By Dara Burke, architect, and Michael White, Northburgh Castle Conservation Group
The Northburgh Castle Conservation group in Donegal used 19th-century paintings held in the Library, as part of their Conservation Plan for the 14th-century Anglo-Norman ruin. The Plan is being launch by the American Ambassador to Ireland Claire D. Cronin, who has family connections to Donegal.
Northburgh Castle 3D model
Northburgh Castle, built in 1305, is an Anglo-Norman castle situated in Greencastle, north Donegal, and described in the Ulster Journal of Archaeology as “the largest and most impressive building of its kind in Ireland”. The castle was built by Richard Óg de Burgo, the 2nd Earl of Ulster, also known as the Red Earl. Northburgh is historically significant, marking the highpoint of Anglo Norman control of Ulster, and bearing witness to Edward the Bruce’s invasion of 1316, the collapse of the Ulster Earldom and subsequent Gaelic revival.
However, the castle has been neglected and the structure has deteriorated in the past 30 years. Because of this, in 2021 we set up the Northburgh Castle Conservation group and then commissioned a Conservation Plan for the castle, part funded by the Heritage Council. In parallel we also commissioned a 3D model of the castle from architect Dara Burke. Dara’s work was supported by some 19th-century paintings of the castle, done by Captain William Smith. These pictures, which are in the Library of Trinity College, the University of Dublin give a clearer picture of the structure of the castle before it became covered in vegetation in the 20th century. They were a constant source of inspiration and reference during the project.
We wanted the model to digitally record the site and the extent of the castle in 3 dimensions. The architect used a drone to scan the structure and capture close up details of the remaining towers at the site. The model was also used to generate scaled plans, sections and elevations of the site and the castle for use in the conservation plan.
3D view showing the model in point cloud model
Apart from recording details of the castle, we also wanted to use the 3D model as a way to promote awareness of the conservation project. We uploaded the final model to the website Sketchfab, where anyone can view and explore the models on any device.
Once the 3d scan was completed it was possible to create scaled two dimensional orthographic projections of the site. It was then possible to digitally overlay a survey carried out by archaeologist DM Waterman in the 1950s.
The original survey was published in the Ulster Journal of Archaeology issue 2 in 1958. It is very interesting to compare the current plan of the castle with the survey drawing as it was clear that the site had deteriorated and had become far more overgrown in the past 30 or 40 years.
And the accuracy of the survey which was carried out without any of the modern surveying equipment we have today is very impressive. By comparing the survey plan to the initial 3D scans, areas were identified which were not immediately obvious due to the trees and ivy that dominate the site in many areas. These areas were then scanned again to add more detail to the overall model.
3D scanning process
The architect Dara Burke created the digital 3d model of the site using drone-based, geo-referenced photogrammetry. Photogrammetry is a process that enables three dimensional measurements to be extracted from two dimensional data such as photos. We used a software application called Reality Capture along with a DJI Mavic drone to capture and organise the photographic images.
The Reality Capture software uses GPS coordinates from the drone flight to geo locate the model for mapping and to establish a rough location for each image recorded. It then analyses the data to detect the alignment and camera pose of each photo. By comparing the difference in parallax between points identified in two images, the software can accurately establish the three dimensional location of those points in space. This process is repeated millions of times to build up a full 3d model of the location being scanned.
The architect carried the scan over a period of 5 weeks during August and September 2021. He made multiple visits were made to the site and around 500-1000 drone photos were captured per visit. An initial scan was carried out to generate a low detail base model. Additional visits were then carried out to add detail in particular areas of interest.
Since the software captures colour information, it was important to capture the site in even lighting conditions. Overcast days are ideal for photogrammetry scanning as there are no hard shadows from the sun. By selecting days to scan with overcast weather and recording the data at broadly similar times of day it was possible to merge the datasets without issues caused by strong shadows or difference in lighting intensity. This method allowed far more detail to be captured than if the project was limited to one day of scanning.
The architect manually flew the drone in a series of orbits around the castle and aerial grid patterns of the overall site to capture a dataset of over 7000 images. Although it is possible to automate the drone flight for photogrammetry, manual control of the drone allows the operator to fly closer to the structure to capture more detail. Additional photos of the basement level and keep were captured using a digital SLR camera on a tripod.
After each scan visit the architect reviewed the new sets of photos then loaded them into the Reality Capture software to add detail to the model. Some sections of the scan such as the basement level were analysed independently and then merged into the main model. The software initially generates a sparse “point cloud” preview of the model which gives a sense of the overall form.
The point cloud is then processed into an 3d mesh with no colour information. The final step is to texture the mesh using the photographic data to generate the colour information for each point. All of this processing is extremely time consuming for the computer and took up 36 hours for the high detail meshing phase.
The model was processed in this way each time there was new data available, adding more detail to the 3d model each time. The final project file contained over 300 million points and consumed over 250gb of hard drive space.
The high detail 300 million point mesh was optimized down to between 300,000 to 3,000,000 points to make it viewable on the web using the Sketchfab online platform. In order to showcase the high level of detail captured, study models were created of smaller parts of the mesh. This allowed the south gate towers and basement level to be shown in high detail in a web browser.
Due to the amount of overgrowth and large trees on at the site some areas were difficult or impossible to scan effectively but the final scan delivered a good overall 3d model of the castle with high levels of detail. The southern gate towers were scanned and processed to a resolution of 1mm per point which is able to clearly show fine detail such as joints in the stone work and cracking around windows. The advantage of using drone based photogrammetry is that the inaccessible tall elements of the towers can be captured in great detail.
Trinity College, the Paintings of Captain William Smith and ‘Verified Views’
William Smith (1778-1832) was a captain in the British Royal Engineer’s Office, stationed in Buncrana, County Donegal. He was responsible for the building of a number of Napoleonic-era towers in Donegal, working under Major-General Charles Ross. He made many paintings of military installations, including a number of drawings and paintings of Northburgh castle in 1806, which he bequeathed to the Library of Trinity College, the University of Dublin. The entire collection is digitised and may be seen among the Library’s Digital Collections. There is also an online exhibition, curated by the Heritage Council’s architectural officer, Colm Murray.
The paintings by Captain Smith give a clearer picture of the structure of the castle before it became covered in vegetation in the 20th century. They were a constant source of inspiration and reference during the project. The paintings clearly show parts of the castle that are currently completely obscured by ivy and trees. While the 3D scan was being refined the paintings were referred to in order to see which parts of the site should be captured in further detail. Once the scan was completed we decided to see if some of the views captured by Captain Smith could be replicated using modern 3d software.
The architect imported the 3D scan into the software system Unreal Engine in order to replicate the views. Unreal Engine was originally developed for use in the computer gaming sector but has been adopted by many other industries in recent years as a powerful visualisation tool that can deal with high detail meshes in real time.
Once the model was set up in Unreal Engine, virtual cameras were created to replicate each of the views captured by Smith. In modern building planning applications a technique known as “verified view” is often used to communicate the visual impact of a new building proposal. This typically involves surveying a camera position in the real world and then creating a virtual camera with the same settings.
Although it is obviously is not possible to determine the exact position of Captain Smiths painting easel, the notes from the paintings made by Smith were used to establish where each painting was created. Together with historic maps and the 3d height data of the ground levels created by the 3D scan, the location for each painting was derived with a good level of accuracy
Cameras were placed at ground level and at eye height to replicate the view as seen by Smith. Once the cameras were in position the paintings were digitally overlaid and the camera positions were finely adjusted to match the paintings as closely as possible.
It was very impressive to see how three dimensionally accurate Smith’s paintings were when matched to an accurate 3d scan. By referring to Smiths paintings and comparing it to the matched 3D views it was also possible to identify clear architectural forms and details that are currently covered in a thick layer of overgrowth. It would be exciting to see the ivy removed from the towers to reveal the architectural features which are currently obscured but clearly visible in Smiths paintings.
To complete the study of the paintings an animated camera was then created that moved along a path pausing at each of the locations of Smiths paintings. The painting for each location was then composited over the 3D view to show how the castle looked from that location at the time the paintings were created in the early 19th century. This video gives a great sense of how dramatically the site has changed over the last 200 years and Smiths paintings are an invaluable historic record in this regard.
You can view the video here – https://www.youtube.com/watch?v=XCj335SpP80
With the model and Conservation Plan now complete in early 2022, the Conservation Group now plans to seek permission to make some physical interventions in an area of the castle where there is a risk of collapsing masonry. The Conservation Plan also suggested additional conservations steps to be taken in the next 2 to 3 years, starting with securing part of the structure of the South Tower Gate.
In parallel to this, we plan to ask Dara Burke to create an additional model of the castle as it appeared when first built in 1305. This will be based on a combination of the current 3D model plus the 1958 archaeological survey and information we have on comparable castles at Hearlech and Caernarfon Castles in Wales.
You can see further information on the castle and its history at https://www.northburghcastle.com/
The 3D models are available at https://sketchfab.com/CaisleanNorthburg
And the video showing the matching of Captain Smith’s paintings with the 3D model are available at https://www.youtube.com/watch?v=XCj335SpP80&t=6s