The 3 farthest outliers from the median of the 13 values are rejected. A line is fitted to the remaining points, providing the surface location and slope directly above the sonar. Dropouts introduced by dense bubble clouds are an issue, and this procedure was developed to minimize the effect. The wave is propagating from right to left. Over time this bubble cloud penetrates to between 2 and 3 m depth.
Arrows indicate estimated vertical velocities of each square at the time of the picture.
A position closer to the surface would both reduce the along-wave cell size and increase the permissible sample rate. For example, from 5 m below the surface horizontal resolution at the surface becomes 10 cm, and sample rates up to 150 frames per second are possible. It would appear that much could be learned about the velocity and turbulence structure under breaking waves by this approach. This trace is characterized by motions near 8 and 5 second periods.