False color mapping to visualize geometric changes between different points in time
After the metrological investigations at the monastery ruins Limburg a.d.H. near Bad Dürkheim in 2007, from 2008 further objects under investigation were geometrically and radiometrically documented, including a Roman sarcophagus in the Benedictine Abbey of St. Matthias in Trier.
After a pilot project to investigate the usability of current 3D measurement technology for the purpose of monitoring historical stone surfaces was successfully completed (use of modern measurement technology for the precise dimensional monitoring of rock surfaces), further outstanding objects could be measured within the framework of follow-up financing and documented over several exhibition periods. This includes a sarcophagus in Trier, where a larger area was to be expected, since the location is problematic due to stronger, moist air currents and leads to greater surface erosion on the surface. In addition to modern photogrammetric equipment, state-of-the-art scanning methods for the close range with object resolutions and accuracies of less than 0.1 mm were used.
In Trier, temporal geometric changes and weathering of the paint application on the sandstone were examined on a special test area (on the sarcophagus) with the help of the stripe projection technique (GOM Atos III). With an object resolution of 0.25 mm and the use of special reference marks, a 0.5 m² area of the sarcophagus surface was recorded from four different epochs. With regard to the reference marks in particular, it should be mentioned that the fastening method used cannot guarantee stability over this long period of time. Furthermore, most of these reference marks have become detached from the object, so that the referencing took place via stable geometric areas at the edge. For further measurements on this object, considerations regarding this circumstance must be made. The four epochs of the fair result in four 3D models describing surfaces that visually document the changes over time in comparison to one another. The time interval shown for the recordings is one year. The coloring of the 3D deviations in Fig. 3 can be understood as follows:
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. This is illustrated in Figures 2 and 4 below.
In order to limit the workflow to an open source tool, the 3D data was evaluated on the basis of QGis. 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.