# A New Sensor System for Accurate 3D Surface Measurements and Modeling of Underwater Objects
## 3 Experiments and Results
### Measurements with Moved Sensor  
First measurements with a moving sensor were performed in air in the laboratory. A constant linear motion with known velocity (0.2 m/s and 1.0 m/s) of the sensor was realized using a traversing track.  
Video images were corrected using the known parameters for \(c,f,ps\), and \(b\) and the estimated quantities \(d=d_{0}=2\) m, \(v=0.2\) m/s (1.0 m/s), and \(\alpha=0^{\circ}\).  
Figure 6 shows the resulting 3D point clouds of the measurement object cylinder, two ball bars, and a plaster bust without and with motion compensation at a velocity of 0.2 m/s and a 375 Hz 2D framerate. Results at 1.0 m/s velocity were not completely satisfying and should be improved in the future.  
\begin{table}
\begin{tabular}{c c c c} \hline \multirow{2}{*}{**Distance [m]**} & **Noise on Sphere1** & **Noise on Sphere2** & **Noise on Plane [mm]** \\ \cline{1-2}  & **[mm]** & **[mm]** & **[mm]** \\ \hline
1.5 & 0.074 \(\pm\) 0.01 1 & 0.074 \(\pm\) 0.01 1 & 0.065 \(\pm\) 0.02 2 \\ \hline
1.6 & 0.059 \(\pm\) 0.01 & 0.073 \(\pm\) 0.02 & 0.075 \(\pm\) 0.02 \\ \hline
1.7 & 0.062 \(\pm\) 0.01 & 0.075 \(\pm\) 0.01 & 0.070 \(\pm\) 0.01 \\ \hline
1.8 & 0.085 \(\pm\) 0.01 & 0.081 \(\pm\) 0.01 & 0.080 \(\pm\) 0.02 \\ \hline
1.9 & 0.071 \(\pm\) 0.01 & 0.096 \(\pm\) 0.01 & 0.090 \(\pm\) 0.01 \\ \hline
2.0 & 0.078 \(\pm\) 0.01 & 0.078 \(\pm\) 0.01 & 0.085 \(\pm\) 0.01 \\ \hline
2.1 & 0.087 \(\pm\) 0.01 & 0.089 \(\pm\) 0.01 & 0.095 \(\pm\) 0.01 \\ \hline
2.2 & 0.114 \(\pm\) 0.01 & 0.093 \(\pm\) 0.01 & 0.115 \(\pm\) 0.03 \\ \hline
2.3 & 0.098 \(\pm\) 0.01 & 0.089 \(\pm\) 0.01 & 0.120 \(\pm\) 0.02 \\ \hline
2.4 & 0.133 \(\pm\) 0.01 & 0.104 \(\pm\) 0.01 & 0.130 \(\pm\) 0.02 \\ \hline average & 0.086 \(\pm\) 0.23 & 0.085 \(\pm\) 0.13 & 0.080 \(\pm\) 0.13 \\ \hline \end{tabular} \({}^{1}\) Standard deviation values on spheres are obtained from two to four independent measurements. 2 Standard deviation values on planes are obtained from ten independent surface positions.  
\end{table}
Table 2: Results of noise determination on sphere surfaces and plane normal depending on object distance; refinement does not have any influence on the noise values.  
Figure 6: 3D measurement results of the reconstruction of the air measurement with sensor velocity of 0.2 m/s without motion compensation (**left**) and with motion compensation (**right**). Note the higher completeness at the margins of the objects with motion compensation.  
Subsequent experiments concerning motion compensation were performed in the water basin. Two different velocities were realized: 0.1 m/s and 0.7 m/s. The maximal sensor velocity of 1 m/s was not yet tested underwater. Motion compensation (MC) was realized manually (for reference), as well as automatically, using the implemented compensation algorithm. Figure 7 shows the results of manual and automatic motion compensation at 0.7 m/s velocity compared to the measurement result without motion compensation (left) with the example of a plastic pipe.  
Table 3 documents the standard deviations of the 3D points from the fitted sphere and cylinder shape, respectively. Motion compensation provides a significant reduction in the standard deviation. Automatic motion compensation gives comparable results to manual compensation.