Motion capture system validation with surveying techniques

Abstract

Motion capture systems are widely used in biomechanics, thus users must consider system errors when evaluating their results. The usually applied validation techniques of these systems are based on relative distance measurements. In contrast, our study aimed to analyse the absolute volume accuracy of an optical motion capture system composed of 18 OptiTrack Flex13 cameras with external reference points that were calibrated using surveying methods. Calibration was based on a geodetic reference network. We demonstrated the benefits of using this method compared to the validation based on relative distance measurements, and we determined the absolute accuracy that describes the difference between the reference and measured coordinates of the markers from the geodetic and OptiTrack coordinate systems, respectively. Camera number dependency of precision expressed by the static detection noise of the marker coordinates was also quantified. The absolute accuracy provided a root-mean-square error (RMSE) of 1.735 mm owing to improper extrinsic calibration, a scaling error that is undetected by conventional validation techniques. The observed noise was in a sub-millimeter range, and showed progressive improvement up to 15 cameras.