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Stereo imaging of the sea surface elevation provides unique field data to investigate the geometry and dynamics of oceanic waves. Typically, this technique allows retrieving the 4-D ocean topography (3-D space + time) at high frequency (up to 15–20 Hz) over a sea surface region of area ~104 m2. Stereo data fill the existing wide gap between sea surface elevation time-measurements, like the local observation provided by wave-buoys, and large-scale ocean observations by satellites. The analysis of stereo images provides a direct measurement of the wavefield without the need of any linear-wave theory assumption, so it is particularly interesting to investigate the nonlinearities of the surface, wave-current interaction, rogue waves, wave breaking, air-sea interaction, and potentially other processes not explored yet. In this context, this open dataset aims to provide, for the first time, valuable stereo measurements collected in different seas and wave conditions to invite the ocean-wave scientific community to continue exploring these data and to contribute to a better understanding of the nature of the sea surface dynamics., Measurement(s)stereo image • wave height • peak wave period • wave directionTechnology Type(s)Camera Device • computational modeling techniqueFactor Type(s)geographic locationSample Characteristic - Environmentsea surface layerSample Characteristic - LocationBlack Sea • Adriatic Sea • Yellow Sea • Iroise Sea Machine-accessible metadata file describing the reported data: 10.6084/m9.figshare.12181158

The use of spatial and spatio-temporal data is rapidly changing the paradigm of wind wave observations, which have been traditionally restricted to time series from single-point measurements (e.g. from buoys, wave gauges). Active and passive 2D remote sensors mounted on platforms, ships, airplanes and satellites are now becoming standards in the oceanographic community and industry. Given the covered area ranging from centimeters to kilometers, such sensors are now a valuable tool for ocean and coastal observations. In this paper, we inter compare spatio-temporal wind wave data acquired with two state-of-the-art techniques, namely the stereo wave imaging and the X-band marine radar. The comparison was performed by operating the two instruments on an oceanographic research platform during a crossing-sea condition. We analyzed the statistical properties of the wave field, and its directional and omni-directional energy distributions. From our analysis, we suggest that stereo data can be exploited to find the best radar Modulation Transfer Function and scale factor needed to estimate wave parameters. Moreover, the fusion of the two systems will allow to broaden the scales covered by any one measurement, and to retrieve reliable directional wave spectra from short (similar to 1 m) to mid-wavelengths (similar to 100 m).

The energy level and its directional distribution are key observations for understanding the energy balance in the wind-wave spectrum between wind-wave generation, nonlinear interactions, and dissipation. Here, properties of gravity waves are investigated from a fixed platform in the Black Sea, equipped with a stereo video system that resolves waves with frequency f up to 1.4 Hz and wavelengths from 0.6 to 11 m. One representative record is analyzed, corresponding to young wind waves with a peak frequency fp = 0.33 Hz and a wind speed of 13 m s−1. These measurements allow for a separation of the linear waves from the bound second-order harmonics. These harmonics are negligible for frequencies f up to 3 times fp but account for most of the energy at higher frequencies. The full spectrum is well described by a combination of linear components and the second-order spectrum. In the range 2fp to 4fp, the full frequency spectrum decays like f−5, which means a steeper decay of the linear spectrum. The directional spectrum exhibits a very pronounced bimodal distribution, with two peaks on either side of the wind direction, separated by 150° at 4fp. This large separation is associated with a significant amount of energy traveling in opposite directions and thus sources of underwater acoustic and seismic noise. The magnitude of these sources can be quantified by the overlap integral I(f), which is found to increase sharply from less than 0.01 at f = 2fp to 0.11 at f = 4fp and possibly up to 0.2 at f = 5fp, close to the 0.5π value proposed in previous studies.

Experiments on wind-waves in a laboratory tank are carried out for a variety of operational conditions. In addition to using a conventional wave gauge to determine the temporal variation of the surface elevation at the sensor location, independent measuring techniques such as 3D reconstruction of stereo video images and 2D laser slope gauge are applied in order to gain information on the three-dimensional structure of the wind-wave field. The consistency of results obtained by different methods under identical forcing conditions is examined to establish the limits of applicability and accuracy of each method. The accumulated results on the spatial and temporal coherence of the surface elevation variation, on the directional wave spectra, and on the probability distribution of the instantaneous surface slope directions demonstrate that the wind-wave field is essentially three-dimensional and short-crested. Published by AIP Publishing.

Large scale particle image velocimetry (LSPIV) has become a reliable and accurate technique to measure water surface velocity field in open channels and rivers. The LSPIV technique is based on a camera view framing the water surface, complemented with image-processing methods to compute water surface displacements between consecutive frames. Using LSPIV, high flow velocities, such as flood conditions, are accurately measured, whereas determinations of low flow regimes is more challenging, especially in the absence of floating material carried by the stream. In fact, in unseeded conditions, typical surface feature dynamics must be taken into account. Indeed, besides surface structures advected by the current, capillary-gravity waves propagate on the surface, with their own speed and direction. In this study, the discrimination between these phenomena is discussed, providing a new method to distinguish and to correct unseeded LSPIV measurements with capillary-gravity wave dynamics. Current velocity obtained with such a correction at low flow speed in an artificial channel and in a natural river is in fair agreement with reference observations indicating the need of accounting for the speed and direction of capillary-gravity waves on the surface to obtain reliable measurements of the current vector field, and, as a further result, of the flow discharge over a river cross-section.

Stereo wave imaging of the sea surface elevation has become an effective instrumentation to gather small- and medium-range 3-D wind wave data. Indeed, fruitful-applications of stereo techniques have provided new insights into directional wave spectra, space-time distributions of wave maxima, and small-scale wave statistics. So far, however, stereo systems have been deployed mainly on fixed structures (e.g. oceanographic platforms or lighthouses) in order to simplify the installation and maintenance procedures. Nonetheless, advances in stereo calibration and processing suggest that stereo deployments are also feasible onboard moving vessels, thus broadening the impact of these observations on the study of wind waves. In this context, this study aims at discussing how the stereo processing designed to gather reliable wave data from fixed structures should be managed to operate on a moving structure. In particular, estimate of stereo cameras orientation and position with respect to the mean sea plane is of utmost importance. We discuss this aspect by using a synthetic sea state and stereo data collected during an oceanographic campaign onboard a research vessel. Results suggest that, without complementary data sources for ship motion compensation, the sea surface elevation field should include at least about sixteen spatial (2-D) waves to gather a robust estimate of the mean sea plane and consequently realistic wave parameters (e.g. the significant wave height). In this respect, our results provide also insight into the uncertainty of estimates in case of a limited number of 2-D waves is collected by the stereo system. Finally, applications of stereo wave imaging on a moving structure are discussed, with particular emphasis on the collection of space-time wave fields for assessment of numerical models and operational wave observation onboard vessels. (C) 2015 Elsevier B.V. All rights reserved.

In the last few years we faced an increased popularity of stereo imaging as an effective tool to investigate wind sea waves at short and medium scales. Given the advances of computer vision techniques, the recovery of a scattered point-cloud from a sea surface area is nowadays a well consolidated technique producing excellent results both in terms of wave data resolution and accuracy. Nevertheless, almost all the subsequent analyses tasks, from the recovery of directional wave spectra to the estimation of significant wave height, are bound to two limiting conditions. First, wave data are required to be aligned to the mean sea plane. Second, a uniform distribution of 3D point samples is assumed. Since the stereo-camera rig is placed tilted with respect to the sea surface, perspective distortion do not allow these conditions to be met. Errors due to this problem are even more challenging if the optical instrumentation is mounted on a moving vessel, so that the mean sea plane cannot be simply obtained by averaging data from multiple subsequent frames. We address the first problem with two main contributions. First, we propose a novel horizon estimation technique to recover the attitude of a moving stereo rig with respect to the sea plane. Second, an effective weighting scheme is described to account for the non-uniform sampling of the scattered data in the estimation of the sea-plane distance. The interplay of the two allows us to provide a precise point cloud alignment without any external positioning sensor or rig viewpoint pre-calibration. The advantages of the proposed technique are evaluated throughout an experimental section spanning both synthetic and real-world scenarios. HighlightsWe propose an optical horizon estimation technique to recover the attitude of a moving stereo rig with respect to the sea plane.Projective distortion is corrected to account for the non-uniform distribution of the reconstructed point cloud.The alignment of each sea surface is improved without using external positioning sensors.

We develop a novel remote sensing technique for the observation of waves on the ocean surface. Our method infers the 3-D waveform and radiance of oceanic sea states via a variational stereo imagery formulation. In this setting, the shape and radiance of the wave surface are given by minimizers of a composite energy functional that combines a photometric matching term along with regularization terms involving the smoothness of the unknowns. The desired ocean surface shape and radiance are the solution of a system of coupled partial differential equations derived from the optimality conditions of the energy functional. The proposed method is naturally extended to study the spatiotemporal dynamics of ocean waves and applied to three sets of stereo video data. Statistical and spectral analysis are carried out. Our results provide evidence that the observed omnidirectional wavenumber spectrum S(k) decays as k-2.5 is in agreement with Zakharov's theory (1999). Furthermore, the 3-D spectrum of the reconstructed wave surface is exploited to estimate wave dispersion and currents.

Image analysis techniques are used for retrieving water surface elevation fields spatially and temporally from CCD-images and CCD-image-sequences. The technique proposed herein utilizes binocular stereogrammetry to recover topographic information from a sequence of synchronous, overlapping video images. The method used differs from the traditional stereo-photogrammetric analysis of a single stereo-pair because the use of video allows for a continuous sequence of stereo-images to be digitally sampled and analyzed. For data acquisition two synchronized progressive-scan cameras were used. A partially supervised 3-D stereo system (called WASS, Wave Acquisition Stereo System) is shown here. It is used to reconstruct the 3-D shape of water surface waves, acquired at frame rate, with small computational time needed. The stereo method is presented, including the derivation of a relationship relating the geometry of the stereo rig and the expected errors. Finally, the 3-D calculated scattered points give the complete spatio-temporal distribution of the water surface elevations. The measurable length-scales depend on the pixel resolution, the triangulation accuracy, and the acquisition frame rate. Limitations in the stereo measurements are also discussed. Two experiments to test and to demonstrate the system took place: one on the Venice lagoon, north of the city of Venice in September 2004 and the second on the coast of California at San Diego in December 2005. For the second experiment, qualitative and quantitative intercomparisons of the stereo-matching and in-situ sensor measurements are presented. All the measurements of water surface waves indicate that the proposed approach is both accurate and applicable for measuring water surface waves. Moreover, shape estimates are accurately and extremely dense both in space and time, and the remote location of the instruments alleviates some difficulties associated with in situ instrumentation.

Stereo video imaging of water surface has become an effective instrumentation to gather wind waves 3-D data from small to medium range spatial scales. Indeed, recent applications of stereo techniques provided new insights of space-time distributions of sea wave elevations, small scale wave statistics, and directional wave spectra. Like most photogrammetric applications, an accurate calibration of the optical acquisition machinery is required to provide a low-noise, precise and reliable surface reconstruction adequate to extract meaningful wavy surfaces. However, for practical open field applications, there is a strong interest to provide a calibration procedure apt to be performed in an uncomfortable environment in which it may be unfeasible to take apart or even physically access the device. Here, we propose a stereo pipeline suitable for 3-D wave measurements from fixed and moving platforms. In particular, within the Wave Acquisition Stereo System (WASS) framework, we contemplated a self-calibration technique for a robust estimation of the stereo extrinsic parameters, a fast dense stereo correspondence algorithm, and a two-step correction of the cameras motion. As for other applications, wave information collected by WASS includes synthetic wave parameters (e.g., significant wave height, wave periods, and directions), wavenumber and frequency-direction spectra, spatial distribution of wave elevations, heights, and lengths. The new pipeline features has been firstly assessed by installing WASS on top of the “Acqua Alta” oceanographic tower in the northern Adriatic Sea (Italy) and comparing WASS measurements against those acquired at the tower with reference instrumentation. Afterwards, the stereo system has been mounted on board the R/V “Urania” during a cruise on April 2013 in the southern Adriatic Sea. During the cruise, to correct for ship’s motion, WASS has been synchronized to the motion unit used for the vessel’s navigation. For validation, sea wave state gathered by WASS has been compared to theoretical models and results from the numerical wave model SWAN. Results presented show that the accuracy of 3-D waves provided by the new algorithms is comparable to that of other WASS applications, although significantly reducing the installation effort and the computational time by more than one order of magnitude. Additionally, encompassing for ship’s motion makes stereo system a perspective instrumentation for operational wave measuring from research and opportunity vessels. The manuscript is completed by a discussion on the limitations and troubleshooting related to the proposed pipeline.

In recent years, the study of the dynamics of ocean waves via stereo vision systems has focused on investigating sea states with wavelengths in the range of 0.01 m to 1 m. In this work, we present two modern stereo variational approaches, viz. the disparity and graph methods, for the estimation of the ocean surface displacements over larger spatial scales of 10 to 100 m. The disparity method estimates the disparity between images as an intermediate step toward retrieving the depth of the waves with respect to the cameras, and the graph method estimates the ocean surface displacements directly in 3-D space. The pros and cons of both methods are compared and their performance is evaluated using experimental video data collected at an offshore platform in the Black Sea.

Stereo video techniques are effective for estimating the space–time wave dynamics over an area of the ocean. Indeed, a stereo camera view allows retrieval of both spatial and temporal data whose statistical content is richer than that of time series data retrieved from point wave probes. We present an application of the Wave Acquisition Stereo System (WASS) for the analysis of offshore video measurements of gravity waves in the Northern Adriatic Sea and near the southern seashore of the Crimean peninsula, in the Black Sea. We use classical epipolar techniques to reconstruct the sea surface from the stereo pairs sequentially in time, viz. a sequence of spatial snapshots. We also present a variational approach that exploits the entire data image set providing a global space–time imaging of the sea surface, viz. simultaneous reconstruction of several spatial snapshots of the surface in order to guarantee continuity of the sea surface both in space and time. Analysis of the WASS measurements show that the sea surface can be accurately estimated in space and time together, yielding associated directional spectra and wave statistics at a point in time that agrees well with probabilistic models. In particular, WASS stereo imaging is able to capture typical features of the wave surface, especially the crest-to-trough asymmetry due to second order nonlinearities, and the observed shape of large waves are fairly described by theoretical models based on the theory of quasi-determinism (Boccotti, 2000). Further, we investigate space–time extremes of the observed stationary sea states, viz. the largest surface wave heights expected over a given area during the sea state duration. The WASS analysis provides the first experimental proof that a space–time extreme is generally larger than that observed in time via point measurements, in agreement with the predictions based on stochastic theories for global maxima of Gaussian fields.

Stereo video techniques are effective for estimating the space-time wave dynamics over an area of the ocean. Indeed, a stereo camera view allows retrieval of both spatial and temporal data whose statistical content is richer than that of time series data retrieved from point wave probes. To prove this, we consider an application of the Wave Acquisition Stereo System (WASS) for the analysis of offshore video measurements of gravity waves in the Northern Adriatic Sea. In particular, we deployed WASS at the oceanographic platform Acqua Alta, off the Venice coast, Italy. Three experimental studies were performed, and the overlapping field of view of the acquired stereo images covered an area of approximately 1100 m2. Analysis of the WASS measurements show that the sea surface can be accurately estimated in space and time together, yielding associated directional spectra and wave statistics that agree well with theoretical models. From the observed wavenumber-frequency spectrum one can also predict the vertical profile of the current flow underneath the wave surface. Finally, future improvements of WASS and applications are discussed.

We present an application of a novel Variational Wave Acquisition Stereo System (VWASS) for the estimation of the wave surface height of oceanic sea states. Specifically, we show that VWASS video technology combined with statistical techniques based on Euler Characteristics of random fields provides a new paradigm for the prediction of wave extremes expected over a given area of the ocean.

We propose a novel remote sensing technique that infers the three-dimensional wave form and radiance of oceanic sea states via a variational stereo imagery formulation. In this setting, the shape and radiance of the wave surface are minimizers of a composite cost functional which combines a data fidelity term and smoothness priors on the unknowns. The solution of a system of coupled partial differential equations derived from the cost functional yields the desired ocean surface shape and radiance. The proposed method is naturally extended to study the spatiotemporal dynamics of ocean waves, and applied to three sets of video data. Statistical and spectral analysis are carried out. The results shows evidence of the fact that the omni-directional wavenumber spectrum S(k) of the reconstructed waves decays as k−2.5 in agreement with Zakharov’s theory (1999). Further, the three-dimensional spectrum of the reconstructed wave surface is exploited to estimate wave dispersion and currents.Copyright © 2011 by ASME

This report describes the implementation of a hyperspectral proximal sensing low-budget acquisition system and its application to the detection of terrestrian vegetation cover anomalies in sites of high environmental quality. Anomalies can be due to stress for lack of water and/or pollution phenomena and weed presence in agricultural fields. The hyperspectral cube (90-bands ranging from 450 to 900 nm) was acquired from the hill near Segni (RM), approximately 500 m far from the target, by means of electronically tunable filters and 8 bit CCD cameras. Spectral libraries were built using both endmember identification method and extraction of centroids of the clusters obtained from a k-means analysis of the image itself. Two classification methods were applied on the hyperspectral cube: Spectral Angle Mapper (hard) and Mixed Tuned Matching Filters (MTMF). Results show the good capability of the system in detecting areas with an arboreal, shrub or leafage cover, distinguishing between zones with different spectral response. Better results were obtained using spectral library originated by the k-means method. The detected anomalies not correlated to seasonal phenomena suggest a ground true analysis to identify their origin.