A COMPARISON OF LASER SCANNING AND STRUCTURE FROM MOTION AS APPLIED TO THE GREAT BARN AT HARMONDSWORTH, UK
Keywords: Surveying, Laser Scanning, Photogrammetry, Structure from Motion, Medieval, Barn
Abstract. The great barn at Harmondsworth near London Heathrow airport, United Kingdom (UK), was built in 1426–7 for the Bishop of Winchester. At 58 metres long and 11.4 metres wide, it is one of the largest ever known to have been built in the UK, and the largest intact medieval timber-framed barn in England. The barn is built almost entirely of oak, although the walls rest on a low masonry sill-wall. Internally the space is divided into a central "nave" with a lower aisle to each side, and is divided along its length into 12 bays. There are three doorways on the east side. For an entirely timber-framed barn, the fabric is exceptionally well preserved. Even the external weatherboarding may be partly original. Following years of neglect, however, there are a number of on-going structural and conservation problems, so in 2011 the barn was bought by English Heritage in order to allow these needs to be addressed. English Heritage is the government agency responsible for the historic sites and buildings in the care of the state of England and is also the UK government's lead advisor on the built heritage.
As one of the first steps in the conservation process the English Heritage Geospatial Imaging and Imaging & Visualisation teams undertook a four-day campaign of survey data collection. This took the form of laser scanning of the interior and exterior of the barn plus the acquisition of photography of the exterior elevations to be used with structure from motion (SFM) software. A comparison of the results of these complimentary yet potentially competing technologies will be given, as well as an evaluation of when they can be successfully used together.
This paper will describe the procedures and problems involved with collecting the survey data and its subsequent analysis. The laser scanning was undertaken using a FARO Focus 3D phase based instrument. Approximately 60 scans were acquired in order to provide as comprehensive as possible coverage given the site circumstances. A repeat visit following the clearance of artefacts and with the benefit of access equipment was required to obtain complete coverage, especially for the top surfaces of the timber frame elements. Initial results from the laser scanning were extremely promising, with some historical events (e.g. a major fire at one end of the structure) dramatically shown in the intensity data. Comprehensive photographic coverage of the exterior of the barn including the roof was obtained using a Nikon D3X mounted on both a 6m telescopic pole and a conventional tripod. A repeat visit was required to address some exposure problems in shadow areas.
A unified control network for both sets of data was obtained through the use of a total station theodolite (TST) with reflectorless electromagnetic distance measurement (REDM), incorporating a closed traverse as well as the acquisition of scanner and photogrammetric targets. The control network therefore permits the direct comparison of the results from both survey methods (allowing for systematic errors). A point cloud generated from the photography, using Agisoft Photoscan structure from motion software, was compared with the registered laser scan points with a view to determining any systematic differences, although these were to a large extent ameliorated by the use of the dense control network.
The resultant data also has potential downstream use within English Heritage for improving our understanding of Building Information Modelling (BIM) as applied to heritage structures rather than new build, and thereby contributing to the formulation of elements of a BIM strategy for English Heritage.
There are also a number of hand-measured survey drawings of the barn in existence. A quantitative as well as a qualitative comparison was made with drawings generated from the laser scan data. In general the later drawings were more metrically accurate but exhibited less understanding of the construction techniques employed. A discussion of the reasons for this is also presented.