Precision measurement technology and weathering monitoring at the Heidenportal of Wetzlar Cathedral
The three-dimensional surface measurements in the course of the weathering monitoring at the Heidenportal of the Wetzlar Cathedral allowed evaluations and comparisons to be made between individual recording epochs. Investigations have also been carried out in the field of the detection of deformations using various surface-based 3D measuring methods, which illustrate the problems and advantages of the measuring methods involved.
The aim is the long-term monitoring of a selected area at the “Heiden Portal” of Wetzlar Cathedral and the associated investigation of the weathering properties of the sandstone used there.
For weathering monitoring, the measurement epoch before the last measurement is used as a reference, orthogonal distances to the current repeat measurement (measurement epochs) are calculated in the same orientation (same coordinate system) and the geometric differences are shown in different forms (3D visualization visualized in color, 2.5D (grayscale). -) representation, profile representation). Evaluation options in a GIS system (QGIS) were considered and implemented, which has the advantage of combining all evaluation steps in one tool and expanding some evaluation options. On the one hand, all 3D models of the individual exhibition eras could be integrated in the form of 2.5D representations, which then served as the basis for different representations of the differences between the eras. On the other hand, profiles of the deviations and areas of certain deviation variables can be determined very easily from these difference representations.
The test area (4.0 m²) has a relief of between one and ten centimeters and is located at a height of 3 m to 4 m above the portal entrance. Due to the high location of the test area, the surveying work was carried out from a scaffolding provided by the client.
With an object resolution of 0.5 mm (2007 & 2012) or 0.25 mm (2017) and the use of special reference marks, a 4.0 m² area of the test area was recorded at three different epochs. With regard to the reference marks, it should be mentioned in particular that long-term stable bolts are sunk into the masonry on the surface, which are to be used for the epoch-by-epoch transformation of the measurement data to the first measurement.
The three measurement epochs result in 3D models describing the surface, which visually document the changes over time in comparison to each other (see Figures 1 to 2).
Blue areas indicate areas of erosion, while reddish areas describe swelling regions. The areas with a green background represent stable regions in which changes are below the measurement accuracy (0.05 - 0.10 mm) and cannot be detected.