RECONSTRUCTION ACCURACY FOR VISUAL CALIBRATION METHOD

Motion analysis is a common technique in biomechanics and sport studies. Since Abdel-Aziz and Karara (1971), the direct linear transformation (DLT) is the most widely used method to analyse human movements. More recently, Drenk et al. (1999) proposed a modified DLT method, called DLT double-plane method (DLT DP), which involves two parallel control planes (rather than a whole 3D structure). With these two calibration methods, a set of coefficients is calculated. This set summarise indirectly the internal and external parameters of the camera. Kwon et al. (2002) and Elipot et al. (2008) have respectively shown that, in aerial and underwater conditions, the DLT DP method can reduce the reconstruction error

MEASURING INSTANTANEOUS VELOCITY IN FOUR SWIM STROKES USING AN AUTOMATIC HEAD TRACKING SYSTEM: A COMPARISON STUDY

This study compared instantaneous swimming velocity from an automated video-based system to a tethered speedometer. Twenty-two state- and national-level swimmers (7 M, 15 F; 14.5 ± 2.5 yrs) swam 25 m of each stroke at maximal intensity. Bland-Altman plots showed good agreement between systems for backstroke and freestyle but poorer agreement for butterfly and breaststroke. The RMS error was also lower in backstroke and freestyle compared to butterfly and breaststroke. The differences in systems may be explained by the different body segments tracked by each system (head vs hips) and with differences being more apparent during butterfly and breaststroke due to the wave-like motion of these strokes. While the automated video-based system is suitable for measuring instantaneous swimming velocity, coaches, sports scientists, and swimmers should be aware of larger discrepancies between systems when assessing butterfly and breaststroke

THREE-DIMENSIONAL INVERSE DYNAMICS LINKED SEGMENT MODEL IN SWIMMING

This study aimed to produce a three-dimensional (3-D) inverse dynamics linked segment model capable of utilising kinematic data of a subject simulating front crawl to determine joint rotations and moments. This model is intended to address the current gap in swimming performance literature concerning the measurement of dynamic loads acting on an athlete during swimming. Joint moments derived from this model are to be used to specify actuator requirements for a biomimetic robotic system which will then be used to replicate human swimming techniqu

Observed basin-scale response of the North Atlantic Meridional Overturning Circulation to wind stress forcing

The response of the North Atlantic Meridional Overturning Circulation (MOC) to wind stress forcing is investigated from an observational standpoint, using four time series of overturning transports below and relative to 1000 m, overlapping by 3.6 years. These time series are derived from four mooring arrays located on the western boundary of the North Atlantic: the RAPID WAVE array (42.5°N), the Woods Hole Oceanographic Institution Line W array (39°N), the RAPID MOC/MOCHA array (26.5°N), and the MOVE array (16°N). Using modal decompositions of the analytic cross-correlation between transports and wind stress, the basin-scale wind stress is shown to significantly drives the MOC coherently at four latitudes, on the timescales available for this study. The dominant mode of covariance is interpreted as rapid barotropic oceanic adjustments to wind stress forcing, eventually forming two counter-rotating Ekman overturning cells centered on the tropics and subtropical gyre. A second mode of covariance appears related to patterns of wind stress and wind stress curl associated with the North Atlantic Oscillation, spinning anomalous horizontal circulations which likely interact with topography to form overturning cells

Can Drake Passage Observations Match Ekman's Classic Theory?

Ekman's theory of the wind-driven ocean surface boundary layer assumes a constant eddy viscosity and predicts that the current rotates with depth at the same rate as it decays in amplitude. Despite its wide acceptance, Ekman current spirals are difficult to observe. This is primarily because the spirals are small signals that are easily masked by ocean variability and cannot readily be separated from the geostrophic component. This study presents a method for estimating ageostrophic currents from shipboard acoustic Doppler current profiler data in Drake Passage and finds that observations are consistent with Ekman's theory. By taking into account the sampling distributions of wind stress and ageostrophic velocity, the authors find eddy viscosity values in the range of 0.08–0.12 m2 s−1 that reconcile observations with the classic theory in Drake Passage. The eddy viscosity value that most frequently reconciles observations with the classic theory is 0.094 m2 s−1, corresponding to an Ekman depth scale of 39 m

A Dataset of Hourly Sea Surface Temperature From Drifting Buoys

A dataset of sea surface temperature (SST) estimates is generated from the temperature observations of surface drifting buoys of NOAA's Global Drifter Program. Estimates of SST at regular hourly time steps along drifter trajectories are obtained by fitting to observations a mathematical model representing simultaneously SST diurnal variability with three harmonics of the daily frequency, and SST low-frequency variability with a first degree polynomial. Subsequent estimates of non-diurnal SST, diurnal SST anomalies, and total SST as their sum, are provided with their respective standard uncertainties. This Lagrangian SST dataset has been developed to match the existing hourly dataset of position and velocity from the Global Drifter Program

A dataset of hourly sea surface temperature from drifting buoys

A dataset of sea surface temperature (SST) estimates is generated from the temperature observations of surface drifting buoys of NOAA’s Global Drifter Program. Estimates of SST at regular hourly time steps along drifter trajectories are obtained by fitting to observations a mathematical model representing simultaneously SST diurnal variability with three harmonics of the daily frequency, and SST low-frequency variability with a first degree polynomial. Subsequent estimates of non-diurnal SST, diurnal SST anomalies, and total SST as their sum, are provided with their respective standard uncertainties. This Lagrangian SST dataset has been developed to match the existing and on-going hourly dataset of position and velocity from the Global Drifter Program

Lagrangian Time Series Models for Ocean Surface Drifter Trajectories

This paper proposes stochastic models for the analysis of ocean surface trajectories obtained from freely-drifting satellite-tracked instruments. The proposed time series models are used to summarise large multivariate datasets and infer important physical parameters of inertial oscillations and other ocean processes. Nonstationary time series methods are employed to account for the spatiotemporal variability of each trajectory. Because the datasets are large, we construct computationally efficient methods through the use of frequency-domain modelling and estimation, with the data expressed as complex-valued time series. We detail how practical issues related to sampling and model misspecification may be addressed using semi-parametric techniques for time series, and we demonstrate the effectiveness of our stochastic models through application to both real-world data and to numerical model output.Comment: 21 pages, 10 figure