The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B7. Beijing 2008 
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However, other features have the potential to provide additional 
information about the imaged scene. The three images in Figure 
2 show three different bridges in Southern Germany. A 
relatively short bridge crossing a freeway is displayed in Figure 
2 (top left). Most of the radar signal on top of the bridge is 
reflected away due to the smooth surface of the tarred road. 
Therefore, the road appears dark in the image and almost no 
insight can be deduced from direct backscatter. However, 
although the bridge surface itself cannot be seen nicely, the 
width of the bridge may be derived from two parallel bright 
lines. They are due to double-bounce effects from the guardrails 
on top of the highway bridge. Such guardrails are typical 
features of highway and freeway bridges. They also appear in 
the top right image and the bottom image in Figure 2. The top 
right image shows a freeway bridge crossing a deep valley close 
to the city of Schwäbisch Hall. In addition to two guardrails on 
each side of the bridge, a third guardrail can be seen in the 
middle of the bridge, separating the two opposed driving 
directions. A typical SAR effect, useful for modelling bridges 
over land, is occlusion. A shadow of the bridge body and the 
pillars is observable in the top right image of Figure 2. Its shape 
depends on the bridge body, the height of the bridge above the 
terrain and the undulation of the terrain. The greater the 
distance between the bridge body and the bottom of the valley 
becomes, the further away is the shadow from the bridge 
(assuming a constant height in the occluded area). This shadow 
distinguishes objects elevated above the ground from non 
elevated objects. It describes the bridge as a three-dimensional 
object, discriminating it from roads. 
3. BRIDGE HEIGHT ESTIMATION FROM OPTICAL 
AND SARIMAGERY 
3.1 Displacement of elevated objects in optical imagery 
One database containing optical imagery covering the entire 
Earth with reasonably high resolution is GoogleEarth. Ortho- 
rectified optical images from both space borne and airborne 
sensors are available free of charge. In case rapid change 
detection is needed during the occurrence of a natural hazard, 
imagery from GoogleEarth can be used as a reference. Since the 
optical images are ortho-rectified, distortions due to terrain 
undulation are significantly decreased. However, elevated man 
made objects stay distorted because they are not included 
within the digital elevation models (DEM) that are used for the 
rectification process. Such DEMs only contain points on the 
bare ground. This effect is shown in Figure 5 while an overview 
of the entire bridge is given in Figure 4. Figure 5 shows a 
section of a long railroad bridge, crossing a valley. The terrain 
height of the valley bottom is varying as it can be seen in Figure 
4 (left) and from the distance between the shadow and the 
bridge in Figure 5. The further away the shadow is situated 
from the bridge, the deeper is the valley. 
The three-dimensional modelling of a scene using one optical 
image and one SAR image is possible due to the different 
perspectives of the sensors (Figure 3). Optical sensors usually 
acquire images with off-nadir angles smaller than 30°. SAR 
sensors are side looking and e.g., military airborne sensors 
acquire imagery with off-nadir angles up to 70°. Hence, two 
different perspectives of the same object on the ground allow 
for the estimation of height values. Additionally, SAR sensors 
measure distances whereas optical sensors measure angles. 
Hence, the points A, B, and C in Figure 3 are projected to Aq, 
Ground 
A B C 
Imaged Ground Area 
Figure 3: Comparison of optical and SAR sensor geometrie 
B 0 , and C 0 by the optical sensor while they are imaged to A R , 
B R , and C R by the SAR sensor. 
Figure 4: Images of a railroad bridge made of concrete-steel 
near the city of Zellingen in southern Germany, (left) oblique 
photo taken out of the aircraft with a consumer grade camera, 
(right) optical image from GoogleEarth. 
Figure 5: Blow-up of a GoogleEarth from Figure 4; compared 
to the two parallel red lines, the distortion of the bridge in the 
ortho-rectified image can be observed. 
The distortion of the railroad bridge in Figure 5 becomes 
obvious by comparing the alignment of the two parallel red 
lines with the bridge body. The greatest distortion occurs at the 
highest elevation of the bridge above the valley bottom. Since 
the image was taken from the south-west, maximum distortion 
occurs towards the north-east.