Quality Magazine

Off Topic: St. Albans Cathedral

February 26, 2008
For a recent remodeling project, laser scanning was used to take a survey of the interior of the central nave and the north and south aisles of St. Albans Cathedral.

St. Albans Cathedral was mostly built in the 13th Century but has gone through many changes since. The nave was partially rebuilt in the 14th century after a collapse. Source: Faro


St. Albans Cathedral (Hertfordshire, UK) was mostly built in the 13th Century but has gone through many changes since. The nave was partially rebuilt in the 14th century after a collapse, and while at first glance it would appear uniform, the detail varies quite dramatically. For a recent remodeling project it was required that a survey be undertaken of the interior of the central nave and the north and south aisles, the general dimensions of which are 64 meters long, 22 meters wide and 20 meters high.

The client’s budget would not allow the creation of a detailed model of the nave, but APR Services Ltd. (London, United Kingdom), a provider of surveying services, was able to demonstrate that an acceptable result could be achieved by combining a point cloud with simple modelled elements, thus reducing the modeling costs.

The Survey

Scanning of the nave was completed in five days using the Faro LS880 Laser Scanner. This scanner was used for two main reasons:

      1. To get the speed and density of scan points necessary to pick out all the detail on the floor.
     
      2. To get good color photography overlaid on the scan data for the visualization.
Scanning was carried out from the floor of the nave for the detailed survey. A full 360-degree scan was carried out in each bay for each of the three aisles to ensure full coverage. To get the high detail obscured from the ground, scanning also was carried out from the narrow elevated walkways high above the central aisle and also from the organ loft. This was necessary to complete the information for the high-level flythrough. All scanning was rigorously controlled using targets, which were recorded by a surveying tool called a total station and tied into existing control from previous surveys. While onsite the data was visually checked to ensure that full coverage of all structures had been obtained and that the photography was acceptable. In total, more than 70 full 360-degree scans were carried out onsite.

Back in the office the data was carefully checked, registered and put onto the correct grid system. All the checking, registration and filtering of the data work was carried out using Faro Scene. The scans were then directly imported and processed in Pointools to clean up the data, removing people and cutting down unnecessary overlap between scans.



A Faro LS880 Laser Scanner was used to create a model of St. Albans Cathedral. Source: Faro

Problems Encountered

The main problem onsite was the general public, because the survey work had to continue around guided tours. It was attempted to carry out as much scanning on the floor of the cathedral each day before tours started and then switch to the upper areas, which would not be affected. It also was possible to restrict the problem by using only the data within approximately 10 meters of each scan and guiding people away when possible.

The other problem was that it was necessary for the cathedral to be cleared to permit both the visualization and also the detailed floor survey to be completed. Unfortunately, it was not physically possible to clear more than a third of the chairs in the nave at one time, as there was no place to store them. This made it essential that all data had to be recorded completely, as there was to be no revisit to the floor after the chairs were returned.

The Deliverables

Two-dimensional drawings were created of the floor and also the bottom 1.5 meter of each wall and column. These were produced using the cloud data directly in CAD software through a plug-in and by tracing from massive orthographic pointcloud images using Pointools that clearly showed the individual stones, joints and major cracks on the floor.

A 3-D visualization was required to show how the new plans would work. From the architect’s sketches, the alterations were made by editing out the scan data in some areas, and in the case of the font by moving it to a new location on a model of the proposed new base.

One of the existing chairs was modelled from the scan data and arrayed around the cathedral to match the proposed new layout. This, along with the new staging and access ramp, was agreed in consultation with the architects via preliminary stills of the main alterations. The final two animations were produced in Pointools from a reduced dataset, one as a walkthrough and one a high-level flythrough.

The final animations clearly showed the alterations and the client could immediately see some possible changes required to the proposed works. A revised movie may well be required at a later date after further consultation has been completed.

Laser scanning can be a valuable method for recording structures. With it highly detailed plans and elevations, as well as a 3-D visualization from one set of data, can be achieved. The data captured is an archive that can be revisited to create additional drawings or movies at any time without returning to site.
    Faro
    (800) 736-0234
    www.faro.com