scholarly journals Sea State from Single Optical Images: A Methodology to Derive Wind-Generated Ocean Waves from Cameras, Drones and Satellites

2021 ◽  
Vol 13 (4) ◽  
pp. 679
Author(s):  
Rafael Almar ◽  
Erwin W. J. Bergsma ◽  
Patricio A. Catalan ◽  
Rodrigo Cienfuegos ◽  
Leandro Suarez ◽  
...  

Sea state is a key variable in ocean and coastal dynamics. The sea state is either sparsely measured by wave buoys and satellites or modelled over large scales. Only a few attempts have been devoted to sea state measurements covering a large domain; in particular its estimation from optical images. With optical technologies becoming omnipresent, optical images offer incomparable spatial resolution from diverse sensors such as shore-based cameras, airborne drones (unmanned aerial vehicles/UAVs), or satellites. Here, we present a standalone methodology to derive the water surface elevation anomaly induced by wind-generated ocean waves from optical imagery. The methodology was tested on drone and satellite images and compared against ground truth. The results show a clear dependence on the relative azimuth view angle in relation to the wave crest. A simple correction is proposed to overcome this bias. Overall, the presented methodology offers a practical way of estimating ocean waves for a wide range of applications.

Author(s):  
Y Pomeau ◽  
T Jamin ◽  
M Le Bars ◽  
P Le Gal ◽  
B Audoly

In a wide range of conditions, ocean waves break. This can be seen as the manifestation of a singularity in the dynamics of the fluid surface, moving under the effect of the fluid motion underneath. We show that, at the onset of breaking, the wave crest expands in the spanwise direction as the square root of time. This is first derived from a theoretical analysis and then compared with experimental findings. The agreement is excellent.


2006 ◽  
Vol 23 (11) ◽  
pp. 1559-1572 ◽  
Author(s):  
Jennifer L. Irish ◽  
Jennifer M. Wozencraft ◽  
A. Grant Cunningham ◽  
Claudine Giroud

Abstract In December 1999, a nonintrusive directional lidar wave gauge (LWG) was field tested at the Field Research Facility (FRF) in North Carolina. The LWG uses proven lidar technology to directly measure water surface elevation from above the water’s surface. Therefore, unlike bottom-mounted gauges, the surface elevation measurement from the LWG is independent of other hydrodynamic processes. The LWG prototype consists of four rangefinders, where each collects water surface elevation time series at a rate of 10 Hz. During the field test, ground truth data were collected concurrently with the LWG data using a biaxial current and pressure meter mounted directly beneath the LWG. Additional ground truth data were available from the existing FRF wave-gauging infrastructure. This paper describes principles of LWG operation and field experiment setup, LWG environmental and technical constraints, and LWG performance in measuring spectral wave parameters.


Author(s):  
Safa M Aldarabseh ◽  
Parviz Merati

Abstract This experiment was done to predict the evaporation rate from the wavy water surface under the different convection regimes ( free, forced, and mixed) at turbulent airflow conditions over a wide range of the ratio(Gr/Re2 ). Evaporation rate from wavy water surface is strongly affected by combinations between wave steepness and main airflow velocity above the wavy water surface. Experimental results show that no pattern can be followed for which combinations of evaporation rate will increase. Thus, only two facts can be noticed: the evaporation rate is larger than that measured under the same airflow velocity conditions with no waves existing on evaporated water surface because the airflow is smooth and attached along the still water surface and when increasing the wave steepness(H/L,H/T), Airflow will separate at the lee side of wave crest near to the bottom of the wave trough. Thus, vortex will generate in the airflow separation region. These vortexes are unstable and cause an increase in turbulence, reducing the water surface's resistance to vertical transport water vapor and increasing the evaporation rate. Also, experimental results show that the evaporation rates are somewhat less than that measured under the same airflow velocity with smaller wave steepness due to air trapped region observed at the leeside of the wave crest near the bottom of the wave trough. Evaporation rate is increasing with increase airflow velocity under the same convection regime.


Author(s):  
R. Agha Taher ◽  
M. Jafari ◽  
M. Fallah ◽  
A. Alavi

Protecting the living environment has become one of the greatest human concerns; sudden increase of population, industry and technology development, unrestrained over consumption of the citizens and climate changes, have all caused many problems for mankind. Shores are special zones that are in contact with three Atmosphere, Hydrosphere and Lithosphere environments of earth. Shore lines are of the most important linear features of the earth’s surface which have an animated and alive nature. In this regard, optimized management of the shores and environmental protection for stable development require observing the shorelines and their variations. Protection of shorelines within appropriate time periods is of high importance for the purpose of optimized management of the shores. The twenty first century has been called the era of information explosion. A time that, through benefits of new technologies, information experts attempt to generate more and up to date information in various fields and to provide them for managers and decision makers in order to be considered for future planning and also to assist the planners to arrange and set a comprehensive plan. <br><br> Aerial images and remote sensing technology are economic methods to acquire the required data. Such methods are free from common time and place limitations in survey based methods. Among remote sensing data, the ones acquired from optical images have several benefits which include low cost, interpretation simplicity and ease of access. That is why most of the researches concerning extraction of shorelines are practiced using optical images. On the other hand, wide range coverage of satellite images along with rapid access to them caused these images to be used extensively for extracting the shorelines. <br><br> The attempt in this research is to use satellite images and their application in order to study variations of the shorelines. Thus, for this purpose, Landsat satellite images from TM and ETM+ sensors in the 35 time period has been used. In order to reach better results, images from MODIS satellite has been used as auxiliary data for the images that are with an error margin. Initial classification on the images was conducted to distinguish water and non water applications. Neural network classification was applied with specific scales on the images and the two major applications were thereby extracted. Then, in order to authenticate the proceedings, Error matrix and Kappa coefficient has been applied on the classified images. Base pixel method of neural network was used for the purpose of information extraction while authenticity of that was evaluated too. The outcomes display the trend of Urmia shoreline has been approximately constant between the years of 1976 to 1995 and has experienced very low variations. In 1998 the lake experienced increase of water and therefore advancement of the shoreline of the lake due to increase of precipitation and the volume of inflowing water to the basin. During 2000 to 20125, however, the lake’s shoreline has experienced a downward trend, which was intensified in 2007 and reached to its most critical level ever since, that is decreasing to about one third. <br><br> Further, temporal and spatial potentiality evaluation of clouds seeding in Urmia lake zone has been studied as a solution for improvement and recovery of the current status of the lake, and an algorithm was proposed for optimized temporal- spatial study on could seeding. Ecological, meteorological and synoptic data were used for timing study of the cloud’s seeding plan, which upon study; it is easy to evaluate precipitation potential and quality of the system. At the next step, the rate of humidity and also stability of the precipitating system can be analyzed using radar acquired data. Whereas extracted date from MODIS images are expressing the spatial position, therefore in order to study the location of the cloud’s seeding, MODIS images of the selected time intervals along with applying MCM algorithm were used to conclude thick clouds. Also, with interpolation of the TRMM data, it is possible to deduce maximum precipitation in the form of spatial arena. One of the data categories that is used both for temporal and spatial analysis is radar images which in addition to time, displays the existing humidity range, movement direction, and positions of accumulated precipitation cores. Therefore, using this algorithm, it is possible to conclude the most optimized spatial position in order to execute the seeding plan.


2021 ◽  
Vol 13 (2) ◽  
pp. 195
Author(s):  
He Wang ◽  
Jingsong Yang ◽  
Jianhua Zhu ◽  
Lin Ren ◽  
Yahao Liu ◽  
...  

Sea state estimation from wide-swath and frequent-revisit scatterometers, which are providing ocean winds in the routine, is an attractive challenge. In this study, state-of-the-art deep learning technology is successfully adopted to develop an algorithm for deriving significant wave height from Advanced Scatterometer (ASCAT) aboard MetOp-A. By collocating three years (2016–2018) of ASCAT measurements and WaveWatch III sea state hindcasts at a global scale, huge amount data points (>8 million) were employed to train the multi-hidden-layer deep learning model, which has been established to map the inputs of thirteen sea state related ASCAT observables into the wave heights. The ASCAT significant wave height estimates were validated against hindcast dataset independent on training, showing good consistency in terms of root mean square error of 0.5 m under moderate sea condition (1.0–5.0 m). Additionally, reasonable agreement is also found between ASCAT derived wave heights and buoy observations from National Data Buoy Center for the proposed algorithm. Results are further discussed with respect to sea state maturity, radar incidence angle along with the limitations of the model. Our work demonstrates the capability of scatterometers for monitoring sea state, thus would advance the use of scatterometers, which were originally designed for winds, in studies of ocean waves.


2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Sakthi Kumar Arul Prakash ◽  
Conrad Tucker

AbstractThis work investigates the ability to classify misinformation in online social media networks in a manner that avoids the need for ground truth labels. Rather than approach the classification problem as a task for humans or machine learning algorithms, this work leverages user–user and user–media (i.e.,media likes) interactions to infer the type of information (fake vs. authentic) being spread, without needing to know the actual details of the information itself. To study the inception and evolution of user–user and user–media interactions over time, we create an experimental platform that mimics the functionality of real-world social media networks. We develop a graphical model that considers the evolution of this network topology to model the uncertainty (entropy) propagation when fake and authentic media disseminates across the network. The creation of a real-world social media network enables a wide range of hypotheses to be tested pertaining to users, their interactions with other users, and with media content. The discovery that the entropy of user–user and user–media interactions approximate fake and authentic media likes, enables us to classify fake media in an unsupervised learning manner.


Author(s):  
Céline Drouet ◽  
Nicolas Cellier ◽  
Jérémie Raymond ◽  
Denis Martigny

In-service monitoring can help to increase safety of ships especially regarding the fatigue assessment. For this purpose, it is compulsory to know the environmental conditions encountered: wind, but also the full directional wave spectrum. During the EU TULCS project, a full scale measurements campaign has been conducted onboard the CMA-CGM 13200 TEU container ship Rigoletto. She has been instrumented to measure deformation of the ship as well as the sea state encountered during its trip. This paper will focus on the sea state estimation. Three systems have been installed to estimate the sea state encountered by the Rigoletto: An X-band radar from Ocean Waves with WAMOS® system and two altimetric wave radars from RADAC®. Nevertheless, the measured significant wave height can be disturbed by several external elements like bow waves, sprays, sea surface ripples, etc… Furthermore, ship motions are also measured and can provide another estimation of the significant wave height using a specific algorithm developed by DCNS Research for the TULCS project. As all those estimations are inherently different, it is necessary to make a fusion of those data to provide a single estimation (“best estimate”) of the significant wave height. This paper will present the data fusion process developed for TULCS and show some first validation results.


2013 ◽  
Vol 117 (1197) ◽  
pp. 1075-1101 ◽  
Author(s):  
S. M. Parkes ◽  
I. Martin ◽  
M. N. Dunstan ◽  
N. Rowell ◽  
O. Dubois-Matra ◽  
...  

Abstract The use of machine vision to guide robotic spacecraft is being considered for a wide range of missions, such as planetary approach and landing, asteroid and small body sampling operations and in-orbit rendezvous and docking. Numerical simulation plays an essential role in the development and testing of such systems, which in the context of vision-guidance means that realistic sequences of navigation images are required, together with knowledge of the ground-truth camera motion. Computer generated imagery (CGI) offers a variety of benefits over real images, such as availability, cost, flexibility and knowledge of the ground truth camera motion to high precision. However, standard CGI methods developed for terrestrial applications lack the realism, fidelity and performance required for engineering simulations. In this paper, we present the results of our ongoing work to develop a suitable CGI-based test environment for spacecraft vision guidance systems. We focus on the various issues involved with image simulation, including the selection of standard CGI techniques and the adaptations required for use in space applications. We also describe our approach to integration with high-fidelity end-to-end mission simulators, and summarise a variety of European Space Agency research and development projects that used our test environment.


Drones ◽  
2018 ◽  
Vol 2 (4) ◽  
pp. 36 ◽  
Author(s):  
Vassilios Krassanakis ◽  
Matthieu Perreira Da Silva ◽  
Vincent Ricordel

The present article describes an experimental study towards the examination of human visual behavior during the observation of unmanned aerial vehicles (UAVs) videos. Experimental performance is based on the collection and the quantitative & qualitative analysis of eye tracking data. The results highlight that UAV flight altitude serves as a dominant specification that affects the visual attention process, while the presence of sky in the video background seems to be the less affecting factor in this procedure. Additionally, the main surrounding environment, the main size of the observed object as well as the main perceived angle between UAV’s flight plain and ground appear to have an equivalent influence in observers’ visual reaction during the exploration of such stimuli. Moreover, the provided heatmap visualizations indicate the most salient locations in the used UAVs videos. All produced data (raw gaze data, fixation and saccade events, and heatmap visualizations) are freely distributed to the scientific community as a new dataset (EyeTrackUAV) that can be served as an objective ground truth in future studies.


Sign in / Sign up

Export Citation Format

Share Document