# Deepwater 3D Measurements with a Novel Sensor System
## 4 Discussion  
In this paper, an underwater 3D sensor system is introduced, which enables high-resolution 3D capture as well as the simultaneous acquisition of color data using active stereo scanning with structured lighting, producing highly accurate and detailed 3D images for the close-range inspection of industrial structures and other objects. Three-dimensional models of industrial facilities such as piping systems, offshore wind farm foundations, anchor chains, and other structures were identified at deep depths of up to 1000 m.  
The newly developed underwater sensor system enables the mobile acquisition of three-dimensional structures in real-time with a high level of detail in both clear water and natural waters such as the Baltic Sea. Up to 50 frames of 3D data can be generated per second. The high 3D measurement accuracy was largely achieved by paying particular attention to the development of the calibration process for the underwater 3D sensor system. The calibration procedure is based on the geometric modeling of visual rays, considering the refraction at media transitions. Additional refinement processes also reduce the systematic measurement error of the sensor system. This system performs the motion estimation and correction of disturbing motion effects as well as the fusion of individual scans into a globally consistent 3D model. For mobile measurements, an improvement in the quality of the 3D point cloud was achieved through automatic motion compensation. When testing in clear water, measurement results were achieved with a very low systematic error. These errors are within the same accuracy range as measurements in air over a comparable measurement volume. The offshore measurements showed an increase in the random length measurement error with repeated measurements at the same measurement distance compared to the water basin measurement. Additionally, there was a slight scaling error of under one percent, which still represents high accuracy. With the successful completion of the project and the realization of the two demonstrators of the deep underwater 3D sensor system, the suitability of the measurement principles used for the precise 3D surface recording and 3D modeling of underwater structures is shown. The mobile mapping of the entire covered area, e.g., the seabed, based on visual odometry was successfully demonstrated. Figure 11: Photograph of three dummy bombs and the 3D reconstruction result of one of the bombs. Figure 10: Photograph (**left**), 3D reconstruction result obtained via VO (**middle**) with mapped color information (**right**) of a clay pot. Unfortunately, influences caused by the vibration of the sensor support system or temperature fluctuations could not be investigated in the experiments. However, measures have already been taken to limit these influences using a carbon rod for the mechanical connection of the measuring elements and the development of a cooling system that dissipates the heat generated by the cameras and the projection unit via the housing to the surrounding water. However, it is to be expected that a potentially significant influence on measurement accuracy remains. Determining the impact of these influences on measurement accuracy should be part of future experiments. Due to the dissimilarity compared to other optical underwater 3D measurement systems, it is difficult to make exact quantitative comparisons in terms of performance and measurement accuracy. In terms of quality, it can be said that compared to other SLAM-capable systems, e.g., the systems described in [10; 38], a significantly higher level of detail can be achieved through the 3D scans from the two measuring cameras. Compared to sonar-based systems [2], the measurement accuracy is likely to be many times higher. However, the speed of large-scale acquisition and reconstruction, for example, of the seabed or a structure, is correspondingly lower. The optical mapping system introduced by [PERSON] et al.