CI PA 2003 XIX th International Symposium, 30 September - 04 October, 2003, Antalya, Turkey
and difficult to record and represent in diagrams and
models.
- the walls, built of coral rag and covered with lime mortar,
display highly eroded, undulating surfaces that pose a major
challenge to automated and visual edge detection and line
drawing.
- the technical and economic limitations of an African
environment render some of the methods and approaches
applied elsewhere in documentation, impractical or entirely
impossible.
Figure 1. Views of the Western wall of the Gereza
1.2 A brief historical account of Kilwa
Kilwa Kisiwani was first settled around 800 AD and by 1320 it
had grown into one of the most prominent trading centres of the
East African Coast. Its splendid buildings include mosques,
palaces and ordinary residential buildings. Kilwa supplied
timber, ivory & other African products to Southern Arabia, the
Persian Gulf and locations as far as India and China. Kilwa’s
trade to the North reached Egypt and other Mediterranean
countries. For quite some time, Kilwa also controlled the coast
to the South as far as Sofala beyond the Zambezi mouth, a
harbour from where gold mined in Zimbabwe was exported.
Around the 15th Century, Kilwa faced growing competition
from Mombassa and Malindi to its North and at the beginning
of the 16th Century it was invaded and subdued by the
Portuguese who built the Kilwa fortress in 1505 to signify their
dominance over the island. When leaving the island after a short
occupation the Portuguese reportedly razed most of the original
fortress and in its “present form the fortress dates to the period
of late Swahili revival and Omani expansion around 1800 AD”.
First structures of the Great Mosque were built in the 11 th
century and expanded significantly in subsequent centuries.
Other than for some reconstruction in the 15 th century, the
present mosque has retained its original form (Sutton 1998).
After some economic revival in the late 18th Century, Kilwa
faced a new period of occupation by Omani rulers based in
Zanzibar at the beginning of the 19th Century. In 1840 the last
Sultan of Kilwa was deported to Oman, thus ending the
dominance of Kilwa. (Chittick 1974).
2. DATA ACQUISITION
Documentation data was acquired in three field campaigns,
during which digital imagery, supported by control point
surveys, was captured. A Cyrax 2500 laser scanner was
available for the last field visit and experimentally employed as
an additional documentation tool.
2.1 Photography
Digital imagery was captured using a Kodak DCS 330 camera
with 14mm and 28mm lenses. The two camera-lens
combinations were pre- and post-calibrated with the lens
focusing ring firmly taped down. The repeatability of the
camera-lens configurations after removal and re-attachment of
the lenses were tested prior to the fieldwork. In a laboratory
experiment both lenses were removed ten times each and the
camera-lens system was recalibrated after each lens change.
The interior orientation parameters proved consistent within the
accuracy requirements of the planned documentation. The
following requirements were considered in the design of the
geometry of the camera stations and the camera orientations:
Multi-image photography had to be acquired to provide
optimal imagery for surface generation by multi-image-
geometrically-constrained matching, the mathematical
model employed in the in-house software employed for the
project.
Cameras had to be oriented with the image plane parallel to
the principal surfaces (walls) to provide optimal data for the
generation of ortho- images
Oblique and rotated photography were required to
strengthen the geometry of the photogrammetric
triangulation
Multiple diagonal photography was needed at both convex
and concave building comers to provide links between
surfaces in rectangular orientation.
Photography from different elevations had to be captured to
obtain texture and detail for the up- and down facing
surfaces of ledges, windows and doors.
Photographs were taken, with the camera handheld, from
ground positions, elevated positions on the fortress structures
and from a six-meter tall ladder. As the site is located between
the equator and the Tropic of Capricorn, it was possible to
photograph South- and North-facing walls in full sunlight by
visiting the site in summer and winter. A total of approximately
1000 photographs of the two structures was acquired, of which
some 500 were finally used for the photogrammetric
triangulation and the creation of the model. A number of
marked control points, distributed over the structure, was
surveyed using conventional survey methods.
Figure 2. Entrance area of the Great Mosque
