Motivation

The Mainz City Hall, a building completed in 1973 according to plans by Arne Jacobsen, has a curtain wall made of Norwegian Porsgrunner marble that is up to 30 m high. In the course of modernization work on the town hall, this façade is to be renewed, since the effects of temperature and humidity mean that many of the approx. 55x75 cm large and only approx. 3 cm thick marble slabs are becoming warped with age. These deflections stress the fastening anchors in such a way that, under unfavorable circumstances, they lead to the panels breaking off from the facade. Since the building is a listed building, special rules apply to modernization. For the modernization work, a detailed inventory of the facade should first be carried out, from which a facade plan can be derived.

Activities

In a joint project with AI MAINZ, i3mainz was involved in drawing up the facade plans. The i3mainz provides the basis for these plans through surveying work. After thorough on-site inspections, a measurement concept was developed that takes into account the special features of the town hall. Due to its partially angled geometry, the building offers a lot of shadows and intersections that make work difficult. Some of the marble slabs are behind the sun protection grilles and are therefore not always fully visible. Neighboring buildings and streets also make it difficult to choose a location.

The recording was carried out using UAV-supported photogrammetry, which was partially supplemented by terrestrial laser scanning. Both methods require reference points on the object, which have been measured using classic tachymetry. Because of the height of the town hall, it was not possible to attach target marks in some areas, so a modern video tachymeter was used to record the spacers on the facade without a reflector. Thus, up to 30 reference points could be determined in a local 3D coordinate system per wall side.

The drone of the i3mainz, which serves as a sensor platform for a high-resolution digital camera, was used for the photogrammetric recording. All facades were photographed from different angles with the camera. This image group was evaluated with SfM algorithms (Structure from Motion) and thus allows a digital reconstruction of the town hall in the form of 3D point clouds.

Results

It was agreed with the city of Mainz that digital orthophotos of the building sides would be created. To do this, the point clouds must first be referenced and cleaned up. A surface model is then created by triangular meshing. The original images are then projected onto this and a digital orthophoto is generated. Based on these orthophotos, AI Mainz mapped the joints and stones.