Photogrammetric reconstruction and 3D visualisation of Bet Giorgis, a rock-hewn church in Ethiopia

mapping Around 1200 AD, a number of remarkable rock churches were constructed in Lalibela, a town in northern Ethiopia. One construction manner stands apart in its uniqueness. This is the rock-hewn monolithic church, which while imitating a built-up structure is actually cut in one piece from the rock and separated from it by an all-around trench. The best known of the monolithic churches is Bet Giorgis (St George’s Church), a UNESCO World Heritage site. The narrowness of the trench around the church makes the photogrammetric image recording demanding. Numerous photographs were necessary for a sufficient coverage. In a first phase of the 3D model generation only monoscopic image measurement was carried out. First results were not satisfactory due to problems in properly defining homologous points in the respective images. Such an ancient building suffers from erosion damages and hence lacks sharp corners and edges. Additional helpful construction features such as parallelism, perpendicularity and planarity can barely be exploited as the building is relatively irregular. For these reasons additional measurements are performed in stereo photogrammetric mode. Still, there is a high demand of visual interpretation and manual measurements are absolutely necessary for producing a comprehensive 3D model. For the 3D model rendering, the ETH-developed visualisation software Disp3D, which employs a view-dependent texture mapping procedure, has been used. The project, which is ongoing, will ultimately result in the production of a fine-detail visually realistic digital model of the church and its immediate surroundings