Solid Modeling for Heritage Documentation
Eric Andrea MNGUMI and Heinz RUTHER, South Africa

Key words: key words: Kilwa, heritage documentation, solid modeling, CAD, 3D models

SUMMARY
As opposed to solid models, line and surface models have been widely used in heritage
documentation because they are easier to build and less bulky. This paper explores the solid
modeling approach as a tool for heritage documentation. As an application example, theprocess of modeling the Gereza, a fortress at Kilwa Kisiwani in Tanzania is discussed. The
authors suggest that a solid (volumetric model) is more suitable than conventional models forthe representation of an open building with an exposed interior structure. Comparative
analysis of line, surface and solid models is given. A combination of laser scan andphotogrammetry data was used as input for the creation of a 3D solid model on CAD.

Workshop – Archaeological Surveys
WSA2 Modelling and Visualization
Eric Andrea Mngumi and Heinz Ruther
WSA2.3 Solid Modeling for Heritage Documentation
FIG Working Week 2004
Athens, Greece, May 22-27, 2004

1/10

Solid Modeling for Heritage Documentation
Eric Andrea MNGUMI and Heinz RUTHER, South Africa

1. INTRODUCTION
1.1 Object Representation for Documentation of Historical Sites
Today, three-dimensional computer models play an important role in engineering, design,
education, entertainment, tourism and many other areas. There are several reasons why
computer models have become the preferred mode of representation. As compared tophysical models, computer models are cheaper to build and can be easily stored and retrieved
when needed. Further, compared to analogue ones, three-dimensional digital models areeasier to modify. In applications to historical sites, computer models provide a virtual model
shop where structures can be studied and experimental reconstruction of partly ruined or
extinct monuments can be carried out.
Three-dimensional modeling for documentation of historical sites is somewhat different
from engineering design in that it follows the reverse engineering approach as opposed toforward engineering. Reverse engineering seeks to produce a credible representation of analready existing object. This involves the acquisition of precise object measurement and thetransfer of measurement to a modeling system. Forward engineering produces a model of anon-existent object as a design process prior to manufacture. In this case, the designer
supplies measurements of the object based on his or her objectives and the design
requirements. In heritage documentation, this is only applied in virtual reconstruction. Apartfrom the reverse-versus-forward-engineering distinction, three-dimensional modeling for
heritage documentation follows the same general pattern manifested in engineering design
representation.
There are several kinds of three-dimensional computer models employed in engineering. The
type of model one chooses to use depends on the purpose of the model. In general, threedimensional computer models are classified as wire-frames, surface models and solid models.
The wire-frame is the first topological level above the point cloud level, making distinct theedges and corners of the object. The surface model is the second level, providing information
not only about edges but also the surface area spanning the edges. A solid model – the thirdlevel up provides information about the boundaries of the object as well as the enclosed
volume thus creating a better understanding of the structure. Wire-frames and surface models
have been widely used in heritage documentation because they are easier to build and lessbulky. In this paper the authors explore the alternative of using solid modeling given theadded advantages this representation has over surface models.

Workshop – Archaeological Surveys
WSA2 Modelling and Visualization
Eric Andrea Mngumi and Heinz Ruther
WSA2.3 Solid Modeling for Heritage Documentation
FIG Working Week 2004
Athens, Greece, May 22-27, 2004

2/10

1.2 The Solid Modeling Advantage
As compared to wire-frames and surface models, solid models are very useful because they
do not only describe the boundary of the object but its interior as well. The data structure of asolid model incorporates topology (connectivity information) documenting how vertices,
edges and surfaces of the model join together. In addition material properties can be assigned
to solid models thus allowing other forms of engineering analysis to be carried out. It should
be noted that, although thus far documentation of historical sites has been viewed in terms ofproviding a basis for visualization, these added advantages will extend the value of the
documentation and are essential to researchers studying historical buildings of from anengineering perspective.

Figure 1: Broken, free standing walls inside the Gereza

In addition to the potentials of solid modeling in general, this paper presents solid modelingas the solution to the challenge of modeling a particular kind of historical buildings. Kilwa’s
monuments were built of coral rag with lime mortar. Over the past centuries most of thebuildings have been reduced to open roofless structures with freestanding walls. The walls
have been badly eroded leaving rough edges and honeycombed surfaces that pose a challenge
to standard CAD modeling. Using an approach based on constructive solid modeling (CSG),
a technique is developed to create a solid models of highly irregular coralline walls based on
object’s surface data. Solid modeling of such complex structures affords researchers a study
model that can be sliced horizontally or vertically to reveal interesting spatial relationships.
2. BACKGROUND: A BRIEF HISTORY OF KILWA KISIWANI
Kilwa Kisiwani was first settled around 800 AD, and by 1320 it had grown into one of themost prominent trading centers of the East African Coast. Its splendid buildings include
Workshop – Archaeological Surveys
WSA2 Modelling and Visualization
Eric Andrea Mngumi and Heinz Ruther
WSA2.3 Solid Modeling for Heritage Documentation
FIG Working Week 2004
Athens, Greece, May 22-27, 2004

3/10

mosques, palaces and ordinary residential buildings. Kilwa supplied timber, ivory and 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 andother Mediterranean countries. For quite some time, Kilwa also controlled the coast to theSouth as far as Sofala beyond the Zambezi mouth; a harbor 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 – the present fortress is largely an extension of
the original structure by Omani rulers who revived Kilwa and ruled there during the 1800s.
First structures of the Great Mosque were built in the 11th century and expanded significantly
in subsequent centuries. Other than for some reconstruction in the 15th 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).
3. DATA ACQUISITION AND PROCESSING
A combined laser and photogrammetry approach, supported by control points determined bytheodolite observations was chosen for the modeling of the Gereza. This hybrid approach has
been found effective and less time consuming in a number of studies. (Borg, 2002, Lapointe,2001). Laserscanning provides detailed surface information, while photogrammetry allows
the extraction of edges and other features.
3.1 Processing Photogrammetry Data
In the early stages of the photogrammetric processing of image data for modeling of theGereza, a data structure was developed to streamline the acquisition and presentation ofphotogrammetrically acquired points. The data structure was instrumental inphotogrammetric digitization of feature points with Australis software (Fraser and
Edmundson, 2000) and subsequently transferring these points to a CAD system for line
drawing. The following workflow was adopted:−

An approximate CAD model was created using tape measurements of the Gereza’s
ground plan.

−

The working model was used to label (code) each point according to its location within
the structure, the feature type and the location on the feature, according to a pre-designed
data structure; for instance, EGl-1 (Figure 6) refers the first point on the left side of thegate in the East wall, or EWPW1_1 refers to vertex 1 of window 1 on Eastern wall,parapet 1. Further points are labeled in a sequence that follows the outline of the window
in clockwise direction, and named EWPW1_2 and so forth. These labels, which identify

Workshop – Archaeological Surveys
WSA2 Modelling and Visualization
Eric Andrea Mngumi and Heinz Ruther
WSA2.3 Solid Modeling for Heritage Documentation
FIG Working Week 2004
Athens, Greece, May 22-27, 2004

4/10

individual points uniquely, are allocated to points during the 3D digitising with theAustralis software. In previous digitising processes, points had only allocated sequential
numbers and were manually linked in CAD. In this approach, the identification ofindividual points in a large densely populated point cloud proved extremely difficult andsometimes impossible. In the newly adopted approach, the file of labeled coordinates,
generated in Australis, was translated into a corresponding Lisp feature file, by means ofan in-house program (Aus2lisp) for the automatic generation of AutoCAD featuredescriptions from an ASCII xyz-file. Once imported into the CAD system, the points of afeature are automatically joined with lines according to the number sequence of theirlabels. The new coding method combined with automated transfer into CAD, proved asignificant improvement on the previous approach and the features of the Gereza’s East
Wall were successfully plotted on AutoCAD with this method.
3.2 Processing Laserscan Data
Before the creation of a model, laserscan point cloudsrequire pre-processing , such as theregistration of adjoining point clouds into a common reference system, the thinning out of
unnecessary points and the elimination of irrelevant for- and back ground points. With
increasing availability of laser scanners it has become important to develop techniques for the
pre-processing of laser scan point clouds and for the creation of solid models from object
surface points. The solid model generation proposed here is meant to be a step in thisdirection.
The data structure coding system applied for photogrammetric feature extraction was
employed to the laserscan point clouds as well. In this case, however, entire wall sections aretreated as units named following the data structure coding pattern.
The speed and efficiency with which 3D data of large surface areas can be acquired throughlaserscanning makes the technique very appropriate for capturing surface detail. An
alternative method of generating dense point clouds are the well established photogrammetricimage matching and feature extraction techniques.
In the hybrid approach photogrammetry provides information about object edges for wire
frame modeling while laser scanning supplies surface detail.
4. SOLID MODELING BASICS“A solid model is a digital representation of the geometry of an existing or envisioned
object.“ (Rossignac). Solid modeling was developed in the seventies in response to the search
for a valid and unambiguous engineering representation that in principle supports any and all
geometric queries that may be asked of the corresponding physical object. (Shapiro, 2001)
The mathematical foundations of solid modeling are Euclidean principles and differential andalgebraic geometry. Data structures and algorithms for efficient solid modeling computations
were developed in the nineteen seventies as a research project at the University of Rochester.
Workshop – Archaeological Surveys
WSA2 Modelling and Visualization
Eric Andrea Mngumi and Heinz Ruther
WSA2.3 Solid Modeling for Heritage Documentation
FIG Working Week 2004
Athens, Greece, May 22-27, 2004

5/10

“The real world artifacts and the associated processes are abstracted by “postulated”
mathematical models. The space of mathematical models and operations serves as thedefinition for the corresponding data type (class) that can be represented on a computer (inmore than one way) by a representation ‘scheme’. Formally a representation scheme can be
defined as a mapping from a computer structure to a well-defined mathematical object.“
Requicha, 1980.
There is a number of existing solid modeling representation schemes. The following list is by
no means exhaustive but provides a summary for the purpose of this paper.

−−

Boundary Representations:
In boundary representation the object is defined as the union of its boundary faces thatshould be non-overlapping. The faces, in turn, are defined by curve functions defining
the edge, and surface functions defining the points spanning the edges. B-rep data
structures therefore document each vertex, edge and face plus their topology.

−

Constructive Solid Geometry (CGS):In this scheme an object is expressed as a collection of primitives i.e simple shapes likecylinders, cones, blocks combined using Boolean operations. Primitives may beinstantiated several times and used with different parameters, positions and orientations.
CSG data structures store objects in form of a tree with Boolean operators (union,intersection and difference) at the nodes and primitives as the leaves. While some nodes
represent Boolean operations others perform translation, rotation or scaling. (Requicha
1980)
Spatial Decomposition
In spatial decomposition an object is represented as a collection of small non-intersecting
cellular units. There are several kinds of spatial decomposition schemes including spatial
occupancy, cell partitioning and octree.

In this work we adopted a hybrid modeling approach based on CGS. Several potentials of
CGS are important in the process of finalizing the model and deriving other representations
from the model. These potentials include Boolean operations, slicing and sectioning.
5. FROM POINT CLOUDS TO SOLID MODELS
Although several methods of solid modeling of complex objects exist, to the author’s
knowledge, none uses surface point information as input. Solid modeling of complex
irregular objects from so-called 3D images is discussed in a number of available literaturesources: Marsan (1999) presents a summary of recent techniques. In medical and similar
applications, 3D images are obtained from CT scans and represent the object completely bydescribing its interior and exterior by means of voxels. A voxel is the smallest cubical unit ofspace or simply a volume pixel. Data acquisition through CT scans, is, for obvious practicalreasons, not suited to the documentation of monuments. Point clouds can be generated either
through photogrammetric processing of images or as a product of laser scanning.
Workshop – Archaeological Surveys
WSA2 Modelling and Visualization
Eric Andrea Mngumi and Heinz Ruther
WSA2.3 Solid Modeling for Heritage Documentation
FIG Working Week 2004
Athens, Greece, May 22-27, 2004

6/10

Figure 2: Photograph(entrance gate to the Gereza’s)

Figure 3. Laserscan image(entrance gate to the Gereza’s)

Figure 3. Wire frame(entrance gate to the Gereza’s)

Figure 4. Solid Model(entrance gate to the Gereza’s)

5.1 The Modeling Process on CAD
In the initial stage of the modeling process the desired model is considered as a solid crust or
shell of the object. The difference between the crust and a surface model is that the crust(shell) is a solid object with some thickness. This solid can be sectioned, sliced, joined toanother solid through the union operation or punched through (windows and doors) using thesubtraction operation. Once the shell has been generated the next task is to fill it with solid
Workshop – Archaeological Surveys
WSA2 Modelling and Visualization
Eric Andrea Mngumi and Heinz Ruther
WSA2.3 Solid Modeling for Heritage Documentation
FIG Working Week 2004
Athens, Greece, May 22-27, 2004

7/10

material to produce a full solid object. Again the filling-in is possible through the use of
Boolean and other solid modeling operations inherent in CGS based systems.
In summary, the solid model is created following these steps:−
A laser scan point cloud is displayed in CAD in dxf format (data exchange format).
−A section of a laser scan point cloud is isolated and saved as an independent file.
−
This new dxf segment of the laser scan point cloud can now be converted into xyz format
and used as input for the creation of a DTM using krigging algorithms – in this caseSurfer software was employed. The ‘density’ of the DTM can be increased, if necessary.
−
A three-dimensional polyline is used to create two triangles from each rectangular cellof the DTM. Each triangular unit is then swept by a given thickness to create a unit solid.
The whole process is automated by as an AutoLisp program. Boolean union is used tojoin individual triangles into a single solid.
6. DERIVING OTHER PRODUCTS FROM SOLID MODELSFor restoration purposes, it is useful to represent a building in detailed 2D drawings in formof plans, elevations and sections, in addition to the 3D model. These 2D representations are
essentially abstractions of the real object and given the right type of 3D model can be derived
directly from the model. In this regard a solid model is superior to wire frames and surface
models because it provides a useful resource from which other representations can beextracted through slicing, sectioning subtraction and other constituent operations of solid
modeling.
Another important advantage of solid modeling for heritage documentation is the ease withwhich the model can be modified and extended in case digital reconstruction should beconsidered. Overall, documenting a historical site using solid models as opposed to wire
frames and surface models offers considerably more potential in addition to visualization.
7. CONCLUSION
Although the documentation of Kilwa Kisiwani is a process in its early stages, certain trends
are beginning to consolidate and show the direction forward. Among these are the advantagesof combining theodolite, laser and photogrammetry surveying techniques,an approach given
full treatment in Ruther 2003 and Borg, 2002. In the course of the Kilwa project, it has been
established that, while wire frame and surface models may sufficiently and economicallycater for visualization needs pertaining to documentation of historical sites, solid modeling isa better approach – one that adequately addresses the needs for documentation as guideline
for restoration work and basis for virtual reconstruction of historical sites. A notabledisadvantage of laser, solid models of complex objects are bulky and more time consuming to
generate than wire frames and surface models. However, with computers becoming more
powerful, it is projected that documentation of historical sites will benefit from adopting asolid modeling approach because of its many advantages.

Workshop – Archaeological Surveys
WSA2 Modelling and Visualization
Eric Andrea Mngumi and Heinz Ruther
WSA2.3 Solid Modeling for Heritage Documentation
FIG Working Week 2004
Athens, Greece, May 22-27, 2004

8/10

REFERENCES
Borg C.E and Cannataci J.A Thealasermetry: A Hybrid Approach to Documentation of Sites
and artefacts
Chittick N. (1974) Kilwa, an Islamic Trading City on the East African Coast. Publication of
the “British Institute in Eastern Africa and “Digital Library (Dictionary of Islamic
Architecture) http://archnet.org/library/dictionary.
Fraser, C.S. & Edmundson, K.L. (2000) Design and Implementation of a Computational
Processing System for Off-Line Digital Close-Range Photogrammetry. ISPRS Int.
Journal of Photogrammetry & Remote Sensing, 55(2): 94-104.
Lapointe R. Laser Scanning Applied to archaeological Heritage
Marsan A.L (1999) Solid Model Construction From 3D Images
Ph.D. dissertation, University of Michigan.
Requicha A.G (1980). Representations for rigid solids: Theory methods and systems. ACM
Computing Surveys, 12(4): 437-464, December 1980
Rossignac J.R and Requicha A.G Solid Modeling
Rüther H, Mtalo G. & Mngumi E, 3D Modelling of Heritage Sites in Africa.
A Case Study in the World Heritage Site of Kilwa Kisiwani, Tanzania.
Proceedings, CIPA 2003 XIXth International Symposium
Shapiro V. Solid Modeling (2001) Technical Report SAL 2001-2, Spatial Automation
Laboratory, University of Wisconsin – Madison.
Sutton J.E.G, (1998) Kilwa: A history of the ancient Swahili town with a guide to themonuments of Kilwa Kisiwani and adjacent islands. AZANIA - Journal of the British
Institute inEastern Africa, Volume 33.
Sutton J.E.G, (1990) A thousand Years of East Africa. Publication of the BritishInstitute inEastern Africa.Sir John Gray ( 1951) Publication of the British Institute inEastern
Africa. Tanganyika Notes and Records, No 31.
CONTACTS
Mr. Eric Andrea Mngumi
School of Architecture, Planning and Geomatics
University of Cape Town
Private Bag
Rondebosch 7700
Cape Town
SOUTH AFRICA
Tel. + 27 72 3699 059
Fax + 27 21 650 3572
Email: mngeri001@mail.uct.ac.za

Workshop – Archaeological Surveys
WSA2 Modelling and Visualization
Eric Andrea Mngumi and Heinz Ruther
WSA2.3 Solid Modeling for Heritage Documentation
FIG Working Week 2004
Athens, Greece, May 22-27, 2004

9/10

Prof. Heinz Ruther
School of Architecture, Planning and Geomatics
University of Cape Town
Private Bag
Rondebosch 7700
Cape Town
SOUTH AFRICA
Tel. + 27 21 3573/7
Fax + 27 21 650 3572
Email: heinz.ruther@eng.uct.ac.za

Workshop – Archaeological Surveys
WSA2 Modelling and Visualization
Eric Andrea Mngumi and Heinz Ruther
WSA2.3 Solid Modeling for Heritage Documentation
FIG Working Week 2004
Athens, Greece, May 22-27, 2004

10/10