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MERGING DTM AND CAD DATA FOR 3D MODELLING PURPOSES 
OF URBAN AREAS 
Siyka Zlatanova Nedkova*, Michael Gruber** and Michael Kofler** 
"International Institute for Aerospace Survey and Earth Sciences 
7500 AA Enschede, The Nederlands 
**Institute for Computer Graphics 
Graz University of Technology 
Münzgrabenstr. 11, A-8010 Graz, Austria 
ISPRS WG IV/1 
Key Words: 3D Urban Database, 3D GIS, 3D CAD, DTM, 3D Modelling 
Abstract 
We present in this paper a method for merging terrain data from DTM and CAD data. The method 
uses data from DTM which are reorganized to a set of 3D polygon in a TIN representation. The CAD 
objects, i.e. buildings and surface features, are fused with DTM after applying a special procedure. 
The used techniques ensure the consistency of the data model in terms of avoiding undesirable gaps 
and occlusions between the DTM and CAD features. A partial simplification of the buildings facilitates 
the process of texturing and manipulating the data. 
In the next steps we will strive fore possible automation of the entire procedure. This shall support a 
more or less autonomous and cheap manner for building 3D photo realistic models of urban aereas. 
1.Introduction. 
Three dimensional (3D) digital models became a 
very important issue in the last few decades due to 
the increasing demand for a realistic presentation of 
the real world. A 3D model gives better presentation 
and facilitates the processes of planning, controlling 
and decision making especially in the urban areas. 
Furthermore modern graphics workstations which are 
available on the market allow already to handle and 
visualize large amounts of data. Since there is need 
for 3D models and sufficient hardware, the task of 
the researchers now is to develop technologies for 
automatic construction of 3D models using different 
data sources. 
Speaking about 3D models, it is always of great 
importance to clarify what is the purpose of the 
model, or in other words, what is the area of 
expected applications. The organization of the data 
in the model can be totally different if one needs 
only a realistic view of the real world with additional 
tools for flying trough, etc., compare to needs, for 
example, spatial analysis to be provided. Another 
aspect which cannot be forgotten is the accuracy of 
the data. Depending on the application, it may 
appear that there is no need for very accurate data 
and there is a possibility for simplification of the 
data sets. This, without any doubts, will facilitate the 
process of data handling. The goal of this paper is 
to contribute a part to the technology for building a 
3D city model, namely the merging of terrain data 
and objects for photo realistic visualization and 
manipulation, which will supply users with: 
311 
e photorealistic presentation with possibilities for 
navigation through the model; 
e abilities to model, design and analyze the data of 
a city scape, e.g. arrange buildings and man- 
made objects, allow queries, etc. 
The investigations are focussed into two important 
tasks. Broadly speaking, the first task is to 
construct geometry and second one is to texture 
the model. The aim of the approach is to make use 
of various data sets comming from different 
Sources, e.g. 2D maps, aereal images, Digital 
Terrain Model (DTM), etc. The joint efforts of the 
researchers at the International Institute for 
Aerospace Survey and Earth  Sciences(ITC), 
Enschede and the Institute for Computer Graphics 
(ICG), Graz will result finally in a automatic or semi- 
automatic procedure for creating, storing, 
manipulating and analyzing 3D city models. 
2. Data sets 
Various methods for constructing 3D models are 
object of studies and different approaches 
concerning data extraction and data merging are 
developed (see [2,5,7,11]). The way of constructing 
the 3D model depends on the data sets which are 
used. For example, the work carried out in ITC, is on 
the construction of models only from aerophoto 
images, applying 3D digitizing for gathering the 
required information. Existing DTM, 2D CAD data 
and aerophoto images are the source data used at 
the ICG. 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996