The Onsite Terrestrial Scanning of Rosslyn Chapel

Rosslyn Chapel is a 15th century medieval stone chapel located in Roslin, Midlothian Scotland. It is internationally renowned for its mixture of ornate pagan and Christian sculpture as well as its links with the Knights Templar, Freemasons and more recently the Holy Grail. Since the 15th century the building has survived vandalism and neglect during the Reformation, been used as a horse stable by Cromwells troops during the English Civil War, unsympathetically renovated during the 1800s and nearly blown-up by the suffragette movement in 1914.

In 1954 the Ancient Monuments Branch of the Ministry of Works issued a structural report on the physical condition of Rosslyn Chapel. It indicated considerable concern regarding the physical condition of the interior, in particular the considerable amount of cyanobacteria growing on the inside of the choir roof and high levels of moisture throughout the building. In an attempt to address the problem, the interior surfaces of the Chapel were scrubbed with wire brushes and a solution of silica fluoride of magnesium initially was applied. An additional coating of a white cementitious paint containing shellac was also administered. Unfortunately the paint acted as a seal, preventing water from naturally moving through the stone.

In combination with a leaking roof, the stone became saturated with water and soluble pollutants, fostering further surface decay. The paint also covered the varied stone colours. In 1995 it was determined that the saturated stone and high levels of humidity had to be immediately addressed and in 1997 a temporary, free-standing steel roof was erected over the entire Chapel to allow it to dry out.

More recently the Chapel has had to endure the effects of a considerable increase in tourist traffic due to the release of Dan Browns novel and subsequent movie The Da Vinci Code.

In 2009 a combined team from the Glasgow School of Art and Historic Scotland terrestrially laser scanned and digitally photographed the site, exterior and interior as part of the on-going conservation efforts at the Chapel. The acquired information will not only provide an precise dimensional information for engineering drawings, it will also produce an accurate 3D record of the Chapels current physical condition.

Comprehensive Digital Survey
Prior to the start of the documentation project it was identified that 3D terrestrial laser scanning was the better technical solution over other forms of documentation techniques such as the measured survey, Total Station or photogrammetry to capture the complex building fabric and the detailed stone carved elements. The scanning technology is capable of capturing and dimensioning both simple or complex terrain or building surfaces.

In addition, the scan system records a reflectance value for the substrate which it strikes. This has significant value for the historic environment where such values of traditional materials such as varying stone types and mortars could be correlated. The information provides useful guidance in materials identification and mapping area of stone repair but imprecise as different angles of reflectance result in different return signal intensity.

The Rosslyn project utilised two a survey-grade Leica ScanStation 2 pulsed high-speed laser scanners and one Leica HDS 6100 phased-based scanner. The ScanStation 2 has a 360-degree horizontal field of view and a 270-degree vertical field of view. It has an accuracy of 6 mm at 50 meters with a range of 300 meters. Most of the laser scanning was taken within 20 meters of the building.

The ScanStation 2 is capable of a point-cloud scan density of 1.0 mm x 1.0 mm at but because of the considerable data overlap from the numerous scan stations, the typical set scan density was 8 mm x 8 mm. The tripod-based ScanStation 2 scanner is capable of acquiring data from the line-of-site of the scan head, approximately 1.4 m off the ground. Exterior scanning was done from the ground but also on the temporary roof walkway and on the Chapel vault.

The HDS 6100 is a phase-based, dual axis system with onboard batteries and an internal hard drive. It offers considerably higher speed scanning than the ScanStation 2 and higher resolution but with only a range of 50+ meters. The ScanStation was chosen for the longer range exterior scanning and a small number of large interior scans with the HDS 6100 used for short-distance infill and interior detail scanning.

Digital Survey – Building Exterior and Property
The ScanStation 2s were usually positioned within 15 to 20 meters of the Chapel. Approximately ten ScanStation 2 scans were performed with an resolution of 8mm in this area. An additional four scans were done outside of the property at the rear of the building to capture the falling terrain, the back wall of the lower chapel and the back of the temporary roof.

The laser documentation of the Chapel exterior had to both address and avoid the imposing temporary steel roof in place to dry out the masonry. With the completion of new roofing now complete, the temporary shed was removed in August 2010. The Chapel documentation project will provide historians and future researchers with a full dimensional understanding of temporary structure in-situ.

Although it was intended to have both the Chapel and temporary shed roof fully documented, it was important to avoid shadows or occlusions cast on to the Chapel by the roofs supporting structural legs. An additional thirty-eight infill scans were required under the shed roof and between the legs and the Chapel. The scan positions were close to the building, approximately 2 to 4 meters, so the short-range, high-speed HDS 6100 was used.

The temporary shed roof was purposely designed to cover but not contact the Chapel walls, it is structurally sound but would move with tourist traffic or high winds. Excessive movement would set off the scanners onboard tilt sensor and the scanning would stop. All of the scanning from this position was done with the HDS 6100 during the early morning with no tourist access.

The scanning of the Lady Chapel and Choir roof areas were achieved by positioning the scanner along the temporary steel roof gangway. The HDS 6100 was used in twelve locations at this height. At this time in the project, the original exterior vault roofing was still in place. Due in part to the visual imposition of the shed roof and the required client deliverables, the use of referenced (nodal ninja) digital photography onto the point cloud was not initially incorporated into this project. Textures were generated using a Nikon D3X digital camera (24.4 megapixel SLR) with a 20mm Nikkor lens.

Digital Survey – Building Interior
In conjunction with the exterior scanning, the interior of the Chapel including the Lower Chapel, the Lady Chapel, the Choir, the Victorian baptistry and organ loft were also done. Particular attention was made to capture the three pillars and the Choir ceiling. It should be noted that although the 1950s surface application of a cementitious slurry had a detrimental affect on the stone interior, the white paint colour and consistent coating throughout provided the nearly-perfect scanning surface.

An initial four scans were done of the interior using the ScanStation II at a 6mm resolution. The scanner was placed in the main Choir area and within the upper organ loft. As the ScanStation 2 has a greater range over the HDS 6100, the intention was to fully capture the detail of the vaulted ceiling as well as acting the primary scan data.

Although the HDS 6100 is capable of scanning to a distance of 40+ meters, the columns along the Choir and Lady Chapel as well as the individual vaulted ceilings along either side of the Choir required individual scanning to avoid occlusions. In total, fifty-six scans of the interior were performed with the HDS 6100 at the high resolution setting with a 6.3 x 6.3mm point spacing at 10 meter range. To enable a seamless connection between the registered interior and exterior point cloud data, two HDS 6100 scans were done at the thresholds of the North, South and Baptistry doorways.

For production purposes the data was reduced and then broken into four groups: interior, exterior-building, exterior-grounds, and the vaulted roof. Initially the sixty-eight interior scans were combined through feature registration within Leica Cyclone software. The registered interior scans were then combined with the exterior scans through feature registration, a connection made through the interior/exterior threshold scanning.

The registered data has since been used within AutoCad via the Leica Cloudworks plugin to develop a series of 2D CAD building sections. The interior and exterior data has also been converted into a very dense, 3D mesh, to generate a series of test 3D animations.

Next Steps
With the temporary roof and roof supports now removed, further scanning will be done to fill in any gaps in the data. It is intended that a number of interior sculptures will be rescanned using the Percepteron sub-centimetre hand scanning system. The Mason and Apprentice Pillar, Green Man, Lucifer, Knight on Horseback, Seven Virtues and Seven Sins will be scanned at a 0.1 to 0.3mm resolution. The data will then be used for public kiosk purposes.

The 3D meshed model will be eventually adapted as a photo-realistic 3D model – showing the evolution of the Chapel from when it was first constructed to present day.

Note: All images Centre for Digital Documentation and Visualisation.