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The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B7. Beijing 2008 
bridges over water. The same two bridges are depicted again in 
InSAR allows for obtaining directly a three-dimensional model 
of the scene. However, calm water surfaces act like a mirror and 
hence the signal to noise ratio (SNR) is poor. Thus, water 
surfaces show poor coherence values leading to artefacts in the 
final Digital Surface Model (DSM). One possibility to more 
precisely estimate the water level is to determine the distance 
between the bottom of the bridge and the water surface. 
Knowing the absolute height of the bridge body, the height of 
the water surface can then be derived. 
an optical aerial image in Figure 1 (centre image). Usually 
is due to double-bounce reflection between the bridge and the 
water surface or vice versa. Triple reflection leads to the third 
stripe where the radar signal first hits the water surface, then 
lower parts of the bridge body, and finally the water surface 
again. The height h of the bridge over the water surface can be 
estimated by measuring the slant range distance As either 
between the first and the second or the second and the third line. 
Knowing the off-nadir angle 0 of the SAR sensor, h is estimated: 
h = As/ cos (0) 
(1) 
However, the exact horizontal position and outlines of the 
bridges cannot be easily derived due to the multiple bounce 
effects. This is done by fusing the three-dimensional InSAR 
scene with an aerial photo taken in nadir view. The double 
bounce stripe in the InSAR data is replaced with the optical 
data and the water surface is replaced with the newly computed 
value (Soergel et al.,2007). 
2.2 Bridges over land 
Bridges over land appear differently in SAR imagery compared 
to bridges over water (Wegner & Soergel,2008). Multiple 
parallel lines at a bridge, typical for bridges over water, do not 
occur in SAR images of bridges over land (compare Figure 1 
(top) and Figure 2). This is due to the different reflectivity 
properties of water and land. Calm water strongly reflects the 
radar signal whereas inhomogeneous terrain and vegetation 
leads to almost diffuse reflection on the ground under the bridge 
body. In case of bridges over land, multiple bounce effects only 
occur at dihedral reflectors on the bridge body and where bridge 
pillars meet the ground. Hence, the bridge height cannot be 
determined directly from the distance of parallel lines and the 
off-nadir angle. 
Figure 1: Test images of bridges near the city of Dorsten in 
southern Germany, (top) InSAR magnitude image pair taken in 
X-Band (illumination direction from right to left), (centre) 
optical aerial image, (bottom) 3D visualization of optical and 
InSAR data after fusion. 
This technique becomes possible because the very calm surface 
of the canal leads to strong signal due to multiple reflections. In 
particular, double and triple-bounce effects are typical for 
bridges over water, illuminated perpendicularly by the sensor. 
Three lines can be observed for each bridge in SAR data of 
coarser resolution. In high resolution imagery such lines widen 
to stripes. The first stripe in illumination direction represents 
the height of the bridge body because most of the radar signal is 
backscattered directly from the bridge body. The second stripe 
Figure 2: SAR test images of highway bridges acquired by the 
MEMPHIS sensor in ka-Band, (top left) SAR image of a bridge 
near Manching (illumination direction from left to right), (top 
right) SAR image of a bridge near Schwäbisch Hall 
(illumination direction from left to right), (bottom) SAR image 
of a bridge near Wolnzach (illumination direction from top to 
bottom). 
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