Self-Calibration and Crosshair Tracking with Modular Digital Imaging Total Station

Abstract

The combination of a geodetic total station with a digital camera opens up the possibilities of digital image analysis of the captured images together with angle measurement. In general, such a combination is called image-assisted total station (IATS). The prototype of an IATS called MoDiTa (Modular Digital Imaging Total Station) developed at i3mainz is designed in such a way that an existing total station or a tachymeter can be extended by an industrial camera in a few simple steps. The ad hoc conversion of the measuring system opens up further areas of application for existing commercial measuring systems, such as high-frequency aiming, autocollimation tasks or tracking of moving targets. MoDiTa is calibrated directly on site using image-processing and adjustment methods. The crosshair plane is captured for each image and provides identical points in the camera image as well as in the reference image. However, since the camera is not precisely coaxially mounted and movement of the camera cannot be ruled out, the camera is continuously observed during the entire measurement. Various image-processing algorithms determine the crosshairs in the image and compare the results to detect movement. In the following, we explain the self-calibration and the methods of crosshair detection as well as the necessary matching. We use exemplary results to show to what extent the parameters of self-calibration remain valid even if the distance and thus the focus between instrument and target object changes. Through this, one calibration is applicable for different distances and eliminates the need for repeated, time-consuming calibrations during typical applications.