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TAP Field Report No. 5 A survey of Shetland's gallow hills Joris ...

TAP Field Report No. 5 A survey of Shetland's gallow hills Joris ...

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Gallow <strong>hills</strong>site’s biography and a wonderful example <strong>of</strong>the cultural meaning <strong>of</strong> places, we need to beaware that they also have their own culturalbiography and may well have turned up lateror may have signiicantly changed over time.In short, the evidence for the alleged places<strong>of</strong> execution needs to be critically assessedfor each site individually. By comparing thenature <strong>of</strong> the sites and their position in thewider landscape, and compare them to places<strong>of</strong> execution in other regions, we might indsimilarities that could give us some more informationabout the history <strong>of</strong> these intriguingplaces. A Geographical Information System(GIS) provides a valuable tool to analysethe topography <strong>of</strong> the sites and their relationshipboth with the physical environment andthe historic landscape.MethodologyMost <strong>of</strong> the sites, which will be discussedbelow, were visited in May and June 2011 toinvestigate their topography and check forthe presence <strong>of</strong> archaeological features. Onthis occasion, panoramic photos were takenin a 360° angle on every site, using a regularcompact digital camera (Appendix II). Sincethe exact location <strong>of</strong> the <strong>gallow</strong>s is not knownfor any <strong>of</strong> the sites, except (perhaps) the oneon Fetlar, the photos were taken from the spot,which <strong>of</strong>fers the widest view <strong>of</strong> the surroundingarea (and hence also presents the mostvisible spot). The single photos were adjustedand stitched together in Adobe Photoshop.Additionally, a viewshed was created forevery site in ArcMap 10.0. A viewshed modelsthe areas, which are visible from one or severalobservation points, based on a digital terrainmodel (DTM). The accuracy <strong>of</strong> the viewshedhence depends on the spatial resolutionand vertical accuracy <strong>of</strong> the used DTM.The viewsheds presented below werecalculated on the basis <strong>of</strong> the OS LandformPanorama DTM <strong>of</strong> Shetland (excluding FairIsle), freely available from Ordnance Survey. 4It has a horizontal resolution <strong>of</strong> 50x50m andvertical resolution <strong>of</strong> 1m. The OS LandformPanorama DTM was created by interpolation<strong>of</strong> 10m contour lines, derived from the Landranger®1:50,000 scale map series, which inturn were generated from stereo aerial photographyfrom the 1970s (Ordnance Survey2010: 9).It is assumed that the visibility <strong>of</strong> the siteand its surroundings are reciprocal, i.e. thatthe site can be seen from all areas, which arevisible from the site. Since the <strong>gallow</strong>s or gibbetswould have risen to a certain height fromthe ground, the observation points were givenan <strong>of</strong>fset <strong>of</strong> 2m. In some cases, a combinedviewshed was created for several pointsaround the spot from where the photos weretaken, to reduce the effect <strong>of</strong> sharp drops inthe DTM at grid cell edges and to achieve abetter correspondence with the actual view.The viewsheds do not only help to locatethe landscape features shown on the panoramicphotos, but also allow for a quantitativeanalysis <strong>of</strong> the view. We can calculate the size<strong>of</strong> the visible area, and also calculate the division<strong>of</strong> the visible land and sea surface. SinceShetland’s vegetation has hardly changed inthe past 3000 years (Bennett et al. 1992), it<strong>of</strong>fers good opportunities for viewshed analysis;trees being rare, it can be assumed thatthe view across the landscape was almost thesame in the Middle Ages as it is now.7

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