Getting the measure of brochs: using survey records old and new to investigate Shetland's Iron Age archaeology

NoBrochs are monumental Iron Age (c.400–200 BC) drystone towers or roundhouses. They are only found in Scotland, particularly the Atlantic north and west. Whilst the structural layout of brochs has long been debated, few measured surveys have been conducted. Three significant broch sites form the tentative World Heritage site of “Mousa, Old Scatness and Jarlshof: the Zenith of Iron Age Shetland” (UNESCO in Mousa, Old Scatness and Jarlshof: the zenith of Iron Age Shetland, UNESCO (2019) Mousa, Old Scatness and Jarlshof: the zenith of Iron Age Shetland. http://whc.unesco.org/en/tentativelists/5677. Accessed 9 Aug 2019). All three sites have undergone new surveys as part of a collaborative doctoral partnership research project. This chapter presents a diachronic perspective using digital documentation techniques to detect stone displacement and weathering at the site of Old Scatness using historic imagery, including photographs from the Old Scatness excavations (1995–2006) and regular condition monitoring undertaken by Shetland Amenity Trust to undertake retrospective digital structure-from-motion (SfM) photogrammetry. Whilst point clouds and 3D meshes were successfully generated from low-resolution digital images, analogue film transparencies without metadata could not produce accurate geospatial data without manually trying to extant reference data. It was possible to detect displacements in stonework over time by comparing two meshes together and measuring the distances between vertex point pairs. The reliability and accuracy of these results were dependent on how well pairs of meshes could be aligned