Ona Vileikis on conducting condition assessments of historic buildings in Itchan Kala, Khiva, using digital technologies
Itchan Kala, ‘the inner fortress of Khiva, is located to the South of the Amu Darya River (known as the Oxus in ancient times) in the Khorezm region of Uzbekistan’. It was the ‘last resting-place’ of the trading Silk Roads caravans coming from the east before crossing the Karakum desert to the south. The site has been inscribed on the World Heritage List since 1990, not only because of its monumental heritage, but due its urban medieval composition and the ‘development of Islamic architecture between the 14th to the 19th century’, including the domestic architecture (https://whc.unesco.org/en/list/543/).
- Coordinates: 41.378333, 60.363889
- Site type: walled inner town
On 26 and 27 February 2020, UCL, together with the International Institute of Central Asian Studies (IICAS) team visited the historic centre of the old Khiva oasis to work with a local team in Uzbekistan. The work was organised in collaboration with the UNESCO Office in Tashkent with the support of the Cultural Heritage Department of the Ministry of Culture of Uzbekistan.
With more than 100 historic buildings on the national list, the site chosen was the mahalla (neighbourhood) Bogbonli Mosque. It was built in 1809 with an open courtyard, a two-column ayvan (open gallery with a flat roof supported by columns) and, unlike most mosques in Khiva, a domed hall. The Mosque had only minor conservation works done in the 1950s and most of its structure is still authentic.
Conservation of the building will take place in the coming years thanks to an EU-UNESCO Silk Roads Heritage Corridors project, but the Bogbonli Mosque has never been accurately recorded. The first documentation of the building dates to 1946, conducted by R. Abdurasulov, and it wasn’t until 1957 that the first architectural measurements were carried out by I. I. Notkin. Supporting informed decisions for the conservation, the aim of the initiative was to improve the documentation and condition assessment of the cultural site by using digital technologies. Documentation is the first step toward a proper management and monitoring strategy. This process could also serve as an example for future work on condition assessment in the region and will remain as a record for future research.
Photogrammetry and 3D Modelling of Bogbonli Mosque
3D in Google Earth shows some historic buildings in Itchan Kala, but the images are low quality – intended only for visualisation and no data for the Bogbonli Mosque is available. Thus, there is a need to capture high quality data to acquire high levels of detail (Figure 1). Aerial and terrestrial photogrammetry were the methods selected to capture the Mosque and to produce high quality orthophotos that will form the basis from which we can produce architectural plans and conduct a damage assessment. In addition, a 360 degree panorama was taken in the interior of the Mosque (Figure 4).
Tools and Equipment
The equipment used in this work was provided by the CAAL project, IICAS and the Cultural Heritage Department of the Ministry of Culture of Uzbekistan. A Canon EOS 6D Mark II with a lens 24-105 and a WEIFENG WF-531T tripod was used to take the terrestrial photogrammetry. For the aerial photogrammetry a UAV from the Ministry was used. It was a Quadrocopter PHANTOM 4 Pro with Sensor 1 CMOS Effective pixels: 20M and a lens FOV 84° 8.8 mm/24 mm (35 mm format equivalent) f/2.8 – f/11 auto focus at 1 m – ∞. A Lenovo Thinkpad X1 Carbon 7 with 16GB RAM was used to process the data during the workshop in a medium quality, but too slow for a high quality or production of one 3D model. Later some orthophotos were reprocessed using a workstation with higher configuration RAM 64, CPU i9-9900K 3.60 GHz and GPU GeForce GTX 1060 6GB. Agisoft Metashape Professional software was used for modelling and AutoCAD 2018 to draw the plans and map the damage.
How? 3D Acquisition Workflow
To start with site reconnaissance, we used the drone to capture the site from above. An orthophoto was created for later comparison. Figure 1 shows the advantage of using aerial photogrammetry, to map the site. In this case, the Google Earth satellite image (left) taken in 2020 has limited resolution in contrast to the orthophoto produced from the drone images captured at 35m high (right). Before the terrestrial survey, we conducted a site visual inspection to understand the damage and identify the critical issues (Figure 2). Then, we started the terrestrial photogrammetry to create orthophotos of the exterior and interior facades.
Figure 3. Training was given to the local team. For example, how we used the colour checker which helps to get the real colour of the object and to adjust for dark or bright photos after colour correction.
Figure 4. 360 degree panoramic images were also taken in the interior of the mosque to get a better visualisation of the state and damage.
Figure 5. The drone was also used to map the historic building at a close range. Training was given to the local team, and everyone had a chance to fly the drone.
Figure 6. Training on using the different software packages for processing images.
3D Processing Workflow
After the data was recorded, the information was processed with the team using Agisoft Metashape. The objective was to use the photogrammetry and create an orthophoto from the facades (Figure 7). We followed this processing workflow:
- First, we did image registration and created a point cloud.
- Then, we created a mesh and as result a textured mesh. The advantage was that the 3D model will enable us to see places you normally cannot reach – such as roofs – and see details you cannot otherwise get to.
- After the orthophoto was created in Metashape, it was brought into AutoCAD. The image served as a base for drawing the façade and creating an accurate analysis of the damage and decay as they were on this day. These images will then be used as base reference points for future monitoring.
Figure 7. image registration; textured mesh; roof comparison; orthophoto imported into AutoCAD and damage mapping.
Challenges Encountered During the Work
Flying a drone is not always easy. Although Uzbekistan seems to have a relatively dry atmosphere, of the three days of fieldwork, the first day it rained, and we could not fly. The following day witnessed the visit of the Prime Minister of Kazakhstan and his delegation to Itchan Kala, so no flights were allowed. Too much light contrast is one condition for good photogrammetry, so we were left with two mornings and one late afternoon for the recording. One of these had to be cancelled because we did not have the necessary permit to fly that day. Finally, we were able to capture what we needed.
We only had one computer with Agisoft Metashape for modelling, so we built the mesh using medium quality. Back at the office we repeated the meshes in high quality, which are highly recommended for a condition assessment (Figure 7).
Now the big challenge is to continue the work at other cultural sites and to transfer this experience towards the preservation of cultural heritage for future generations.
by Ona Vileikis