The Stour Valley Project, England: a cropmark landscape in three dimensions
IntroductionThe 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).
Air Photographic SourcesAir 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 mappingSelected 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).
Viewing sites in the landscape and view-shed analysisThe 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
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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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