March was a very busy month for the Digital Documentation team. Much of our time has been spent laser scanning the Palace block at Edinburgh Castle for a scan-to-BIM case study by Historic Environment Scotland. The idea is to create an BIM or AIM (asset information model) of the Palace block to be used for the subsequent management of various aspects of the building’s function.
Laser scanning is the primary method of survey for the building fabric (architectural elements), but it will be supplemented by additional surveys. The laser scanning survey is guided by the Asset Information Requirements document, which specifies areas of interest/priority and required levels of detail per category. The goal is to achieve 100% coverage of both exterior and interior spaces.
According to the AIR document, some of the least “glamorous” areas of the building, such as boiler rooms, electrical cupboards, switch rooms, and attic spaces have a higher priority for the model than some of the high-profile exhibition areas for example. This means that a lot of our time on site has been spent crouching inside tiny cupboards and boiler rooms surrounded by pipes and cables. These areas tend to be confined spaces full of equipment, difficult to navigate (or even to set up the scanner) and usually need a lot of scans to get good coverage and avoid shadows or data voids. Although we do not have to scan every last surface inside a boiler room, the AIR document makes it clear that at least the general dimensions and position of all equipment will need to be visible in the resulting point cloud.
In this project, multiple scanners have been used. The Leica P40 produces clean, crisp data with considerably less noise over great distances, but is very difficult (or impossible) to set up inside any of those tight cramped spaces. Therefore, all the cupboards, boiler rooms, switch rooms, etc were scanned with a Faro Focus 3D, which has a much shorter range and produces noisy data, but is significantly smaller, a lot lighter, and can easily fit inside any of those spaces. The same scanner was used in the attics, which were accessible through a folding ladder and generally very tight spaces, difficult to navigate when carrying any sort of equipment. Using the Faro proved very successful in those areas (on a photographic tripod or sitting directly on the floor).
Laser scanning an attic at Edinburgh Castle Palace with a Faro Focus 3D. Ghostbusters uniform to be worn at all times, as per the risk assessment report.
Registration of the data has been progressing along with the field work. More on that to follow very soon…
The 3D documentation programme for Edinburgh Castle included laser scanning the interior of the Great Hall and adjacent rooms and circulation areas. The data capture phase for this part of the Castle had already been finished, but the scans had not yet been registered. My job was to register 6 P20 and 36 Faro scans (mostly freescans) from the interior of the Great Hall and several adjacent rooms and circulations areas, using Leica Cyclone.
After importing all the scans to a Cyclone database (Faro scans exported as PTX files from Scene – although now Cyclone supports FLS files), it was decided that a certain level of cleaning the scans would improve the results of the cloud alignment algorithm. Only a few targets had been used, so the main registration method was cloud-to-cloud. Using clean clouds for registration was especially important for those scans taken inside the Great Hall, which was at the time open to the public and therefore full of people.
The process was time-consuming, but the results justified the effort: the improvement over the results of the cloud optimisation algorithm were significant (RMS error dropping from 0.014 to 0.007 or similar). The auto-add constraints options was used, but some of the constraints added automatically were disabled, as the overlap in some cases was very low (only a few thousand points) and they did not seem to improve the overall registration. Based on the registration diagnostics, the mean absolute error was 0.001 m, but based on visual inspection, measurement, and some control targets, the errors are in fact closer to 3-4mm, which is well within the expected tolerance of the survey.
One interesting area covered by the scanning was the “laird’s lug”, a device allowing the lord of the castle to spy on his guests or would-be conpsirators, which consists of an opening on the wall over the fireplace in the Great Hall which is linked through some convoluted tunnels to a room on the third floor, currently used as an office.
Creating a final registered point cloud for the entire Edinburgh Castle seems like a giant 3D puzzle. The next step for me will be to join the Great Hall interiors to a registered point cloud of the French Prison and underground vaults leading all the way up to the Crown Square, which has already been created in Cyclone).