Three dimensional measurement of textured surfaces using digital photogrammetric techniques

Doctoral Thesis


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University of Cape Town

The deep-level gold mines on the Witwatersrand (South Africa) are located at depths of over 3,000m below surface. Mining excavations follow tabular reefs that are kilometres in extent, but only a few centimetres thick. Due to the great depths of these excavations and the large overburden of rock, immense pressure is exerted on the rock mass being excavated. In order to relieve this stress in the rock mass being mined, the rock is "pre-conditioned" by pre-fracturing the rock face with large blasts. This has the effect of pushing the critical stress load further into the rock mass, thus increasing safety at the rock face. To better understand the rock behaviour as a result of by pre-conditioning blasts, it is desirable to quantify the deformations of the rock face. Digital photogrammetry provides an ideally suited method of monitoring these deformations, as the necessary equipment is portable and easy to use underground, where the conditions are extreme with temperatures approaching 40°C and humidity levels close to 100%. Digital photogrammetric techniques are also highly accurate, and can be used to detect relatively small three-dimensional movements. The determination of the three-dimensional (XYZ) co-ordinates of the rock face, represented by densely spaced individual points, by means of digital photogrammetric techniques, is reported in this thesis. The novel measurement system developed comprises the following components: * establishment of a stable reference co-ordinate system; * image acquisition; * camera calibration and exterior orientation calculation; * feature extraction; * multi-image matching and space intersection; and * surface modelling. The details of the techniques developed and implemented in order to generate the necessary object space co-ordinates are discussed. Sub-millimetre accuracy point determination, as required for deformation analysis was achieved and a sample of the underground test result data is presented. The final analysis of the underground test data made it apparent that digital photogrammetry is highly suitable for the determination of digital terrain models of the rock surfaces, for subsequent deformation analysis. The relative speed of the process and the convenient size of the equipment makes the technology especially suitable to the demanding underground mining environment. The difficulties, which were experienced as a result of the highly restrictive environment, were overcome through careful planning and pre-analysis. It can thus be concluded that this approach is not only feasible, but it meets the stringent demands of the underground mining industry, as is confirmed by the high accuracy of the final surface point co-ordinates achieved. In addition to being implemented in the underground mining industry, the measurement system developed was used for the mapping of several other textured surfaces. In particular it was used in mapping the 3.6 million-year-old hominid trackway located at Laetoli (Tanzania), the imprints of cheetah and rhinoceros for the purposes of conservation, and for the archaeological documentation of a shipwreck found off the Cape coast (South Africa). All of these applications are discussed and the results achieved presented.

Bibliography: pages 133-156.