Development and calibration of an image assisted total station

There exists an increasing demand for higher accuracy, faster processing and ease of use of modern total stations. The purpose of my work is to combine the strength of traditional user controlled surveying with the power of modern data processing to satisfy the needs. The combination of the user’s experience and a higher degree of automation retains the efficiency of a manually operated theodolite and enhances the reliability and accuracy of measurements through automation. The user identifies his targets mainly by their ‘structure’, which he usually interprets as simple geometrical shapes. Such ‘primitive’ features, however, can be handled effectively by algorithms to either identify and measure single points or to guide the instrument to areas of interest. Thus the main goal is to find the 3D coordinates of a non-cooperative but structured target by using a theodolite together with an imaging sensor. The surveyor no longer has to rely on active or cooperative targets like prisms, and this new freedom facilitates his work tremendously. However, the integration of 2D image sensors requires additional calibration effort. My thesis presents a prototype of such an “Image Assisted Total Station” (IATS), models the imaging process and outlines the calibration procedures. Image assisted measurements of artificial markers are compared with traditional measurements. The main effort, however, is focused on applications with natural objects: I try to assess the precision in terms of repeatability, the usability and the comfort of semi-automatic measurements. A Leica Total Station of the TPS1100 Professional Series is modified into a prototype of an IATS. A 2D CCD sensor is placed in the intermediate focus plane of the objective lens, replacing the eyepiece and the reticle, and an autofocus unit to drive the focus lens is implanted. The image data from the sensor are transferred to a PC using a synchronized frame grabber. To maintain the mechanical stability, the connecting cables transmitting the video signals are guided through the hollow tilting axis. The pixel size of 9.8 μm (Hz) × 6.3 μm (V) corresponds to viewing angles of 2.7 mgon (Hz) × 1.8 mgon (V). To fulfill the specified precision requirements of 0.5 mgon, a resolution of better than 0.2 pixels is required. Traditional optical total stations measure ‘on-axis’ objects, i.e. determine both pointing angles of the reticle crosshair. In case of an IATS viewing angles can be assigned to all CCD pixels inside the optical field of view. To describe the relation between sensors pixel coordinates and the angular viewing angles in the object space, a mathematical model is needed, which describes the optics used, and which specifies the contributions of various sources to the overall error budget. In particular, the optical mapping model has to include the theodolites tilting axis errors, the collimation error, the pointing error of the optical axis, and the vertical-index error. Further errors result from a displacement of the projection center from the intersection of the standing and tilting axis and from the optical distortions of field points. The semi-automated measurement process is based on a permanent interaction between user and instrument. The user supervises the measurement sequence while the IATS executes the measurements. For example, the surveyor proposes a pattern – a geometrical ‘primitive’ – which adequately represents the object of interest. The processing software estimates the posi-