CMRT09: Object Extraction for 3D City Models, Road Databases and Traffic Monitoring - Concepts, Algorithms, and Evaluation 
occlusion areas lead to mismatches during the image matching 
algorithm and subsequently to errors in the resulting surface 
model. 
3. DATA SET AND STUDY AREA 
The satellite Ikonos is able to rotate the CCD Linear Array 
sensor up to an angle of 26° off-nadir, so the satellite can take 
images of the same location from two different view points on 
the same orbital track. Next to along track stereo pairs, it is also 
possible to create stereo couples out of images from the same 
area but taken from a different orbit at a different date. These 
are so-called across track stereo pairs. This approach to form 
couples has some disadvantages. The most important ones are 
radiometric differences and changes of the ground surface due 
to the time gap between acquisition of the imagery. A triplet is 
constructed out of an along track Ikonos stereo pair taken in 
March 2002 and a third image taken in May 2005. The third 
image can be considered as a nadir image. Selection criteria for 
the near vertical image were multiple: overlap with stereo 
couple, cloud-free acquisition, minimal time interval and 
optimal stereo constellation. Despite the big time interval, the 
2005 Ikonos image was chosen to be the most optimal 
candidate. 
N 
Meters 
\ 0 4000 
Figure 1. High resolution study field, indicated by the red 
polygon. The extent of the HR study area covers the 
overlapping area between the 3 Ikonos images. 
The Ikonos STEREO product imagery, which comprises of a 
forward and backward image acquisition and the GEO Ortho 
Kit 2005 image are panchromatic, resampled to a spatial 
resolution of 1 m by the image provider and provided with the 
Rational Polynomial Coefficients (RPC) camera model file. 
Further characteristics of each image of the triplet can be found 
in table 1. 
Image ID 
Acquisition 
date 
Elevation 
angle 
Collection 
azimuth 
Sun 
elevation 
angle 
A (Forward) 
1/03/2002 
67.59° 
1.6° 
39.1° 
B (Backward) 
1/03/2002 
75.59° 
214.1° 
39.1° 
C (Nadir) 
16/05/2005 
80.93° 
23.5° 
65.5° 
Table 1. Characteristics of the three VHR satellite images 
acquired over the study field. 
Part of the mega city Istanbul, Turkey is chosen as test field for 
the project, mainly because it is a city characterized by an 
intense urban growth. The city is very compact and 
concentrated along the Bosphorus strait. The high resolution 
test area covers the overlapping area between the Ikonos 2002 
stereo pair and the 2005 image and covers an area of 
approximately 60 km 2 , containing Istanbul’s historic peninsula 
and going up to the north to the urban fringe. It concerns a 
densely built-up area with a height range of 220 m with the 
lowest point at sea level and geo-morphologically characterized 
by a hilly landscape. 
4. SURFACE MODEL GENERATION 
In following subsections, the successive steps of the applied 
methodology for city surface model generation, based on 
(tri)stereoscopic VHR satellite imagery, are elucidated. The 
emphasis is especially laid on those phases were research is 
done to cope with the complexity of an urban environment. 
4.1 Tri-stereoscopic approach 
Instead of the standard stereo mapping with two images a tri- 
stereoscopic approach is followed. Generation of a DSM using 
more than two overlapping images has some interesting 
characteristics. First of all, this approach strengthens the image 
orientation because of the redundancy in the geometric 
reconstruction. Points in object space can be calculated by the 
best fit of N convergent image rays instead of two. Secondly the 
redundancy leads to a more robust matching, as mismatches and 
a unique solution, in case of multiple matching candidates, can 
be easier identified. In the stereo case, an object point cannot be 
matched if it is located in an occluded area on one or both 
images. In the tri-stereoscopic case, the third image is taken 
from a different viewing angle. Consequently this leads to a 
shift of the occluded areas in the image and enlarges the chance 
of a successful match. 
Processing of the Ikonos triplet is mainly done with a 
photogrammetric software platform, called SAT-PP. SAT-PP is 
able to perform image matching on more than two images 
simultaneously (Zhang & Gruen, 2006). This is in contrast to 
most photogrammetric software packages that are only able to 
match two images at the same time. 
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