Rescue-Scanning in Zanzibar

Figure 1: Screenshot of the 3D model of the front façade of the Palace Museum.

The tropical island of Zanzibar on Africa’s East Coast, the embodiment of exotic dreams and a reminder of the largely lost world of Indian Ocean trade is still trying to hold onto its history and its Swahili culture. Stone Town, the historical part of Zanzibar’s main city, with its colourful shops and crowded markets, its unique architecture, its narrow lanes and its elaborately carved wooden doors was inscribed in the UNESCO World Heritage List in 2000. While UNESCO World Heritage status provides a measure of protection for an inscribed site, it cannot prevent natural or even man-made slow or fast deterioration or eventual destruction. Many of Stone Town’s buildings are examples of such deterioration and in spite of substantial efforts by international institutions, such as the Aga Khan Trust for Culture, many historical buildings show cracks and other signs of threatening structural failure. Prominent among these are buildings along the waterfront close to Stone Town’s harbour.

The Zamani research group of the University of Cape Town (www.zamaniproject.org), which had documented two 19th Century Persian baths and the more recent Beit el Amani in Stone Town, was tasked by the World Monuments Fund (New York) with the documentation of Beit el-Sahel, the structurally endangered 19th Century Sultans Palace, today used and known as the Palace Museum.

The three-storey Palace is surrounded by other historical buildings such as the iconic House of Wonders, a Persian Bath and a Mosque. Much of the Sultan’s furniture, paintings, clothing and other artefacts are now on public display in the museum to present the life and times of the Zanzibar Royal family.

Of special interest to museum visitors seem to be the rooms and paintings which relate to the, historically rather insignificant, but sad fate of a Zanzibar Princess who married a European merchant.  In 2014 the Palace Museum was, due to its poor condition, added to the World Monuments Fund’s Watch List. The seriousness of the threat to the palace is obvious from the many cracks of its outer walls and from the recently installed large supporting beams on the outside of the building, which are – somewhat desperately- trying to stabilise the structure.

The Scanning Project

The task of the Zamani team was to spatially and photographically document the building with two principal deliverables, firstly, an accurate 3D model of the entire inside and outside of the building and secondly the creation of ortho images of all walls of the more than 50 rooms as well as all outside walls.

In the interest of a holistic documentation, photographic panoramas of each room and a panorama tour through the more interesting parts of the building were also created. Secondary products of the scanning were ground and roof plans, sections and elevations. The 3D model and the ortho-images were used to detect and monitor any structural deformations and structural problems and to measure and analyze cracks. The ortho-images also assisted the heritage architect during on site recording of conditions.  The final outcome of the project will be a condition survey and a conservation.

To achieve these objectives, the site was documented with laser-scanning technology, 360 degree panoramas and photogrammetric images. A  Z+F 5010C and a Z+F 5006 were used to capture 345 scans of the building and a few thousand photos were taken for structure-from-motion applications.

The final results were a coloured point cloud of the Palace Museum, textured 3D computer models at a resolution (surface point spacing) of 2cm, a panorama tour for visualisation, ground plans of each floor, and orthogonal images of all walls of each room of the museum.

Figure 2: Textured orthophoto of the South Wall of the Palace Museum

It is the documentation policy of the Zamani project for all its projects to go beyond the coloured point cloud product and to create meshed models for all sites and fully texture surfaces wherever possible or relevant.  The textured surfaces are then used to create ortho images.

Data Capture and Challenges

The Palace consists of three floors with approximately 15 – 20 rooms per floor. Laser scanning the rooms of the Museum proved to be a challenge as the display rooms were filled with valuable and often fragile artifacts such as Indonesian and ceremonial furniture, large banqueting tables, huge paintings and Chinese ceramics. In order to record each room thoroughly and capture data for each of the walls of the rooms all the furniture and the paintings had to be moved to the centre of the rooms.  Museum staff formed a “moving” team which co-ordinated with the scanning. For each room in sequence, all items were moved to the room centre, scanning was completed, often with repeated rearrangement of the moved furniture, and the room was put in order again to minimise the impact on impatient museum visitors.

In addition to the display rooms there were many others used for storage or office space. The storage rooms , probably not opened for years,  contained heavy furniture and were often very densely packed Clearing out these rooms proved to be a major undertaking and slowed the scanning team down considerably again having to synchronise with the moving teams and often give a helping hand .

Another challenge was scanning the outside walls of the museum. The Southern and Eastern walls were bordered by small alleyways and it was not possible to scan from a distance to capture the entire walls; therefore scanning had to be done from neighbouring buildings to capture this data. One of the buildings was a school and the laser scanner had to be placed to scan out of the classroom window while classes were in session. The laser scanner team was roped in to present impromptu English lessons.

Current tourist reports on internet travel pages claim that the back half of the building has collapsed, however this is likely to be an exaggeration and could not be confirmed by the time this article was submitted for publication.

Figure 3: Screenshot of the 3D model of the Palace Museum with the top of the first floor removed