A correlation based target finder for terrestrial laser scanning

Many calibration and registration methods for optical sensors require highly accurate and robust detection of markers. To date, many different approaches for detecting markers have been developed. However, all of these share certain disadvantages, depending on the optical sensor and their application. We have developed a novel approach for high-accuracy target point detection based on normed cross-correlation from symmetric patterns. The method has been specifically developed for terrestrial laser scanning, but may also work for other types of optical sensors due to its vision based properties. Our method is robust to noise and invariant to rotation, translation and perspective projection. It is scale invariant and also works in varying distances. In addition, the cross-correlation function allows quality control. Another advantage of this method is that only a few parameters need to be adjusted. Consequently, it is applicable in field test scenarios. The method does not require a certain target pattern. The only constraint on the target type is that after projecting it perspectively, a region around the target center has to remain symmetric and the approximate position of the target center has to be known. Our results demonstrate the robustness and accuracy of this method which we have validated in field applications.