3D City Modelling for Mobile Augmented Reality 
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The commercial modelling software package PhotoModeller is used for computing the 3D position of all measured points and 
reconstructing the surfaces. This approach involves a lot of manual work for selecting corresponding points on two and more images 
and connecting the points composing one triangle. It requires significant time and efforts to obtain the shape of a single building. 
However, since the measurements are performed and conducted by the operator, the reconstruction can be fully controlled. The 
output of the reconstruction is a 3D model in a model coordinate system. This model is then geo-referenced in PhotoModeler using 
control point co-ordinates derived from GPS and aerial photogrammetric measurements. The final co-ordinates of the 3D model in 
the national geodetic system are obtained by an integral LSA of all the measurements using software package BINGO. These are the 
photogrammetric measurements of the terrestrial and the aerial imagery, and control points measured with GPS. All the 
measurements on the aerial images are performed with the software package SoftPlotter. 
In-house software (see Figure 3: 3D reconstruction software I) processes topologie data from PhotoModeller and geometric data 
from BINGO in order to obtain the correctly structured geo-referenced data. Unfortunately non-of the export file formats contains 
complete information about geometry and topology. Processing two export files from PhotoModeller, i.e. an ASCII file and VRML 
file, and one export file from BINGO, completes the structuring. The ASCII file contains the measured points with their IDs and 3D 
model co-ordinates. The VRML file contains the topology (description of the triangles) of all the surfaces reconstructed in 
PhotoModeller. According to the VRML syntax 1) the coordinates of the points (vertices of the faces in VRML) are stored only once 
in the description in VRML (in the IndexetFacetSet node), and 2) the faces can be grouped according to the objects they belong to. 
The BINGO file provides the geo-referenced coordinates of all the measured points. The in-house software unites coplanar triangles 
that share one edge in rectangular faces considering a given distance threshold. Thus, one or more rectangular faces represent every 
façade of a building (see Table 1). 
Table 1: Project characteristics 
3D object 
Photos 
Points 
(all) 
Model 
points 
Triangles 
Rectangles 
Aula 
27 
316 
240 
259 
178 
Physics 
20 
241 
54 
54 
28 
Mechanics 
40 
392 
122 
141 
78 
Art 
9 
64 
56 
98 
50 
All 
96 
1013 
472 
552 
324 
Table 2 shows the results of the LSA. The first three columns represent RMS of the object points in the local co-ordinate system. The 
good result, i.e. centimetre accuracy is indication for the appropriate selection of images and measured points. The second three 
columns show the co-ordinate precision resulting from the adjustment by BINGO in the national co-ordinate system. The observed 
decrease in the precision is basically due to lower resolution of the aerial images, and visibility and location of measured points used 
for geo-referencing. For example the Art monument is relatively small and white, surrounded by grass (i.e. dark object), which 
complicates the measurements due to the high contrast. The results given in bold represent the RMS precision of the objects after the 
common adjustment of all the measurements performed on all models. These results can be considered as an evaluation of the 
relative position of the individually reconstructed models. The improvement of the RMS precision achieved is due to rejection of 
some ambiguous measurements of control points. 
Table 2: Average RMS precision values of the reconstructed buildings 
3D object 
Model co-ordinates 
Geo-referenced co-ordinates 
RMSX 
(mm) 
RMSY 
(mm) 
RMSZ 
(mm) 
RMSX 
(mm) 
RMSY 
(mm) 
RMSZ 
(mm) 
Aula 
23.3 
8.9 
19.8 
77.7 
76.5 
76.7 
Physics 
34.8 
14.3 
36.3 
71.8 
67.6 
73.7 
Mechanics 
20.7 
11.1 
25.8 
152.0 
98.1 
84.7 
Art 
15.1 
7.4 
16.0 
92.8 
92.1 
132.0 
All 
67.1 
66.1 
77.5 
Clearly, using the procedures described above, we ensure the decimetre accuracy and the topological structuring required by the 
UbiCom augmented reality application.