Photogrammetric Reconstruction and 3d Visualisation of Bet Giorgis, a Rock-hewn Church in Ethiopia

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. 1 THE ROCK CHURCHES OF LALIBELA The town of Lalibela lies in the province of Wollo in northern Ethiopia, some 640 km from Addis Ababa. Lalibela is internationally renowned for its rock-hewn churches. Their creation is ascribed to King Lalibela, one of the last kings of the Zagwe dynasty. All 12 churches in the town are thought to have been constructed within a 100-year period around 1200 AD. Of the three basic types of rock-churches in Ethiopia, built-up cave churches, rock-hewn cave churches and rock-hewn monolithic churches, the last, which is of current interest, is unique to the Lalibela region. Arguably the most significant of Lalibela’s four strictly monolithic rock-hewn churches is Bet Giorgis (the Church of Saint George), which is regarded as being the most elegant and refined in its architecture and stonemasonary. Fig. 1 shows the 12 x 12 x 13m Bet Giorgis standing within its 25m square trench. Fig. 1: Bet Giorgis rock-hewn church, Lalibela. Fig. 2: Ornaments at upper windows and weathered corners of cornices in the foreground Legend has it that Bet Giorgis was built only after King Lalibela was reproached by Saint George (the national saint of Ethiopia) for not having built a house for him. King Lalibela’s response was to build a church, the construction of which, legend tells, was supervised by Saint George in person. As is apparent from Fig. 1, the ‘construction’ of a monolithic rock church was in fact an excavation, the procedure being to first cut free a block of stone in the volcanic tuff, after which stonemasons chiselled out the church, shaping both the exterior and interior. The extent of the detail involved in this process can be appreciated from Fig. 2, which shows one of the upper windows of the east facade. Bet Giorgis is positioned in its deep pit on a sloping rock terrace with the church being accessed via an entrance trench and tunnel. Around the walls of the courtyard in the pit there are caves and chambers which house both today’s priests and the graves of pious former pilgrims and monks. The cruciform church rises approximately 12m from its triple-stepped supporting platform, and it has three west-facing doorways (characteristic of Ethiopian churches), nine ‘blind’ lower level windows and 12 upper-row windows. The interior of the church follows the cruciform floor plan and on the roof there is a relief of three equilateral Greek crosses inside each other (Fig. 1). The roots of Lalibela rock churches are thought to lie in Axumite architecture and in the early Christian basilica, yet while they may reflect a blending of eastern Mediterranean Christianity and Axumite tradition, they are also a truly unique contribution to Ethiopian Christian heritage. In recognition of their significance they have been accorded UNESCO World Heritage status (Ruther et al., 2001). 2 PHOTOGRAMMETRIC DOCUMENTATION 2.1 Project Motivation As an aid in the long-term preservation of Bet Giorgis, and as a contribution towards making this remarkable heritage site more accessible in today’s ‘virtual world’, a project was undertaken to photogrammetrically document the church. The project, which was initiated by Heinz Ruther, one of the authors, with support of government agencies in Addis Ababa and encouragement from UN affiliated agencies, has as its ultimate aim the creation of fine-detail visually realistic digital models of both Bet Giorgis and other Lalibela rock churches. With direct support being given to the project by the Ethiopian Mapping Authority, work commenced in October 2000 with a field trip to Lalibela (Ruther et al., 2001). 2.2 Data Acquisition Three fundamental data sources were involved in the Bet Giorgis documentation. The first of these was existing 1:10,000 scale aerial photography covering the church surroundings. The second was ground survey data to facilitate control for the aerial imagery and to provide a tie between the local XYZ reference coordinate system used in the close-range photogrammetry and the geodetic network. The third was terrestrial imagery, to facilitate photogrammetric triangulation, geometry model generation and texture mapping for the visually realistic Bet Giorgis model. For the close-range modelling of the church and the pit, three comprehensive sets of imagery were recorded at camera station intervals of a few metres around the top and the bottom of the pit. Three digital cameras were employed, a Kodak DCS330, a DC210 and a SONY Cybershot, as well as a Leica R5 35mm semimetric analogue camera for possible comparative studies. For the SONY Cybershot calibration, parameters from a prior laboratory testfield calibration were employed. Both the DCS330 and the DC210 were calibrated in the field via self-