Kinematic Analysis of the Reconstruction Error of A Calibration Volume for 3d Analysis in Swimming

Abstract

The purpose of this study was to assess the accuracy and reliability of the three-dimensional reconstruction - above and below water - of a calibration volume for three-dimensional analysis in swimming. The calibration volume (3x3x3m) was placed in a 25m x 12.5m x 2m swimming pool and was recorded simultaneously by 4 under and 2 above water synchronized cameras. To assess the number of control points required to maximise the accuracy of threedimensional coordinate reconstruction, 12 markers in the calibrated space were digitised over 10 fields for each underwater and above water camera views. Seven series of 12 markers, using 8, 12, 16, 20, 24, 28 and 30 control points were digitised. Direct linear transformation methods were used to estimate the marker locations on the volume. Comparison among different numbers of control points showed that the set of 20 (underwater) and 16 (above water) points produced the most accurate results. The average root mean square errors were (x, y and z, respectively): (i) 4.85mm, 2.52mm and 7.43mm (set of 20 digitised underwater points) and (ii) 4.11mm, 5.69mm and 3.90mm (set of 16 digitised above water points). The standard deviation in underwater cameras was 1.22mm, 0.33mm and 3.47mm, and 1.57mm, 2.63mm and 2.35mm for above water cameras (for x, y and z, respectively). The calibration volume was found to have high accuracy and reliability.