Delights and Challenges When Scanning in Africa

A hidden Ethiopian treasure history of Yemrehanna Kristos

Yemrehanna Kristos is one of Ethiopia’s lesser known and, maybe for that reason, better preserved churches compared to churches of Lalibela. Built by a Zagwe king-priest of the same name, around the 12th century, this church is an outstanding example of a late Axumite design. It was, and still is, an important centre of Orthodox Christianity where pilgrims converge, some coming from as far Egypt, and some chose Yemrehanna Kristos as their last resting place and their bare skulls and remaining bones can be found at the back of the cave containing the church and so-called palace. The church predates the Lalibela churches by about 100 years and they inspired the design of Lalibela rock churches.

Figure 1 Scanner if front of the the wood-stone layered facade of Yemrehanan Kristos

Build with alternating layers of wood and plaster, and adorned with square windows fitted with geometric symbols, like the swastikas and other symbols from the Late Axumite period, this church is located inside the 30 m by 30 m cave.

A second building, referred to as the “Palace”, and of a similar wood and plaster configuration, is also located inside the cave.

Although the church and palace are, thanks to their position inside the large cave, well protected, they now show signs of severe damage. Multiple structural cracks and deterioration of painted artwork is threatening this unique site.

Framework of project

Figure 2 Inside of Yemrehanna Kristos (from panorama). The “Holy of Holies” is hidden behind the curtain, two closed sacred rooms are on the left and right

The World Monuments Fund, New York, is in the process of assessing the physical state of the church, namely its structural stability and the state of the wall paintings adorning its pillars, ceiling and roof, with a view for possible restoration interventions. The Zamani Project, which has a long standing working relationship to the WMF, felt privileged to be trusted with the spatial documentation of the church complex. It is intended to use high resolution ortho-images and laser scan models to locate and measure cracks and other defects. To create the ortho-photos, a high resolution textured laser scan model of the church has to be created.

Data Capture & Challenges

Over a hundred laser scans were captured using a Z+F 5010c, over a period of 3 days.  The data captured had to be timed to avoid prayer periods during which no scanning could take place. While laser scanning the site was a routine operation from a technical perspective, there were some obstacles as a result of the holy nature of the site. The team was not allowed to work without the presence of the Church Committee, which carefully observed that no desecration occurred during the scanning. The severity of these restrictions became especially obvious when the scanning team asked for permission to access the ‘Holy of Holies”, the restricted section which contains the Tabot, the symbol of the Arc of the Covenant.  This area is only accessible to deacons and high priests and no exception was made for the scanning team.

During the scanning of the rock churches of Lalibela for the WMF in 2005 to 2008 (https://www.youtube.com/watch?v=NqnuO4n0Yao) the Zamani team trained a deacon to operate the Leica 3000 and Leica 6000 scanners and it was possible to scan some of the “Holy of Holies” of Lalibela rock churches by directing scanning operations ‘from behind the curtain” of the forbidden areas.

Figure 3 Screen print of 3D model of the facade of Yemrehanna Kristos prior to texturing

This was not possible in Yemrehanna Kristos because of the complex inside of the sacred area, which required scanner positioning by an experienced person. An attempt to have a Deacon take photos for structure-from-motion also failed. Thus the sacred area remains undocumented. It is however anticipated that these rooms can be scanned later when the Tabot is removed for the restoration period. Access to the fragile church’s roof was also not possible and it might be necessary to build a scaffold for later scanning. Also problematic was scanning the many pillars inside the church as the upwards facing parts of their capitals were not accessible thus creating data voids.

The very dark conditions at various spaces in and around the church would have requested extremely long exposure times for the inbuilt laser scanner camera and resulted in potentially lower quality image data. It was therefore decided to use photography captured with a professional DSLR camera to texture the laser scans.

Twenty-five HDR panoramas were acquired to further document the church and its surroundings.

The fascinating landscapes and the rural life which, on the surface, appeared to be little changed for hundreds of years as well as the welcoming population was enough to make the trip another memorable experience in a long string of memorable field campaigns in Africa and the Middle East. When the Zamani project started in 2004, nobody in the team would have expected that laser scanning can give access to such unique and unforgettable experiences.

by Heinz Ruther