Strict geometric modelling concepts for underwater laser triangulation
Final Report Abstract
In this research project a measurement principle was developed which allows highly accurate optical underwater 3D measurements. The method is based on the established method of laser triangulation, in which a laser line is projected onto an object and this is observed by a camera. The laser line is deformed depending on the geometry of the object. By automatically detecting the deformed laser line in the image, the three-dimensional geometry of the object along the line can be derived. For this purpose, the system must be calibrated. In the calibration, the exact position and orientation of the camera and laser to each other is determined so that a 3D coordinate can be calculated by intersecting the laser plane with the image measurements. Applying laser triangulation techniques for 3D point coordinate determination underwater requires several modifications of the geometric and stochastic models for sensor modelling and calibration. The components of the triangulation sensor must be installed in a waterproof glass housing, which is then immersed into the water. If the camera and laser are in a common housing, they must be mounted oblique to the housing surface in a way that the laser light sheet and the camera's optical axis intersect near-orthogonally at the object. The laser light is refracted twice at the air – glass – water interfaces. This refraction has to be modeled strictly geometrically using the methods of multimedia photogrammetry. For the spatial resection of image observation and laser light sheet, Snell’s law has to be strictly considered. The refraction of the laser plane is modeled by individual sub-beams, each of which is refracted at the interfaces. In order to be able to calibrate the exact position and orientation of the camera and laser relative to the interfaces, a special calibration procedure was developed. A calibration field consisting of two planes is used. The upper plane has some gaps that allow light to pass through to the lower plane. Thus, from the connection of the gaps in the line on the upper level and the line pieces on the lower level, individual sub-beams within the laser plane can be modeled. The measurement principle was developed with the goal of enabling highly accurate measurements of 3D underwater geometries in a close range. Possible applications are surveys using an unmanned water vehicle platform, which can for instance be used for the provision of riverbed data as a basis for flood simulations in small catchments, in ecosystem monitoring, underwater habitat mapping or in sedimentation process monitoring. It may also be used to survey model water bottoms in hydromechanical laboratories. Beyond this, underwater laser triangulation also depicts an interesting 3D measurement tool on autonomous submarine vehicles for technical purposes such as pipeline welding, or for underwater archaeology. The developed measurement principle was evaluated in a laboratory test. For this purpose, a prototype sensor system was set up and used to measure various objects in a small water tank. The tests showed that underwater measurements with an accuracy of 0.4 mm in a range of up to 15 cm can be achieved with the developed method. This is consistent with the theoretical accuracy estimates that were also carried out in the project and qualifies the technique as a tool for a wide range of underwater 3D measurements.
Publications
- Strict geometric Calibration of an Underwater Laser Triangulation System. International Archives of Photogrammetry, Remote Sensing and Spatial Information Sciences (2021), No. XLIII-B2-2021, 689–692
Sardemann, H., Mulsow, C., Maas, H-G.
(See online at https://doi.org/10.5194/isprs-archives-XLIII-B2-2021-689-2021) - Accuracy Analysis of an Oblique Underwater Laser Lightsheet Triangulation System. PFG – Journal of Photogrammetry, Remote Sensing and Geoinformation Science (2022) Volume 90, Issue 1
Sardemann, H., Mulsow, C., Maas, H-G.
(See online at https://doi.org/10.1007/s41064-022-00196-x)
