As part of the ongoing Conservation Management Plan project with the Scottish Fisheries Museum, Wessex Archaeology Scotland has been engaged in photogrammetric recording of the vessels within the museum’s collection. This recording allows 3D scale models of the vessels to be produced in Agisoft Photoscan. The results will be used alongside existing plans of the vessels to check for changes in the shape, state and condition of the individual boats, or in the case of some, to allow scaled plans to be drawn up using both the models produced and on-site recording.
The premise behind photogrammetry is relatively simple, but requires a large amount of processing power and dedicated software. The software (in our case Agisoft Photoscan) compares areas of similar colours, light and lines within the selection of photographs to find point matches between the dataset, which are then used to hang the wider pictures across. The more of these points there are, the more complete, the finer quality and the more accurate the end model will be. The amount and time of processing therefore increases exponentially to the number of photos put into the model, with any new photo having to be compared to all of the others by the software.
It is vital for the process to have as complete a photographic coverage of the subject as possible. An efficient way of doing this is to choose an easily recognisable starting point and work around the object in defined steps at a constant height and distance (where possible) from the object until the start point is reached again. Then start again at the next height to capture the next ring of images. By keeping a systematic approach to the photography, we are able to ensure we have all the angles covered when the images are put into Photoscan.
For the larger complex vessels within the museum fleet, it was necessary to ensure there was coverage not only of the exterior of the vessels but also of the interior. Some of these, such as the Newburgh Salmon Coble, contained complex machinery which required an extra series of photographs to make sure they were fully recorded. With three of the vessels being indoors, it was easy to regulate the amount of light around the subjects, with extra lighting being used to ensure dark areas, particularly towards the keel and floors of the boats were illuminated. After all the more black areas in the photographs, the fewer points of recognition the software can pick up on. This is equally true of over-exposed white areas within photographs, which proved problematic in the photogrammetric survey of White Wing which was outside in occasionally glaring sunlight during the survey.
Ideally the lighting and the subject, along with the surroundings, should be constant for the period of the survey ie, people or items should not suddenly appear within the view of the camera for some photographs of an area and then disappear for other photographs of the same area. This is not disastrous if it does occur, but again reduces the number of potential matching points for the dense point cloud.
Once the photographs have been collected, they are put into Photoscan, and a series of processes applied to them, culminating in a scaled 3D model of the object, which can then be covered with a computer-generated textured mesh made up of the textures and colours from the photographs. These models will there be traced off, also using a set of reference photos for guidance and a set of field plans, sketches and notes for extra detail, to form accurate deck plans, sheer plans and cross sections of the vessels within the fleet. These will become the record of how the vessels appear now, and where previous plans are available, the changes to and potential deterioration of the vessel will be noted.
Even where no previous plans exist, as with the Maggie, it is clear from the model that this vessel has had a tough working life, with the shape in plan being slightly twisted and multiple repairs being obvious. It is vitally important to note these adaptations and changes prior to further conservation work being completed, so that an accurate understanding of the vessel’s fabric is known.