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The sea level data observed by tide gauge stations are important basic observation information for monitoring sea level change and determining mean sea level. This paper analyzes the difference in relative sea level trends of tide gauges in different observation periods by sliding period method and time period accumulation method, discusses the influence of time period selection on the calculation of relative sea level change, and uses data of tide gauge stations on the east coast of the United States to supplement and verify the analysis results. The experimental results of tide gauges in China show that the maximum difference of relative sea level change rate in different observation periods is 27.5 mm/a, and the maximum difference of annual sliding results in adjacent years is 7.1 mm/a .With the accumulation of years in the observation period, the relative sea level change rate tends to increase at a constant speed, and in order to obtain the relative sea level change rate with the MSE (mean square error) of 0.5 mm/a, the observation years required by the tide gauges are 22− 28 years, and the determination of the relative sea level trends with the MSE of 0.2 mm/a requires 40− 54 years, which is consistent with the supplementary experimental results of tide gauge stations in USA. [ABSTRACT FROM AUTHOR]

Utilizing the data of water level, observed by tide gauges in Hong Kong, China from 1962 ‒ 2017, the variation of sea level, tidal harmonic constants and other character values of water level in the sea area are determined and analyzed in this paper. The results show that the average changing rate of mean sea level is 1.06mm/a, however, both the trend and scale of each tide gauge’ s mean sea level changing rate are different. For example, the rate of Chi Ma Wan is (-3.85 ± 0.61) mm/a, while the rate of Waglan Island is (4.12 ± 0.76) mm/a and the the rate of Tsim Bei Tsui is (0.32 ± 0.32) mm/a. From 1962 to 2017, Sa and Ssa of Quarry Bay, calculated annually, have poor stability, and the changing trends of harmonic constants of shallow water constituents are obvious relatively. Diurnal and semidiurnal constituents are nearly stable, but in 2004―2008 the amplitudes of M2, O1 and K1 vary from other years. The level of lowest normal low water and lowest astronomical tide increase in the rate of 0.18 mm/a and 0.27 mm/a respectively. [ABSTRACT FROM AUTHOR]

With the technical developments of poisioning and depth detecting, nowadays, the accuracy of sounding results is highly improved than before, while the effect of water correction still stays the same, which makes it become the main restraint for bathymetry precise. It is key to determine tidal time difference between tidal gauges in time - difference method, which is commonly used to conduct water correction. However, the influence of temporal variation of tidal time-difference on water correction is usually ignored, and the time period of observations to determine tidal time - difference is also selected freely. Focusing on these problems, the phenomenon and cause of temporal variation of tidal time-difference are analyzed from the perspective of the merging theory of tidal constituents. Daily determined and hourly determined tidal time differences are compared in value and meaning and the error formula of water correction in time-difference method is educed. Data experiments are conducted in one sea area to show the contrast effect of water correction by using daily and hourly determined time-difference of tide. The research reveals that it is more precise and reasonable to use hourly determined tidal time-difference, than to use daily determined one, which is currently utilized for water correction. [ABSTRACT FROM AUTHOR]

A method of detecting outlier of multibeam sounding with back propagation (BP) neural network is proposed in this paper for the complexity of bathymetric data of a ping. This paper constructs a training and learning algorithm for complex curve of multibeam single ping data for curve fitting based on the mapping function from input to output of BP neural network. Then it inspects the results from the previous steps lengthways by the correlation analysis of data of adjacent pings, and a vertical check to locate and remore outlier is also proposed. The experiment is conducted using the real bathymetric data, where there is a shipwreck in the middle. And also the result is compared with the combined uncertainty and bathymetry estimator (CUBE) algorithm, which is a popular method in detecting outlier of multibeam sounding at present. The experiment proves that the method proposed in this paper can detect the outlier more effectively. [ABSTRACT FROM AUTHOR]

In this paper, a method for inversion of sound velocity profile in multibeam survey is pre-sented. Firstly, the sound velocity profile samples are analyzed with empirical orthogonal function (EOF) and the reconstruction coefficients are calculated by sound velocity perturbation matrix and previous order of EOF. Secondly, the reconstruction coefficients of EOF are estimated by simulated annealing algorithm, the target function is constructed with the seafloor distortion which is obtained by multibeam sounding. Finally, the inversion of sound velocity profile is reconstructed. The analysis of examples show that the inversion of sound velocity profile is closer to the real sound velocity profile and the seafloor corrected with inversion of sound velocity profile is closer to the real seafloor. [ABSTRACT FROM AUTHOR]

In this paper, a method of re-calibration for roll bias of multi-beam sounding system is proposed to solve the faltiness of multi-beam sunding transducer calibration. Based on the sounding data of smooth terrain, the roll bias angle of transducer, the dip of terrain, can be calculated by using the method of weighted least squares. The obtained bias angle can be used for roll bias correction of transducer in vessel model. The conclusion shows that the corrected bathymetry data in the overlapping area of adjacent two swaths is more consistent than before, the accuracy of multi-beam bathymetry data has been significantly improved. [ABSTRACT FROM AUTHOR]

In this paper, mode vectors via EOF (empirical orthogonal function) are obtained based on real sound velocity profiles; by correcting the terrain distortion, the reconstruction coefficients of sound velocity profile are determined by GA (genetic algorithm) and then the inversion result of sound velocity profile is obtained. Results show that terrain distortion caused by the sound velocity errors can be effectively corrected by the inversion result of sound velocity profiles. Meanwhile, the accuracy and processing efficiency of multi-beam bathymetry data was significantly improved. [ABSTRACT FROM AUTHOR]

The vertical datums for hydrography and their relations were analysed and the technical approaches for expressing a chart datum on the height datum and Earth ellipsoidal surface were demonstrated. Among the three approaches for vertical datum transportation, developing a set of transportation models recent research has only focused on those relying on the single and discrete tidal station observations and calculations. The accuracy of the mean sea level(MSL) and the chart datum defined with the lowest normal low water was determined while different time spans were estimated with long period tidal station observations. This study indicates that the MSL under 5 cm can be obtained with two years of observations, and a CD under 7 cm root mean squares, along the coast of China. As an example, the CD model over the South China Sea and its separation from the height datum and Earth ellipsoidal surface are established through tide, mean sea surface, and sea surface topography models. Suggestions for unification tidal station CD calculation and establishment of a network for vertical datum control are put forward. [ABSTRACT FROM AUTHOR]

Out the error source in tradition of multi-beam sounding system adjustment of post-processing methods are greatly influenced by factors such as sound velocity error limitation of application of adjacent strips the central beam is put forward to build each ping the topography of the trend line as a prior information, using Kalman filtering under the influence of the integrated error of sound sounding data of systemic error correction method. Firstly, the central beam data of adjacent bands with relatively high accuracy of the sounding data were extracted, and the trend line of the regional seabed topography of the area under which the multi-beam was located was constructed roughly. Secondly, the deviation of the observed value and the deviation of the trend line of the seabed topography are obtained by using the data of the overlaps between the central beam of the detection line and the main measuring line. Finally, based on the deviation obtained, the construction trend line is used as the prior information to correct the data using Kalman filtering, and the accuracy of the corrected data is analyzed and evaluated. The experimental results show that the system error of the edge beam can be corrected effectively by Kalman filter. [ABSTRACT FROM AUTHOR]

The covariance matrix that least-squares collocation is used to the downward continuation of airborne gravity data is ill-conditioned, which influences the reliability and precision of computation results. To solve this problem, the least squares collocation-Tikhonov regularization method for the downward continuation of airborne gravity data is proposed. The functional relationship between airborne gravity data and ground gravity data is established by using the global covariance function model, and the Tikhonov regularization method that the Generalized Cross-Validation (GCV) method is used to select the regularization parameter is introduced, which can improve the ill conditioned covariance matrix and inhibit the amplification impact of ill matrix on observational error. Based on the EGM2008 gravity model, the simulation experiments that airborne gravity anomalies are downward continued to calculate corresponding ground gravity anomalies in mountainous, hill, and sea areas are designed, and the results validate the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

Objective: The research on topography and landform of the Yangtze estuarine waters is of great significance to human social and economic activities such as river management, river construction and water transportation. Establishing an accurate and seamless chart datum and its transformation model with other vertical datums will directly affect the acquisition of high ⁃precision topographic and geomorphic data at the water-land junctions and the unified comprehensive management and analysis. Thus, this paper studies the establishment of seamless chart datum and its transformation model with other vertical datums for estuarine waters, combining with geoid, sea surface topography and three-dimensional numerical simulation of tidal wave motion. Methods: Firstly, the unstructured-grid, finite-volume community ocean model (FVCOM) was used to simulate tidal level and calculate the tidal range at each grid node with the underwater topographic data and the open boundary water level forcing data. According to the correlation between tidal range and chart datum, the chart datum value of each grid node was deduced using tidal range ratios from tide gauge stations with known datums, and a regional seamless chart datum was then established. Secondly, the mean sea surface topography corresponding to a long time scale at each grid node was calculated with the daily sea surface topography product provided by the Copernicus marine environment monitoring service (CMEMS). The mean sea surface topography model was further established by the minimum curvature interpolation. Meanwhile, the geoid in the corresponding area was calculated by the global ultra ⁃high ⁃ order gravity field model Earth gravitational model 2008 (EGM2008) with a spatial resolution of 5′ × 5′. Thirdly, the geoid model was established through bi ⁃ quadratic interpolation. Finally, according to these three constructed models and the geometric relationship of three kinds of vertical datums in space, unification to a consistent spatial resolution was performed and the separation value between chart datum and the ellipsoid reference datum at each grid node was calculated. The vertical datum transformation model be⁃ tween the seamless chart datum and ellipsoid reference datum was established by the minimum curvature interpolation. Results: Using this method, we carried out the experiment at the typical estuarine waters, south branch of the Yangtze Estuary, and established the seamless chart datum and its vertical datum trans⁃ formation model. The global navigation satellite system (GNSS) leveling data from three long-term tide gauge stations were used for verification and accuracy evaluation of the model. The results show that the ab⁃ solute errors of the vertical datum transformation model at these three long-term tide gauge stations were − 19.8 cm, −25.8 cm and −27.0 cm respectively, which were all higher than the evaluated error (12.4 cm) of the model. The main reason is that the Yangtze estuarine waters have complicated tidal wave characteristics and they are nearshore waters, which leads to poor accuracy of the mean sea surface topography model in this region, thus further reducing the accuracy of the transformation model as a whole. Despite this, the accuracy of the transformation model still meets the requirements of International Hydrographic Organization standards for hydrographic surveys. Conclusions: Therefore, the research method of this paper are feasible and reliable, especially in far-sea areas. When the accuracy of satellite altimetry is further improved in the nearshore areas, the proposed method in this paper can be extended to nearshore and inland waters. [ABSTRACT FROM AUTHOR]