Trim Castle is situated on a slight bend in the River Boyne in Trim, Co. Meath where it has dominated the landscape for over 800 years. This castle has seen an extensive and colourful history but by the late seventeenth century it was falling into disrepair; with the north tower of the keep collapsing around this time. In 1822 the castle was sold and since than many of the structures within the curtain wall were torn down, with the stones repurposed.
Archaeological excavations were carried out in 1971-1974. In 1981 it became a pitch-and-putt course and in 1994 was used a set location for the film Braveheart. From 1995-1998 the Office of Public Works (OPW) carried out a multi-million Euro programme of excavation and conservation, under the directorship of Alan Hayden. The primary goals of the project were to preserve the keep, curtain wall, and gates and make the site publicly accessible – an ambition that was realised in 2000.
In summer 2016 Ithaca College (New York) and the Irish Archaeology Field School, with support from the OPW and the local community, partnered with the goal of 3D scanning the Barbican Gate and Curtain Wall.
In summer 2016 and 2017 a team from Ithaca College (New York), led by Prof. Michael ‘Bodhi’ Rodgers, partnered with the Irish Archaeology Field School, OPW and local community to 3D scan the castle, Barbican Gate and Curtain Wall. At historic sites such as Trim Castle, 3D laser scanning can document every small detail of the archaeological landscape. Laser scanning not only records every detail down to the centimetre and millimeter level it allows us to look at the site from different perspectives by removing trees and/or modern features and taking slices or profile cuts in areas of interest without having to physically alter the landscape. Put differently it is possible to digitally reconstruct and deconstruct both the modern and relict landscapes, allowing a more layered examination of space through time. This application can prove particularly useful at sites like Trim Castle, where much of the original site no longer remains.
3D laser scanners work by sending out pulses of laser light, which travel out from the scanner, reflect from the object being scanned, and return to the scanner. The scanner records the time it takes each laser pulse to travel out and back from the object they reflect from. This two-way travel time is converted to a distance and the scanner’s precise horizontal and vertical angular measuring tools facilitate the conversion of each pulse to an X, Y, and Z location in space for each spot the laser reflects from. Modern laser scanners pulse the laser 50,000 to 1,000,000 times per second. While pulsing this fast they spin the laser in the vertical and horizontal directions to cover 360 degrees horizontal and 270 degrees vertical. The scanners cannot ‘see’ 360 degrees in the vertical because they cannot image beneath them where they sit upon a tripod. Most scanners also have a digital camera onboard, which is used to take a full dome of photographs. These photographs are used to map an RGB color to each point recorded using the laser. Photographs can also be taken with a standalone camera and registered to the laser scan post-acquisition.
The data from each scan location are loaded into the Leica program Cyclone. Because each scan location has coordinates within the local grid system the data from different locations automatically come together to form a 3-dimensional data set called a point cloud. The photographs are adjusted to ensure uniform exposure and color vibrancy, and then extraneous data are removed from the point cloud. The point cloud can now be manipulated to look at features of interest.
But how might the data be manipulated, and to what end? The 3D scanning at Trim has generated a detailed model comprised of billions of data points, which can be used in a host of innovative ways relating to heritage management, archaeological research, outreach and education, remote touring, virtual reality modelling etc. As with more traditional archaeological work, in practice the data will be most effectively used in a holistic fashion drawing from a range of disciplines and expertise. All data has been given to the custodians of the Castle, the OPW.
During summer 2017, the project focused on scanning the interior of the keep and details from the castle grounds which had not been scanned in the 2016 season, such as the undercroft and the dry moat. The summer 2017 scanning went much more quickly due to the addition of a second, faster, scanner, the Leica P40.
The project has resulted in a highly accurate digital record of Trim Castle and its surrounds, which can be utilised as a baseline for monitoring the structures, and as a research tool for analysing the buildings. For example, 3D modelling has allowed a clone of the south tower to be placed in position of the collapsed north tower, creating a virtual environment whereby a researcher can position oneself at the windows and see what the view from these rooms might have been like in the past.
The scan data can be used could be used in a host of imaginative ways, relating to education, preservation, monitoring, and interpretation of historical monuments. For example:
- Real-world measurements can be captured easily of architectural features, such as the relative heights between different towers or the thickness of a wall from one room to the next.
- Data can be used to create digital reconstructions of how structures once looked.
- Scans can be repeated at regular time intervals (months or even years) in order to monitor slumping in relation to the surroundings.
- Point clouds can also provide almost instant cross-sectional views and floor plans.
- The scan will permit ‘virtual visitors’ being able to access the castle through accurate digital models.
The full results of the survey have been communicated to the OPW, both in terms of regular updates and the submission of scan data. The project results have been communicated to both the local and academic community through a series of presentations. Finally, the results of the 3D scan have been published in the 2018 Autumn issue of Archaeology Ireland.
For further information see also Rodgers, M., Bouricius, R., Shine, D. Mandal, S. and Stull, S. 2018. Laser-Scanning Trim Castle. Archaeology Ireland, 32 (3), 34-39.
Thanks to the OPW and, in particular the staff and guides of Trim Castle for providing access to the monument. Thank you to the field team and the people of Trim for welcoming us into their community and for their enthusiasm for the project.