial Analyst extensions of Arcview allow the inter-visibility of monuments to be studied by selecting a position on, or above the DTM, and carrying out “line of sight” (LOS) or view-shed analysis. The former determines what is visible on a surface, looking in a certain direction from a single point, while the latter shows areas on a surface visible from one or more observation points. View-shed analysis is generally more useful as an aid to studying an archaeological landscape as it reveals other sites which share visibility with the observation point. The process results in a “flood diagram” which shows areas visible from that position and areas which are obscured by terrain (Fig. 5). Cumulative view-shed analysis (CVA) can be carried out using a number of different positions, or sites, to create a model of inter-visibility. While the model does not take into account vegetation cover, it is a valuable tool for the analysis of the relationship between sites and their topographical setting. The principles and methodology for CVA have been described in a study of the inter-visiblility of long barrows in the Stonehenge and Avebury areas (Wheatley 1995). The technique requires only a suitable DTM and the location of sites in the landscape, and can therefore be easily applied, and is likely to produce archaeologically significant, and often unexpected, results. Complex studies can be quickly carried out over large areas and on contours where it would be difficult to judge inter-visibility by contour lines alone. It is also invaluable in areas where modern development may obscure visibility and remove the possibility of checking visibility on the ground.
In addition, TINs can be viewed used as the basis for 3D visualisation, and viewed in three dimensions, along with the cropmark plots and other data, in perspective from a selected static 3D position, or in rotation (Fig. 6). This feature is useful for viewing sites in their topographical setting, and the software allows for vertical exaggeration of the DTM (which can reveal topographical subtleties) and control over the suns azimuth and altitude. In addition, when viewed from ground level, the models display the spatial organisation of monument complexes within their topographical setting. The potential for this technique as an aid to landscape interpretation is evident, but is more likely to be significant when combined with a morphological study of the sites mapped. The project has involved simple morphological analysis of sites within the “Monument Complexes” class, allowing distributions and cumulative view-shed analysis to be carried out on selected site-types. It is hoped to include details of this process, along with results, in a future AARGnews and the AARG web-site.
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