4D Mapping of Sea State by Stereo Photographic Imaging
- Antonie Oosterkamp (Uni Research Polytec) | Sigmund Clausen (Uni Research Polytec) | Torleif Lothe (Uni Research Polytec)
- Document ID
- International Society of Offshore and Polar Engineers
- The 29th International Ocean and Polar Engineering Conference, 16-21 June, Honolulu, Hawaii, USA
- Publication Date
- Document Type
- Conference Paper
- 2019. International Society of Offshore and Polar Engineers
- waves, measurement, ocean, optical methods, image recognition
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- 24 since 2007
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Spectral and spatial data from waves are traditionally gathered with single point instruments like buoys or wave gages. These provide accurate timeseries of the sea surface movements but are limited to a singular location. To track the sea surface movement over a larger area is possible with satellite, lidar or radar imaging. The major challenge with these methods is the lack of spatial accuracy. Here optical systems have their merits. This work has the objective to further the principle of stereographic measurement of ocean waves to a practical applicable method. We describe the development of a field employable instrument and practical image recognition algorithms. The results of initial tests and experience with the practical use of the method are presented. Future work will focus on investigation of the accuracy and dependency of the system and to obtain real time analysis.
The spectral and spatial data from waves are traditionally gathered with single point instruments such as buoys or, wave gages. These provides accurate timeseries of the movement of the sea surface at singular locations. Over the years, significant progress has been made in developing the theory and methods to deduce overall wave dynamics from such singular point data.
To track the movement of the sea surface over a larger area can be done with satellite, lidar or radar imaging. The major drawback of these technologies is their lack of spatial accuracy. In this respect, optical systems are regarded to have a performance advantage.
Stereographic imaging of ocean waves is described and discussed several publications, some from as early as the beginning of the previous century (Laas, 1905, Mason, 1942). During the last three decades, major advances have been made in digital image acquisition and image processing, making stereographic wave measurements a viable technique today.
Recent publications in the field show a progression of the state of art towards practical useable systems. MacHutchon and Liu (2007) demonstrated a system with three cameras. The camera resolution is 640x480 pixels. A so-called area-feature hybrid matching algorithm is used to correlate image pairs to derive a point cloud representing the sea surface. This algorithm contains two steps. First specular reflection is dealt with, to establish matches between corresponding points of the left and right camera images. Secondly, a pixel-based box search is conducted and the matches assessed with normalized cross-correlation. The resulting disparities can then be used to construct a 3d image of the sea plane. Benetazzo (2006) also describes a similar method to derive the 3D sea surface from matched pairs of stereo images. Benetazzo (2006) also uses an algorithm based on finding individual corresponding pixels through cross-correlation. Instead of a regular box search, a so-called pyramidical process (hierarchical) is used, starting with a few pixels on a regular grid on one the camera frames. This grid is refined in subsequent steps. For each pixel of the grid, the corresponding pixels are sought at the equivalent row of the other image. The pixel with the highest cross-correlation coefficient is chosen as the corresponding point, but only when it is above a threshold value. Benatazzo (2006) tested his methodology with both 480∗640 resolution cameras and 1008x1008 (1 MP) resolution video cameras. The distance between the cameras was 1.88 m. In subsequent trials, Benetazzo et al. (2012) used a system of two cameras mounted on an oceanographic platform. The distance between the pair of 5 MP cameras was 2.5 m and measuring a 100x100 m area. From the measurements the wave spectra were computed. To calculate the wave spectra, virtual probes were used on in the x-y plane of the surface displacement time series. Benetazzo et al. (2012) found the accuracy to be comparable to traditional wave measurement instruments. Benetazzo et al. (2012) also concluded that the wave spectrum can be derived from the reconstructed spatial sea surface, and that by Fourier transformation of the space time data set, wave dispersion and currents can be estimated. Statistics of crest to through heights further showed good agreement with the Forristall and Tayun-Fedele distributions. Benetazzo et al (2016) discuss the use on moving platforms (vessels) and the necessary calibration procedures. In their publication they emphasize the importance of appropriate systems to transfer the 3D wave data from the camera’s image plane to the sea reference plane.
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