1071 
BRIDGE HEIGHT ESTIMATION FROM COMBINED HIGH-RESOLUTION OPTICAL 
AND SAR IMAGERY 
J. D. Wegner*, U. Soergel 
Institute of Photogrammetry and Geoinformation, Leibniz University Hannover, Nienburger Str.l, 30167 Hannover, 
Germany - (wegner, soergel)@ipi.uni-hannover.de 
Commission VII, WG VII/6 
KEY WORDS: SAR, High Resolution, Urban, Fusion, Change Detection, Infrastructure, Detection 
ABSTRACT: 
Today’s airborne (Memphis, AeS-1, Ramses) and space borne (TerraSAR-X, CosmoSkyMed, Radarsat) SAR sensors provide very 
high resolution imagery independent of daylight and cloud coverage. Space borne systems achieve geometrical resolutions of down 
to one meter while airborne sensors are capable of acquiring images with sub metric resolution. In this kind of data, urban objects 
like buildings and bridges become visible in much detail. However, due to the side-looking SAR sensor principle, layover and 
occlusion hamper the interpretation particularly in urban scenes. One possibility to overcome this drawback is the use of additional 
information from high resolution optical imagery. In this paper, first findings of a long term project using both optical and SAR 
imagery for the modelling and extraction of bridges are presented. The focus is on bridges because they play a key role as 
connecting parts of man-made infrastructure and are of high importance in case of rapid natural hazard response. Differences 
between bridges over water and bridges over land are explained. Furthermore, concepts for estimating bridge heights from of a 
single SAR image and by means of combined optical and SAR imagery are derived. 
1. INTRODUCTION 
Up-to-date airborne SAR sensors provide geometrical 
resolutions of well below half a meter. Because of the high 
level of detail provided by such imagery, the acquisition of 
urban scenes and furthermore the extraction of elevated objects 
like buildings and bridges becomes a promising application 
field. However, the SAR typical disadvantages, due to the 
sensor’s side looking perspective, cannot be overcome easily. 
Effects like layover and occlusion are developed to their full 
extent particularly in urban areas. Such principal drawbacks can 
at least partly be alleviated by combining SAR imagery with 
spatial information, for example, retrieved from topographic 
maps or optical images. In this study, SAR images are 
interpreted in combination with optical images since they are 
usually available even for remote regions. However, the 
viewing geometries of SAR and optical sensors lead to different 
appearances of the same object in the corresponding images. 
Hence, simply overlaying images of these two different kinds of 
sensors will lead to displacements. The higher an object is 
elevated above the ground, the greater is the displacement 
between the imaged object in the SAR image and the imaged 
object in the optical image. Thus, elevated objects have to be 
modelled carefully taking into account both the SAR and the 
optical viewing geometry in order to come up with reasonably 
accurate results. 
The focus of this paper is on the modelling and automatic 
extraction of bridges. Bridges are a key feature in urban 
infrastructure and have high importance especially in case of 
time critical events, such as natural hazards or political crisis. 
They are linking roads and railways, enabling rapid emergency 
response in regions, e.g., hit by flooding or land slides. It is of 
high interest to retrieve information about the bridges current 
condition in a crisis situation. Lack of daylight or clouds 
covering the region of interest often circumvent the acquisition 
of optical images. Due to their relatively long wavelength and 
the active imaging mode, SAR sensors capture images almost 
insensitive to the weather conditions and independently of 
daylight. By comparing optical images dating back to before the 
hazard and SAR images acquired during or after the hazard, 
automatic change detection can be conducted. 
The following paragraphs will give an insight into first findings 
of a long term research project. SAR scenes of bridges over 
water and of bridges over land will be compared and SAR 
specific effects will be explained. Concepts for the three- 
dimensional extraction of bridges from one SAR image and one 
optical image will be presented. 
2. BRIDGES IN INSAR AND SAR IMAGERY 
The appearance of a bridge in a SAR image very much depends 
on the environment surrounding the bridge. Generally, two 
different kinds of bridges are thus distinguished: bridges over 
water and bridges over land. In the following two paragraphs, 
the differences between bridges over water and bridges over 
land are discussed. 
2.1 Bridges over water 
The characteristics of bridges over water in SAR imagery have 
already been discussed in detail for instance in (Robalo & 
Lichtenegger, 1999; Soergel et al.,2007; Cadario et al.,2008). 
Figure 1 (top image) shows an InSAR amplitude image of two 
* Corresponding author