Historic Churches 2021

BCD SPECIAL REPORT ON HISTORIC CHURCHES 28 TH ANNUAL EDITION 31 YGT and the York Minster stone yard in investigating new technologies of mutual interest and benefit. The glaziers and masons have frequently worked in close collaboration on Minster projects, ensuring that maximum advantage is taken of high level access whenever costly scaffold structures are in place. The research work conducted by Laura pointed to a number of companies which were able to demonstrate different approaches to how the measuring requirements of historic glass and stone could be met in the digital realm. LASER SCANNING Our first investigations looked into the capabilities of laser scanning and photogrammetric 3D scanning. We were able to arrange several on-site demonstrations of a range of different scanners to look at the possibilities in practice. We learned how hand-held portable units, based on photogrammetric scanning technology, could produce very interesting and accurately-rendered 3D models, with the possibility of gaining dimensions from the resulting files. A demonstration of a laser tracking system showed us the extraordinary precision that could be achieved in the gathering of dimension data from 3D files. Despite being hugely impressed by what the 3D scanning technologies could achieve, we realised that our own needs were far simpler than the complex capabilities of these systems. We were looking for a user-friendly system that could accurately measure the spaces and capture the shapes of window traceries, and create a simple and very accurate line drawing in two dimensions. We actually had little need to create a huge point cloud digital file, or an accurately rendered 3D model, although these impressive tools are excellent for documenting historic material for use in interpretation. Our next demonstration was of a quite different piece of technology, a coordinate measuring machine called a Proliner. CMM TECHNOLOGY A coordinate measuring machine (CMM) is a device that plots the geometry of an object from a series of three-dimensional coordinates. These are measured using a probe connected to the machine (directly or indirectly) while its tip touches one of the series of points to be measured. In some cases the probe operates on rails in two or three axes, with distance to the point measured in each axis. In others the probe is held on an articulated arm, with accurate angular measurements calculating the coordinates of each point in 3D. The Proliner is manufactured by the Dutch company Prodim. Like other forms of CMM it works by taking individual measurements in 3D space to create an accurate 1:1 drawing of any shape, but in this case the steel probe or ‘pointer’ is on the end of a retractable steel cable that is calibrated to measure objects and structures up to five metres away, in all directions. The cable is calibrated to within a 0.5mm margin of accuracy. The measuring unit can be used with a dedicated tripod in order to fine-tune the starting position of any new project. By keeping the unit in a fixed position, the measuring wire can then be extended and the pointer set to the object – in this case the stone window surround. The dimension is then taken by pressing a button on a dedicated remote control device or in an app on a mobile phone, and the operator can take as many measurements as necessary to make each template drawing. As the device measures in 3D space, not only can the stone surround be measured but so too can other features on different planes, such as the exterior stone profile (if it differs from the interior) and any metal bars connected to the stonework. As the device contains a self-calibrating inclinometer, each template drawing can include a spirit level position to ensure that panels and supporting bars are set accordingly. A portable coordinate measuring machine, the Prodim Proliner, being used by Zoe Harrigan on window CHn5: above, the angle of the cable and distance to the steel pointer are accurately measured by the device to plot the position of each point in 3D; and top, the form takes shape on the display screen.