Edinburgh Castle: Great Hall

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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.

after cleaning
P20 scan inside the Great Hall before cleaning.
before cleaning
Same scan, after cleaning.

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.

Cyclone registration diagnostics.

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.

lairds lug.jpg
View of the point cloud showing the laird’s lug.

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).






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