Long Range Scanning of St Michael's Cathedral Ruins


Coventry St Michaels Cathedral was built in the late 14th century and was one of the most beautiful and largest parish churches of its time in England, until it was destroyed during the Second World War.

3D Scanners were asked to produce a 360 degree record of the ruins.

3D scanning allows us to save, maintain and recover heritage objects. Laser scanners are non-contact measurement devices that capture millions of discrete points of real world objects or complex environments in only a few minutes. The resulting images are an assembly of millions of 3D measurement points, known as point clouds, which can be aligned to create a polygon mesh.

A Focus long range scanner was used to capture St Michaels Cathedral as this type of scanner provides a range from approximately 0.6m up to 130m and has a freedom of view of 360 degrees, producing an efficient means of capturing data in such a complex environment.

The screen shots below show the point clouds with the points coloured to the corresponding colours of the scans on the aerial photo.

The photo above shows the scan areas that were captured. Fifteen scans were taken in total, with the first being a full 360 degree scan covering the whole area.

The engineers placed reference target datums (spheres and markers) all over the Cathedral. After all preparations were completed, the long range scanner was placed in the middle of the ruins and a 360 degree high-quality reference 3D image was captured. This first 3D scan contained most of the data and information about placement of the datums. These markers were used to align together multiple scans from their own coordinate system into one “Primary” aligned coordinate system. After completing the reference 3D scan, we moved the long range scanner along the ruins to fourteen additional positions, allowing us to align together each scan when processing the project. The goal of the additional scans was to collect as much data in this particular range with higher intensity of points per scan.

Data

  • Total number of points taken from scanning object = 250 million.
  • Scanning time = 3.5 hours.
  • Length of St Michaels Cathedral = 293 feet

Back at the 3D Scanners office, PolyWorks 2014 software was used to process the points in IMAlign.

The following images are screenshots of the points captured scanning the area. Each group of scans has been colour coded to correspond to the photograph above, highlighting the area captured by each scan.

The screen shots show the point clouds, with the points coloured to the corresponding colours of the scans on the aerial photo.

These screen shots show the north & south ends of the cathedral ruins in point clouds. The points are coloured to the corresponding colours of the scans on the aerial photo.

The brown coloured screenshot show texture information which has been added to the polygon mesh. This information is captured whilst scanning the cathedral.

The screen shots show the point cloud data and also the polygon mesh with IMTexture being used. The polygon mesh has not been cleaned at this stage.

The screenshot above shows the window area after it has been cleaned up in PolyWorks software.

If higher detail is required, a laser arm scanner would be used to capture more detail on specific areas. The existing data on the detailed areas captured with the long range scanner would then be used along side the laser arm scanner for alignment purposes.

The polygon model is an accurate visual representation of St Michaels cathedral which will be used for years.