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Hybride photogrammetric and geodetic surveillance of historical buildings for an urban tunnel construction Image acquisition Two digital cameras Kodak DCS 460 with 14mm lenses have been used for photogrammetric image acquisition. The achievable accuracy in object space has been estimated by SXYZ=q.mb.Sx'y' A mean image measuring accuracy of sx'y'=0.2 pixel resp. 1.8mm has been expected for the used point targets. Although much higher measuring accuracies are possible for this type of camera, a lack of accuracy has been taken into account due to uncertain exterior conditions. Especially uncontrollable light conditions might lead to a worse image quality. Assuming a mean image scale of mb=500 and a design factor of q=1.2, the above specified object accuracy of sXYZ=1.0mm can be achieved. Data storage of images is performed directly on PCMCIA disk drives built in the DCS 460 cameras. On site the images have been stored on a laptop computer additionally in order to ensure a direct analysis of the image quality. The target points have been manufactured as special adapters that could be mounted into caburized screw dowels. The reproducability of the adapters inside the holes has been investigated beforehand, and has been proven to better that 0.1mm. For the actual target centre retro-reflective material has been used (Figure 4). In addition to the reference points a large number of temporary tie points has been added. Due to the limited space for the selection of image stations and due to the height of the buildings, numerous images have been taken from a stroke riser (Figure 5).  Figure 5: Image acquisition out of a stroke riser Image configuration Each object has been measured according to an individual image configuration. In general, camera station have been selected in a way that every object point has been imaged in at least five photos in order to meet the accuracy specifications.  Figure 6: Angle measurement  Figure 7: Example of a geodetic network of measured directions In order to create a long-term stable reference system, the object point which is farest from the ditch has been selected as origin of the local object co-ordinate system. Using the 3-2-1 method known from industrial metrology the co-ordinate axes have been defined without any constraints. Parallel to the actual measurement of the buildings a signalised test field has been set up on site in order to provide the possibility of re-verification and calibration of the cameras. Figure 8 shows some examples of metric images of different buildings. Definition of scale and datum of the object co-ordinate system was performed by precise leveling and geodetic angle measurements using an industrial theodolite measuring system based on precision theodolites Leica T2002 (Figure 6). Targeted precision scale bars (Brunson), again taken from an industrial metrology system, have been used to define reference distances in object space. As an example, Figure 7 shows the geodetic network of directions for the measurement of control points, and the configuration of photogrammetric tie points as well. 
Figure 8: Examples of metric images Results Image processing system The measurement of image points has been carried out by the program Ax.Ima (AXIOS 3D Services) that consists of sophisticated algorithms for the determination of ellipse-shaped targets even under poor image quality conditions. The accuracy of these algorithm has been shown in previous investigations (Luhmann, 1996). The bundle adjustment program BINGO (Kruck, 1983) has been used for image orientation and three-dimensional point determination. Here all geodetic observations have been integrated with weights according to their accuracy. Achieved accuracy The average measuring accuracy of the geodetic measurements amounts to 0.1mm for precise leveling and also 0.1mm for 3-D point determinations using the theodolite system. Consequently these observations are available with superior accuracy with respect to the photogrammetric measurements. After the combined bundle adjustment mean residuals of image co-ordinates of about 0.9mm have been achieved. The standard deviations of object point co-ordinates vary between 0.5mm and 0.8mm in X-, Y- and Z-direction. Compared to independently measured reference distances a mean uncertainty of length measurement of 1mm is resulting. Summary A combination of photogrammetric and geodetic methods has been applied to a complex and extensive task of evidence documentation. The specified accuracy of 1mm in object space has been achieved. The main technical problem has occurred with the difficult conditions on-site and hard to access buildings. Image processing has been performed by precise image operators. The measured image co-ordinates have been processed in a combined bundle adjustment including all geodetic observations. For the following measuring epochs the transformation of measured points is mainly based on differences between the epochs, because the reference points have been determined with superior accuracy using precise leveling and a theodolite system. References
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