Site Prediction |
The recording of Bet Giorgis, a 12th century Rock-Hewn church in Ethiopia
Dept. of Geomatics
University of Cape Town, South Africa
Dept. of Geomatics
University of Melbourne, Australia
Armin Gruen & Thomas Buehrer
Institute of Geodesy & Photogrammetry
Around 1200AD, King Lalibela, one of the last kings of the Zagwe dynasty that ruled in Northern Ethiopia for 200 years, created a number of remarkable rock churches which remain to this day as functional places of worship. They also constitute symbols of paramount religious, cultural and architectural significance to the people of Ethiopia. Of the basically three types of these rock churches, one appears to stand 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) in the town of Lalibela. Given the national and international significance of this World Heritage site, a project was recently initiated to undertake a photogrammetric documentation of Bet Giorgis. This project is expected to culminate in the production of a fine-detail visually realistic digital model of the church and its immediate surroundings. The work has entailed initial image recording and photogrammetric triangulation to produce a numerical frame of reference (completed in October, 2000), and attention is now being paid to line and surface model generation, orthorectification, texture mapping and visualisation. This paper reports on the aims of the Bet Giorgis project and discusses progress made to date.
The Rock Churches of Lalibela
The town of Lalibela lies in the province of Wollo in northern Ethiopia, some 640 km from Addis Ababa. Other than on market day, Lalibela is not much more than a quiet mountain village, yet it is internationally renowned for its rock-hewn churches. The creation of these churches 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. The denomination of the still functioning churches is Ethiopian Orthodox, which is part of the Coptic Christian Church headquartered in Alexandria.
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. Figure 1 shows the 12 x 12 x 13m Bet Giorgis standing within its 25m square trench.
Figure 1: Bet Giorgis rock-hewn church, Lalibela.
Legend has it that Bet Giorgis, which stands apart from the two main groups of rock churches in Lalibela, 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 Figure 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 Figure 1, as well as from the cut-away sketch plan of Bet Giorgis (Figure 2).
Figure 2: Interior layout of Bet Giorgis.
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 with semi-palmette cross motifs. The interior of the church follows the cruciform floor plan (Figure 2) and on the roof there is a relief of three equilateral Greek crosses inside each other (Figure 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.
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 the first author, 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. All necessary imagery for the photogrammetric reconstruction was recorded during this time, along with the gathering of supplementary data necessary for a comprehensive mapping of the Bet Giorgis site.