For testing the applicability of concepts and available tools
we have built a prototype 3D-GIS. Chapter 4 provides an
outline of its components and presents results from pilot
studies which have been conducted. At this stage the
emphasis of our studies is on geometry, leaving aside
semantic modelling which is very much dependent on the
specific application context of the GIS.
Based on the problems encountered in the pilot studies, the
paper concludes with design considerations for future digital
plotters which should offer higher 3D productivity.
2. 3D-GIS
2.1 Requirements
A 3D municipal information system should support civic
activities ranging from administration to planning and
construction for city (re)development, planning and
management of utilities, security assurance, environmental
management, and conservation. The promise of 3D-GIS is
better accessible inventories, extended spatial analysis,
refined simulations, and more appealing visualization than
either 'flat' GISs or solitary CAD models can grant. Moreover,
a 3D-GIS could also support automation of
photogrammetric object reconstruction by providing
information for model based image analysis. 3D analysis
capabilities could also be exploited to efficiently plan and
conduct field completion (see Felus et al, 1996).
This broad spectrum of application presents multifarious
requirements for the urban 3D-GIS these include:
€ provision of means to construct the digital model
representing geometric and semantic properties of
objects from various inputs and handling implied
uncertainty
integrity and maintainability of the database
support of spatial analysis in different user contexts,
including basic operations such as:
measure spatial extensions of objects (volume,
surface area, line length)
answer topologic queries (pipe passing through the
building?)
search and retrieve objects of certain properties
zoning, route optimization, lighting studies, etc
support of simulations (eg environmental impact
analyses) and design (virtual updating of the existing
situation by designed objects)
effective visualization (perspective and orthogonal views,
user (interactively) defined viewing direction, viewing
point and stretch, photo realistic computer graphics and
photo true pictures)
promotion of different abstraction
resolution)
levels (multi-
A discussion of application areas of an urban GIS and
requirements for its 3D model can be found, eg in (Fórstner
et Pallaske, 1993). Pilouk (1996b) gives an overview of
possible system architectures.
860
2.2 Data Model
The analysis of current GIS and CAD capabilities and
software tools offered (see Bric et al, 1994) let us to decide
to first look for rigorous spatial concepts for data modelling
and on such a solid basis, see which available tools can be
utilized and how. Focusing on urban scenes and
considering the above requirements, we have selected a
vector data model based on Molenaar's FDS.
The comprehensive 3D data model comprises geometric
and semantic description of objects and allows for the
analysis of 3D topology. It can be handled by a single
DBMS. It is downward compatible with 2D-FDS so that data
from existing GISs can be incorporated after being
converted to this structure. It follows a 3D boundary
representation because of the desired visualization
capabilities. Photogrammetry offers accurate and economic
techniques for the acquisition of vector data on urban
topographic objects and provides for the input for mapping
photo textures onto surfaces of objects. These raster data
can be integrated into the vector model. Since we are
mainly interested in object reconstruction, we can limit our
review of FDS to the representation of geometry without
elaborating on the modelling of semantics.
The 3D-FDS groups objects to classes in a thematic sense
and distinguishes four types of objects according to
geometry: point, line, surface, and body object (see figure 1).
The representation of geometry aims at separating its three
aspects: topology, shape and size, and position. Positional
information is contained in the geometric primitive node
with its x,y,z coordinates. The one-dimensional geometric
primitive is arc. Arcs constitute line objects and boundaries
of faces. Faces are two-dimensional primitives, they
constitute surface objects and boundaries of bodies. Line
objects, surface objects, and solid objects (bodies) have
shape and size. To avoid ambiguity in the data model, arcs
that constitute the boundary of a face, must be relatively
oriented; the auxiliary geometric primitive edge is
introduced for this purpose. Hence, /eftand right body to a
face can be defined. The topologic relationships between
the objects are defined through the geometric primitives
and the links between them which are shown in figure 1.
This diagram, together with a set of conventions, represents
the basic data model. The conventions are rules that must
be observed when structuring data; eg, faces must not
intersect or touch, an arc cannot intersect a face, line
objects have no branches, etc. When introducing the
conventions that arcs are straight lines and faces are planes,
they would not need any shape parameters. The data model
as shown in figure 1 supports the handling of a large variety
of queries and can be extended to build composite objects
(see Bric et al, 1994). Moreover, it can be upgraded to
structurally integrate a digital terrain relief model (DTM), see
(Pilouk et Tempfli, 1994).
The 3D-FDS can be seen as an extension of the edge based
boundary representation used in solid modelling. Since any
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996