Application of optical techniques to surveying

This thesis addresses the problem of acquiring spatial data concerning points on the surface of structures such as underground tunnels and sewers. These data can usefully provide knowledge of deformation, shape, area, volume, and position of structures. Such data can be further analysed to give insight into clearances, deterioration, flow rates and in-fill volumes or can be used to give knowledge of the present state of structures and their position. Few systems address the problem of reliably acquiring this data in a manner that is fast and accurate while remaining flexible, adaptable and robust. This thesis considers a solution to the problem of fast and accurate spatial data acquisition concerning commonly found structures using the technique of optical triangulation with a linear array camera and diode laser light source. Optical triangulation is a technique that has not fully matured for medium range measurement with few systems having been developed and little research material produced. However, the research carried out for this thesis shows that providing all the factors that contribute errors of measurement are understood, then a fast, robust and high accuracy system can be developed. The development of the optical triangulation technique for use in surveying was addressed through a programme of prototype development, testing, and refinement. Three prototypes were built that demonstrated the reliability, accuracy, speed and robustness of this technique. The errors associated with the a triangulation measuring system when applied to surveying application is considered from the intrinsic errors which are the same for any triangulation system and the extrinsic errors which are particular to the use of this system in surveying situations. A calibration bench was constructed for consideration of the triangulation system which was automatic and used an interferometer to provide high accuracy measurement of the performance of the triangulation system. Calibration and interpolation trials were conducted and the results analysed. An analysis of the subpixel accuracy achieved with the discrete pixel CCD imagers has been performed and an analysis made. One of the main disadvantages of optical triangulation when applied to the range 0-5 metres is that of non-linearity. A method of correction has been developed and analysed which is believed to be novel and makes a significant improvement to the measuring system. The conclusion of this research is that an improved system of measurement has been produced which has a number of novel features. Trials show that the measuring system could be developed commercially to provide a solution to measurements of structures within the range of the device and with greater accuracy than comparable equipment designed for the same purpose