Single Beam Echosounder Research Articles

A robust fusion terrain-aided navigation method with a single beam echo sounder

The underwater terrain-aided navigation (TAN) method with a single beam echo sounder (SBES) can achieve long-term underwater navigation. However, due to the limited bathymetric measurements collected using a SBES, the accuracy and robustness of the TAN method are affected by a variety of factors, such as the richness of terrain features and the gradient of underwater terrain. Consequently, the SBES-TAN methods with gridded or contour priori maps cannot yield robust navigational results for long-range autonomous underwater vehicles (AUVs). To address this issue, this paper proposed a robust fusion TAN (RFTAN) method with a SBES on the basis of particle filter (PF) theory to locate a vehicle accurately. In the proposed method, both terrain positioning results provided by gridded-TAN and contour-TAN were fused to weigh a particle, with the fuzzy-theory-based fusion weight division (FWD) method. Playback experiments were conducted using 25-h-length field data collected from an at-sea experiment, and experimental results show that the proposed method can provide both robust and accurate long-term navigational results for AUVs.

Assessment of Target Strength Value of Demersal Fish in Lancang Waters Using Simrad Single Beam Echosounder

Hydroacoustic technology is excellent at determining accurately and in near real-time the objects located near the seabed. Therefore, acoustic technology is widely recognized for fisheries resources mapping, including demersal fish. The objective of this study was to estimate the value of demersal fish target strength (TS) in the waters of Lancang Island, Kepulauan Seribu region. A Single Beam Echosounder Simrad EK-15 with a frequency of 200 kHz was employed for data collecting in the waters around Lancang Island. The integration layer width was set at 3 meters with a horizontal integration distance of 200 meters. The average target strength (TSc) values obtained from the integration cell ranged from -55.52 dB to -42 dB with an average of -50.30 dB. Furthermore, the seabed depth influenced the distribution of TSc values, with higher TSc values in deeper habitat locations.

First Calibrated Methane Bubble Wintertime Observations in the Siberian Arctic Seas: Selected Results from the Fast Ice

This paper presents the results of an acoustic survey carried out from the fast ice in the shallow waters of the East Siberian Arctic Shelf (ESAS) using a single beam echosounder. The aim of this paper is to demonstrate an improved approach to study seafloor seepages in the Arctic coastal zone with an echosounder calibrated on site. During wintertime field observations of natural rising gas bubbles, we recorded three periods of their increased activity with a total of 63 short-term ejections of bubbles from the seabed. This study presents quantitative estimates of the methane (CH4) flux obtained in wintertime at two levels of the water column: (1) at the bottom/water interface and (2) at the water/sea ice interface. In winter, the flux of CH4 transported by rising bubbles to the bottom water in the shallow part of the ESAS was estimated at ~19 g·m−2 per day, while the flux reaching the water/sea ice interface was calculated as ~15 g·m−2 per day taking into account the diffusion of CH4 in the surrounding water and the enrichment of rising bubbles with nitrogen and oxygen. We suggest that this bubble-transported CH4 flux reaching the water /sea ice interface can be emitted into the atmosphere through numerous ice trenches, leads, and polynyas. This CH4 ebullition value detected at the water/sea ice interface is in the mid high range of CH4 ebullition value estimated for the entire ESAS, and two orders higher than the upper range of CH4 ebullition from the northern thermocarst lakes, which are considered as a significant source to the atmospheric methane budget.

Variability in acoustic backscatter and fish school abundance at a shallow water CCS site

Carbon capture and storage (CCS) is one approach used to reduce carbon dioxide (CO2) emissions to the atmosphere. Subsea CO2 storage sites must have low risk of leakage and have minimal impact on the marine environment. Single beam echosounders (SBES) are a good technology for detecting CO2 leakage in the water column for application in a marine Measurement, Monitoring and Verification (MM&V) framework. Characterisation of active acoustic data is needed to determine whether levels of water column backscatter enable CO2 gas leaks to be detected, and to understand acoustic false positives caused by naturally occurring acoustic signals (e.g., fish schools). We used acoustic data collected by SBES fitted to two mobile and two fixed sensor platforms over four years to characterise the site of the proposed CarbonNet CCS Project subsurface CO2 storage reservoir. We observed diel and seasonal variability in water column backscatter and fish school abundance. Water column backscatter was highest at night and in Austral summer and autumn periods but was at a level that would allow small CO2 gas leaks to be detected. Fish abundance was highest during the day and in Austral summer and autumn. To reduce the likelihood of acoustic false positives, surveys designed to detect CO2 gas leaks using active acoustic technologies could prioritise data collection at night and during the months where fish school abundance is lowest. Understanding these sources of natural variability can guide survey strategies and response actions.

Usporedba podataka hidrografskog premjera s podacima o dubinama mora dobivenih iz javnih izvora

A hydrographic survey is a standardized procedure for collecting data for the production of nautical charts and publications. It is a lengthy and costly procedure, so the survey is carried out depending on the capabilities of hydrographic organizations. It is known that relatively large parts of the world's oceans are very poorly covered by hydrographic surveys. To increase the amount of data collected, the International Hydrographic Organization (IHO) has introduced the concept of crowdsourced bathymetry (CSB). Under the CSB concept, all vessels meeting certain minimum technical requirements (carrying a global navigation satellite system and a single beam echo sounder) can participate in voluntary bathymetric data collection. The paper analyzes the method of collecting bathymetric data from CSB. The depth data collected as part of the CSB are compared with official data displayed on electronic navigational charts (ENC) in the United States of America. Four sea areas were selected in which 104 depths were compared at the same positions, and categorization was also made according to the criterion of navigational importance, i.e., the category zones of confidence (CATZOC). By comparing the official depth data from the hydrographic survey with the depth data collected from public sources for the same positions, their mutual relationships were established, from which it can be concluded that the CSB data, despite its limitations, is a very valuable supplement to the existing official data.

Coastal Bathymetric Sounding in Very Shallow Water Using USV: Study of Public Beach in Gdynia, Poland.

The bathymetric surveys executed with a use of small survey vessels in limited water areas, including offshore areas, require precise determination of the geospatial coordinates of the seabed which is a synthesis of, among others, determining the position coordinates and measuring the depth. Inclination of the seabed and the declining depth make manoeuvring of the sounding vessel, e.g., a hydrographic motorboat or Unmanned Survey Vehicle (USV), in shallow water impossible. Therefore, it is important to determine the minimal depth for the survey resulting from the draught of the sounding vessel and the limits of the sounding area. The boundaries also result from the dimensions of the sounding vessel, its manoeuvring parameters and local water level. Type of the echosounder used in the bathymetric survey is a decisive factor for the sounding profile planning and the distances between them and the survey vessel for the possibility performing the measurements in shallow water. Electronic Navigational Chart (ENC) was used to determine the water area's boundaries, and the safety contours were determined on the basis of the built Digital Sea Bottom Model (DSBM). The safety contour, together with the vessel's dimensions, its manoeuvring parameters and the hydrometeorological conditions, limit the offshore area in which the measurement can be performed. A method of determining boundaries of the survey performed by a USV equipped with SingleBeam EchoSounder (SBES) on survey lines perpendicular to the coastal line are presented in the paper in order to cover the water area with the highest amount of measurement data, with the USV's navigational safety taken into consideration. The measurements executed on the municipal beach served verification of the DSBM.

Bathymetric Reconstruction From Sidescan Sonar With Deep Neural Networks

In this article, we propose a novel data-driven approach for high-resolution bathymetric reconstruction from sidescan. Sidescan sonar intensities as a function of range do contain some information about the slope of the seabed. However, that information must be inferred. In addition, the navigation system provides the estimated trajectory, and normally, the altitude along this trajectory is also available. From these, we obtain a very coarse seabed bathymetry as an input. This is then combined with the indirect but high-resolution seabed slope information from the sidescan to estimate the full bathymetry. This sparse depth could be acquired by single-beam echo sounder, Doppler velocity log, and other bottom tracking sensors or bottom tracking algorithm from sidescan itself. In our work, a fully convolutional network is used to estimate the depth contour and its aleatoric uncertainty from the sidescan images and sparse depth in an end-to-end fashion. The estimated depth is then used together with the range to calculate the point's three-dimensional location on the seafloor. A high-quality bathymetric map can be reconstructed after fusing the depth predictions and the corresponding confidence measures from the neural networks. We show the improvement of the bathymetric map gained by using sparse depths with sidescan over estimates with sidescan alone. We also show the benefit of confidence weighting when fusing multiple bathymetric estimates into a single map.

Analisis Batimetri Guna Perencanaan Pengerukan Kolam Pelabuhan Kalibaru Tanjung Priok Menggunakan Single Beam Echosounder

The Kalibaru Port of Tanjung Priok, North Jakarta, is one of the ports that is used for shipping and loading and unloading activities. Bathymetry information is needed to support one of the activities in the port, namely in terms of shipping. The purpose of this research is to map the bathymetry in the harbor pool area and to find out the areas that need to be dredged in the Kalibaru Tanjong Priok harbor pond. This research was conducted on May 24, 2022 in the pond area of ​​Tanjung Priok Harbor, North Jakarta. This research was conducted using a single beam echosounder using a Garmin GPSmap 420s tool to determine the depth of the harbor pool. Data processing using Ms. excel, Auto CAD land desktop, surfer to generate depth contours and calculate dredge volume. The volume calculation uses the average end area method. The calculation result of dredging volume is the sum of the cross-sectional area multiplied by the section distance. The results showed that the depth of the Tangjung Priok harbor waters ranged from 3 to 17 meters and the dredged volume reached 165,319 m3.

Sonar investigation of the old Nubian Villages in the bottom topography of Nasser Lake, Egypt

Underwater sonar survey using Side Scan Sonar (SSS) and Single beam Echo-sounder (SBES) was used for the investigation of the old Nubian archaeological villages buried within the bottom topography of Nasser Lake. The research objective involved emphasizing the efficiency of marine geophysical techniques in the detection of the submerged old Nubian Villages in Nasser Lake behind the High Dam, the study also aimed to study riverbed topography and determining the old course of the Nile River in the area. The resulted bathymetric maps showed that the two former banks of the Nile River in the study area were characterized by two distinct natures, while the shoreline has retreated with different extents starting from 250 m at the southern regions up to 3 km in the northern parts. The high-resolution sonar images of the bottom uncovered distinctive acoustic patterns according to difference in the returned backscatter strength. Submerged Nubian houses were detected along some areas of the eastern bank, which were found either standing over elevated rocky cliffs or built on multiple tiered levels. Precise target measurements were applied on the detected houses and different dimensions were obtained, which suggest that these houses were built according to the availability of suitable land to construct such dwellings.

An expeditious low-cost method for the acoustic characterization of seabeds in a Mediterranean coastal protected area

Posidonia oceanica meadows are ecosystem engineers which, despite their ecological relevance, are experiencing habitat fragmentation and area decrease. Cartography and information on the ecological status of these habitats is key to an effective maritime spatial planning and management for habitat conservation. In this work we apply an acoustic methodology to map benthic habitats (substrate and vegetation) in an archipelago of the Natura 2000 Network close to the coast of Murcia (SE Spain) where dense and sparse areas of P. oceanica, and patches of Cymodocea nodosa appear over a sandy and had bottom. The methodology uses dual frequency information (200 kHz and 38 kHz) acquired with a single-beam echosounder to develop a bathymetry, and performs sea bottom and vegetation supervised classifications, using video and scuba diver observations as groundtruthing data. Sea bottom was classified from acoustic features of the first and second 200 kHz echoes into 5 substrate classes using a random forest classifier: sand, fine sand, coarse sand, hard bottoms and hard bottoms with sandy patches. The vegetation was classified from features extracted from the "above-bottom" part of the echo (height and backscattering intensity) in both frequencies, resulting also in a 5 class classification: C. nodosa meadows, dense P. oceanica meadows, dispersed P. oceanica meadows, dense P. oceanica with sand patches, and no-vegetation; according to the random-forest Gini index, 38 kHz features were the most informational variables for this classification. The validation accuracies of both classifications were 85% (substrates) and 70% (vegetation), close to accuracies reported in the literature when using a similar number of classes. The results of this article (including bathymetric, and substrate and vegetation thematic maps), together with the acoustic methodology described and used, are contributions that can improve the continuous monitoring of Mediterranean seagrasses.

Evaluation of a crowd sourced bathymetric approach

Crowd-Sourced Bathymetry is the process of generating a harbor chart through collecting, enriching, processing, and aggregating depth (and other) measurements from a host of vessels, using standard navigation instruments, while engaged in routine maritime operations. This study explores the accuracy and limitations of one particular approach towards utilizing Crowd-Sourced data, comparing the results obtained from DockTech’s approach to that of a classic MBES survey. As DockTech is a for-profit startup that sells their product to various stakeholders in the maritime supply chain, a balance between protection of proprietary interest and scientific collaboration is necessary. As such, while specific details in regards to both chosen methodologies and their particular evaluations cannot be discussed, a general overview will be provided and the results of DockTech’s solution will be evaluated. This study is based on depth measurements collected by service vessels, active on a daily basis in Ashdod Port over the course of two months. Tug and Pilot boats use stand-alone GNSS navigation and navigation class Single Beam echo sounders for safety purposes when traversing the port and maneuvering between the wharfs. These depths, averaged over a relatively rough grid, were used to produce a chart of part of the port. Data from the service vessels did not include RTK GNSS navigation and hence ellipsoid referenced surveying techniques were not used in lieu of water level monitoring. This chart was then compared to another chart produced from a professional hydrographic survey of the port, using state of the art equipment and strict hydrographic control. The differences were then analyzed according to the latest edition of the IHO S-44 standards. Recommendations were suggested for some relatively simple measures which could enhance the accuracy achieved and the reliability of such a chart, at least for harbor maintenance purposes.

Measurement and Analysis of Acoustic Surface Scattering for Oil Concentration

The life of maritime flora and wildlife are substantially impacted by oil's effects, and this has an immediate impact on livelihoods. The impact disrupts the lives of contaminated coastal communities. Because of the potential losses caused by oil pollution, the government enacts policies governing environmental conditions in order to maintain or improve the quality of the polluted sea. The purpose of this study was to calculate the acoustic backscattering value of oil using a single beam echosounder and compare the measurement results from a single beam echosounder with the results of gravimetric analysis. The data acquisition was carried out in March 2021 at the Marine Acoustic Laboratory, Department of Marine Science and Technology, Faculty of Fisheries and Marine Sciences, IPB University, using the SIMRAD EK-15 frequency of 200 kHz and oil used as a source of pollution, namely the dexlite type (density of 880 kg/m3). The results showed that the value of Surface Backscattering Strength (SS) decreased when contaminated with oil, namely -63.76 dB and -65.90 dB, but when oil was added back, the SS value increased to -50.81 dB. The relationship between oil concentration and surface backscattering strength shows a negative correlation between the variables, and the level of the relationship between variables is 0.85.

Modeling and Analysis of Sea-Surface Vehicle System for Underwater Mapping Using Single-Beam Echosounder

Detailed knowledge on the shape of the seafloor is crucial for many researchers. Bathymetric data are critical for navigational safety and are used for underwater mapping. This study develops a sea-surface vehicle (SSV) system for underwater mapping by using both bathymetric data from a low-cost single-beam echosounder located on the SSV, and the navigation data of the SSV. The navigation of the SSV was obtained using a global positioning system (GPS). The effect of changing bathymetric and navigation data due to external disturbances such as wind and waves on the map was analyzed. The sea-bottom slope angles, which are effective in changing bathymetric data, were estimated and corrected in relation to the estimated angles in a particular mapped area for more accurate underwater mapping. Additionally, the effects of the grid range of the mapped area, beam angle of the echosounder, and position of the echosounder on the underwater mapping were analyzed. These analyses were based on simulation data, and were performed in a MATLAB, HYPACK, and Global Mapper environment. An underwater map was also obtained in the Kozlu/Zonguldak area, Black Sea by using a single-beam echosounder located on the SSV. This map was improved by estimating sea-bottom slope angles and the corrected bathymetric data to obtain a more accurate underwater map of the area. The experimental and simulation results were compared, focusing on the sea-bottom slope changes, sea-surface disturbances, bathymetry grid range changes, and draft effects.

Mesophotic Reefs of the Largest Brazilian Coastal Protected Area: Mapping, Characterization and Biodiversity

Mesophotic reefs are poorly known worldwide despite their great ecological relevance and management importance for coral reef conservation strategies. To aid in filling this gap, we conducted a pioneering, large-scale survey, covering a total of around 315 miles in length, in the largest Brazilian coastal Marine Protected Area (MPA) Costa dos Corais. From the digital bathymetry model (30 to 50 m depth) generated by a multibeam echo sounder, we selected areas of greater geomorphological diversity for a detailed investigative expedition of mesophotic ecosystems. Various sampling techniques were used: single-beam echo sounders for detailing the relief, a remotely operated underwater vehicle (ROV) for habitat type investigation, baited remote underwater video (BRUV) for collecting images of the fish community, and scuba diving to perform transects describing the benthic and fish community. We analyzed reef environments from 20 to 68 m deep. As a result, we present the mapping and geomorphological characterization of two compartments of mesophotic reefs at 21–45 m depth and an image library of mesophotic ecosystems with the species description and indications of whether it is a new record in the region. Biodiversity data were collected covering eight species of sponges, with greater abundance for Ircinia spp., Aplysina spp., and Xestospongia muta; eight from corals, mainly Siderastrea spp. And Montastrea cavernosa; and 68 species of reef fish, with the Labridae family (including Scarinae—11 species) being the richest. Our results demonstrate the importance of mesophotic reefs for MPA Costa dos Corais reef biodiversity and, with that, the need to protect these areas through the application of local conservation strategies, such as the creation of “no-take zones”.

Marine priority habitat mapping in a Mediterranean conservation area (Gyaros, South Aegean) through multi-platform marine remote sensing techniques

The aim of this study is to present the results of the first complete marine habitat mapping through marine remote sensing techniques in Gyaros Island, a remote island in the Cyclades archipelago with a great historical and ecological value. Gyaros Island is of great biological importance and, for this reason, since 2011, is part of the NATURA 2000 network and, by 2019, is characterised as a marine protected area (MPA). More than 80 km2 of seafloor area were inspected through a multi-platform marine remote sensing technique survey through two individual expeditions in 2014 and 2017. The remote sensing survey was conducted utilising side scan sonar, sub bottom profiler and multi- and single-beam echosounders. In addition to the remote sensing survey, an extensive ground truth network was established utilising a underwater towed camera and VanVeen sediment grabber. These resulted in the area being fully covered with high-quality data. Through these, a classification of the area was performed based on three distinct seafloor habitat mapping schemes (Expert, European Nature Information System and NATURA HD). The survey and the classification revealed that over 50% of the seafloor is covered by Posidonia oceanica meadows and coralligenous and other calcareous bio-concretion priority habitats. Based on the results of this work, Gyaros MPA was also established and different protections and conservation zones were set.

Deteksi Batimetri Perairan Dangkal di Pulau Menjangan, Provinsi Bali Menggunakan Citra Landsat

Remote sensing-based research in Indonesia using satellite imagery frequently faces the challenge of cloud coverage due to the tropical country. One spatial data that can be extracted from satellite imagery is bathymetry. However, cloud-covered water bathymetric extraction still needs to be examined. This study aims to understand the ability of Landsat 7 ETM+ acquired on 29 July 2013, and Landsat 8, acquired on 24 July 2020, as the representative of non-cloudy image compared to Landsat 8, acquired on 9 August 2020, as the cloudy image. Stumpf algorithm was applied, including a statistical approach of linear regression analysis with in-situ data measurement from Single Beam Echo-Sounder (SBES) to derive the absolute bathymetric map with several classes of depth ranging from 0 – 2 m up to 10 m. To assess the accuracy, RMSE and confusion matrix was used. The result shows that Landsat 7 ETM+ yields the highest R2 with 0,52, while the lowest total RMSE (8,167 m) and highest overall accuracy of about 69% from the confusion matrix was achieved by the cloudy image of Landsat 8. Nevertheless, the highest absolute depth value yield by Landsat 8 non-cloudy image with 16,1 m. This research confirms that the highest R2 value does not always produce the best model, but it is still promised to be used. Furthermore, the quality of the imagery based on its percentage of cloud coverage is affecting the resulted model.

Improved Hydrographic Surveying Accuracy with the Use of GPS/IMU and Single Beam Echo Sounder

Single beam echo sounders (SBES) mounted on unmanned surface vehicles (USVs) are widely used in hydrographic surveying, particularly for detecting water depth and digital depth mapping, among other things. Because of conditions such as ocean waves, the relative level arm of each component, and errors in each component device, using only SBES and GPS sensors leads to significant errors and poor accuracy. Now the SBES sensor's accuracy has improved. However, this is insufficient. A new real-time kinematic (RTK) positioning technique enables horizontal and vertical coordinate precision of 2 to 5 centimeters. Additionally, the precision of the survey is dependent on the USV's movements. USVs use inertial measurement units (IMUs) to provide information on the system's acceleration, velocity, position, and orientation. To reduce errors and produce trustworthy results, it is necessary to process data from various devices using a well-integrated algorithm. The purpose of this article is to discuss how to develop an algorithm that combines sensors such as GPS RTK/IMU, SBES, and other similar devices. This method takes into account both the rotations and relative level-arm of sensors. Finally, analysis of the data indicates a significant increase in the accuracy of the integrated algorithm when compared to the conventional techniques.

Modelling topo-bathymetric surface using a triangulation irregular network (TIN) of Tunga Dam in Nigeria

Dams are built to store the water flowing from upstream to downstream. Sedimentation and siltation are some of the major problems affecting the storage capacity of dams. For effective management, bathymetric and topographic data are used to assess this challenge. In the Mambila Plateau of Taraba, Nigeria, the Tunga Dam is a multifunctional reservoir that serves as a small hydropower, irrigation and domestic use dam. Nonetheless, it is not operating to its full potential, leading to issues such as frequent stoppage of the turbine, low irrigation activities and a shortage of water supply for domestic use. To determine the basin’s approximate present volume, a topographic and bathymetric survey was conducted using a differential global positioning system (DGPS)-Hi-Target V30 and a single beam echosounder to acquire the real-time data. The data were processed, and the digital elevation model (DEM) of the study area was modelled using a triangulation irregular network algorithm (TIN). The deepest point of the dam was found to be 21.25 m, and the volumetric capacity was assessed based on the elevations. The tessellation data format adequately represents the reservoir DEM for future purposes to better enhance the reservoir capacity. Hence, the research suggested that dredging should be carried out to boost the basin’s capacity. Likewise, an embankment can be constructed around the dam to enhance the basin’s storage capacity. The dredged material can be used to achieve the barrier’s building, which will reduce the overall cost.

Evaluation of Satellite-Derived Bathymetry from High and Medium-Resolution Sensors Using Empirical Methods

This study evaluates the accuracy of bathymetric maps generated from multispectral satellite datasets acquired from different multispectral sensors, namely the Worldview 2, PlanetScope, and the Sentinel 2, in the bay of Elounda in Crete. Image pre-processing steps were implemented before the use of the three empirical methods for estimating bathymetry. A dedicated correction and median filter have been applied to minimize noise from the sun glint and the sea waves. Due to the spectral complexity of the selected study area, statistical correlation with different numbers of bands was applied. The analysis indicated that blue and green bands obtained the best results with higher accuracy. Then, three empirical models, namely the Single Band Linear Algorithm, the Multiband Linear Algorithm, and the Ratio Transform Algorithm, were applied to the three multispectral images. Bathymetric and error distribution maps were created and used for the error assessment of results. The accuracy of the bathymetric maps estimated from different empirical models is compared with on-site Single beam Echo Sounder measurements. The most accurate bathymetric maps were obtained using the WorldView 2 and the empirical model of the Ratio Transform algorithm, with the RMSE reaching 1.01 m.

Simplified Method of Determination of the Sound Speed in Water on the Basis of Temperature Measurements and Salinity Prediction for Shallow Water Bathymetry

The aim of this paper is to present a method of determining sound speed in water, based on temperature measurements executed by means of a laboratory low-cost thermometer with a probe provided with a long cable. It has been assumed that the salinity variation in respect to depth, found in a shallow water area, has insignificant impact on the sound velocity distribution determined by the temperature changes. The salinity data were obtained via the Internet service from the closest measuring station that registers surface water parameters. The sound speed in water was determined based on the formulas widely adopted in hydroacoustics and compared with the results obtained from the measurements executed by means of a Conductivity/Salinity Temperature Depth (CTD/STD) probe. The impact of inaccuracy in determining the sound speed in respect to the SingleBeam EchoSounder (SBES) immersion depth, i.e., a method commonly used by unmanned surface vessels in seaport measurements, was estimated. The measurements were taken in water areas of the Baltic Sea of low salinity and then verified with measurements in the Mediterranean Sea representing quite high salinity. The method is an alternative for calibrating the SBES the bar check way and has the capacity to meet the requirements in respect to its application in hydrographic surveys.