Browsing by Subject "Photogrammetry"

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    Relative orientation with limited control in close range Photogrammetry
    (1982) Rüther, Heinz; Adams, L P
    In close-range photogrammetry, situations can arise in which it is difficult or impossible to establish a network of control points as required for a conventional absolute orientation procedure. The thesis investigates the replacement of the traditional control network by a few control distances measured between well-defined artificial markers or natural feature points. The measured distances must then serve to reduce deformations suffered by the photogrammetric model in the orientation procedures. All investigations are based on analytical rather than analogue photogrammetry. After a review of the concepts of rotation matrices, least squares adjustment and the generation of synthetic image co-ordinate observations, the study is executed in three major steps. A test field of high precision is established by means of space intersection and a camera calibration method for close-range cameras is developed which combines perspective projection with geodetic observations of the lens system parameters. Thus a problem inherent in many camera calibration methods, namely the exact determination of the perspective centre, is largely overcome. Deformation characteristics related to error in elements of interior and relative orientation are determined by the controlled introduction of errors into these elements. The deformations are presented in tabular and diagrammatical form. An analysis of the deformation leads to the conclusions of theoretical and practical relevance for close-range photogrammetry. As a result of the deformation analysis mathematical models are introduced which utilise the measured distances for the reduction of model deformations. The efficiency of homogeneous scaling, affine scaling and convergency correction, as applied individually and in various combinations, is tested. A mathematical formulation of the converging correction as a restraining condition in a least squares adjustment is developed for this purpose. It is shown that a convergency error is less relevant to close-range photogrammetry than generally assumed and that characteristic model deformations in close-range photogrammetry have the character of affine scale errors. Throughout the thesis algorithms are developed which make it possible to execute all computations on computers with limited memory capacity. A program sample for the relative orientation adjustment is given in Appendix IV to demonstrate the memory saving techniques. Finally the results of the investigation are applied to the survey of shoulder height of African elephants in their natural habitat. Equipment and field work are described and results reported.
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    Open Access
    A study of water wave reflection using close range photogrammetry
    (1989) Petzer, J M; Kilner, F A; Rüther, Heinz
    The intention of this investigation is to investigate the various forms of water wave reflection to a high degree of accuracy. Close range photogrammetry is the technique that is used to measure the water surface profile, as it produces an accurate and comprehensive analysis of the water surface profiles. Conventional photogrammetry techniques (photography) were used in preference to near real time photogrammetry (digital). Although near real time photogrammetry has the advantage of a far higher rate of data acquisition, it does not achieve the same degree of accuracy as can be achieved by conventional photogrammetry, the technique finally used for this investigation. For the generation of the desired wave patterns, certain equipment was developed and modified. To test the various angles of incidence a moveable reflecting wall was built. Due to the small size of the wave basin, an efficient wave absorber was required to absorb the reflected wave generated when oblique wave reflection was investigated. It was observed that a very poor quality wave was being generated by the wave generator, as a result of its flexibility. Consequently the wave generator was stiffened considerably which improved the wave generated. Interesting information was obtained from the analysis of the standing wave. A coefficient of reflection of 1,6 at the reflection wall was obtained, this places new emphasis on the relationship between the standing wave and overtopping. The oblique wave reflection resu1ts corresponded well with theoretical predictions, while no well-defined trends were established for the mach wave. The data for the mace wave did however suggest that previously established trends for the mach wave may not be correct. Close range photogrammetry produced accurate results, and is an excellent method for water surface profile measurement. The results obtained showed that the wave generator was not generating a pure wave, which lead to unknown errors in the results of spot heights. Conventional photogrammetry is a slow process, so not enough data was acquired to adequately analyse the reflection trends. This suggests that in order for the trends to be well-established, near real time photogrammetry should be used once these systems have developed sufficient accuracy.
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    Surface capture using near-real-time photogrammetry for a computer numerically controlled milling system
    (1989) Wildschek, Reto; Sass, Andrew; Rüther, Heinz
    During the past three years, a research project has been carried out in the Department of Mechanical Engineering at UCT, directed at developing a system to accurately reproduce three-dimensional (3D), sculptured surfaces on a three axis computer numerically controlled (CNC) milling machine. Sculptured surfaces are surfaces that cannot easily be represented mathematically. The project was divided into two parts: the development of an automatic noncontact 3D measuring system, and the development of a milling system capable of machining 30 sculptured surfaces (Back, 1988). The immediate need for such a system exists for the manufacture of medical prostheses. The writer undertook to investigate the measurement system, .with the objective to develop a non-contact measuring system that can be used to 'map' a sculptured surface so that it can be represented by a set of XYZ coordinates in the form required by the milling system developed by Back (1988). This thesis describes the development of a PC-based near-realtime photogrammetry system (PHOENICS) for surf ace capture. The topic is introduced by describing photogrammetric principles as used for non-contact measurements of objects. A number of different algorithms for image target detection, centering and matching is investigated. The approach to image matching adopted was the projection of a regular grid onto the surface with subsequent matching of conjugate grid intersections. A general algorithm which automatically detects crosses on a line and finds their accurate centres was developed. This algorithm was then extended from finding the crosses on a line, to finding all the intersection points of a grid. The algorithms were programmed in TRUE BASIC and specifically adapted for use with PHOENICS as an object point matching tool. The non-contact surface measuring technique which was developed was used in conjunction with the milling system developed by Back (1988) to replicate a test object. This test proved that the combined system is suitable for the manufacture of sculptured surf aces. The accuracy requirements for the manufacture of medical prostheses can be achieved with the combined measuring and milling system. At an object-to-camera distance of 0.5 m, points on a surface can be measured with an accuracy of approximately 0.3 mm at an interval of 5 mm. This corresponds to a relative accuracy of 1:1600. Back (1988) reported an average undercutting error of 0.46 mm for the milling system. This combines to an uncertainty of 0.55 mm. Finally, the limitations of PHOENICS at its prototype stage as a surface measuring tool are discussed, in particular the factors influencing the system's accuracy. PHOENICS is an ongoing project and the thesis is concluded by some recommendations for further research work.