Table of Contents

• Introduction

• What is RTK: Basics of Real Time Kinematic Positioning

• Conventional Methods and Challenges of Hydrographic Surveying (Sounding)

• Techniques and Benefits of Hydrographic Surveying Using RTK

• Points to Note for RTK Hydrographic Surveying (Challenges and Countermeasures)

• Use Cases of RTK Hydrographic Surveying

• Accuracy and Validation of RTK Hydrographic Surveying

• Future Prospects of RTK Hydrographic Surveying

• Possibility of Simple Surveys with LRTK

• FAQ

Introduction

Hydrographic surveying (bathymetry, depth measurement) on water has traditionally required advanced specialist skills and considerable effort. However, in recent years, advances in high-precision GNSS positioning technology known as RTK (Real Time Kinematic) have made precise surveying from a boat increasingly accessible. In response to the question, "Is hydrographic surveying possible with RTK?", this article summarizes the answer and important points to note based on expert perspectives and field experience. From the basics of RTK technology, to methods of applying it to hydrographic surveying, potential challenges and countermeasures, concrete use cases, accuracy verification results, and future outlook, we provide a comprehensive explanation. This will be useful not only for technicians interested in GNSS surveying on water, but also for those considering adopting the technology.

What is RTK: Basics of Real Time Kinematic Positioning

First, let’s cover the basics of what RTK is. RTK (Real Time Kinematic) positioning is a technique that uses satellite positioning systems such as GPS, GLONASS, and Michibiki (QZSS) to achieve centimeter-level high-precision positioning in real time. Standalone GPS positioning typically has errors on the order of several meters, but RTK exchanges observation data between a base station (reference) and a rover in real time via radio or communications networks to correct satellite signal error sources. As a result, it features the ability to measure current position with horizontal accuracy of a few centimeters (a few inches) and vertical accuracy of a few centimeters (a few inches).

An RTK positioning system generally consists of the following components:

• Base station (reference station): A GNSS receiver set up at a known precise coordinate. It generates differential correction information based on phase data received from satellites. Traditionally, users often installed their own base stations, but nowadays one can also utilize electronic reference stations installed across regions or services that provide correction data over the Internet (network RTK such as Ntrip).

• Rover (mobile station): The GNSS receiver carried at the survey points. It receives correction information sent from the base station and applies it to its own observations to calculate high-precision coordinates. Rovers typically include a survey-grade antenna/receiver and a radio or communications modem (for mobile networks) to receive correction data.

• Data communication: The communication means connecting the base and rover. In the past, low-power radios or UHF radios covering several to a dozen kilometers were common, but recently receiving correction data over the cellular network (VRS, mountpoint methods, etc.) has become more widespread.

The strength of RTK lies in being real-time and high-precision. Surveyors can obtain high-precision coordinates on site immediately, reducing the burden of post-processing while improving work efficiency without sacrificing accuracy. However, RTK also has caveats: accuracy degrades if the distance from the base station becomes too great, and if the radio link is interrupted, a fixed solution (Fix) must be reacquired. These and other detailed issues will be discussed later, but first let’s examine conventional hydrographic survey methods and how they change with RTK adoption.

Conventional Methods and Challenges of Hydrographic Surveying (Sounding)

Hydrographic surveying refers to measuring the underwater terrain and depth in lakes, rivers, and seas. In Japan this is also called sounding (deep–shallow surveying), and it is indispensable for securing navigation channels, port construction, dam and river maintenance, dredging verification, and so on. Conventional hydrographic surveying has required addressing several challenges distinct from land surveying.

• Ship positioning and guidance: Historically, to accurately determine the survey vessel’s position, surveyors used total stations onshore to track prisms or guided boats in river cross-section surveys using wire ropes and distance markers. With the spread of DGPS (differential GPS), satellite positioning of vessels increased, but horizontal accuracy on the order of several meters can be insufficient for narrow waterways or small-scale construction.

• Depth measurement: Depth is measured with acoustic echo sounders (single-beam echo sounders or multibeam echosounders). Because these instruments alone provide only relative distance to the seabed, tidal corrections were required afterward. Traditionally, tide gauge stations were installed near the survey area to record water level changes during the survey, and depth data were corrected for tidal variations afterward. Such surveys requiring tide measurements necessitate additional personnel and equipment, increasing cost and labor.

• Efficiency and manpower: Conventional methods involve many steps—establishing control points, observing tides, guiding the vessel—so survey teams tended to be large. For example, tasks were divided among the person operating the total station on land, the tide observer, and the vessel operator managing the echo sounder and navigation. These preparations and coordination consumed time and impeded efficiency.

• Data accuracy and reliability: Even small errors in vessel guidance can be hard to detect later in depth data. If plotted positions are shifted on a map, visual inspection often cannot reveal the discrepancy, so results depended heavily on the operator’s workmanship. Wave-induced heave and roll also introduce scatter in single depth measurements. Traditionally, operators mitigated this by surveying the same line multiple times and averaging depth values, but real-time error assessment was difficult and ensuring accuracy was challenging.

As described above, conventional hydrographic surveying involved considerable effort and complex corrections. Next, we will see in detail how these problems can be addressed by introducing RTK technology.

Techniques and Benefits of Hydrographic Surveying Using RTK

Applying RTK’s high-precision positioning to hydrographic surveying has made significant progress in solving issues of conventional methods. In RTK-based hydrographic surveying, real-time high-precision positions (X, Y, Z) obtained from an RTK-GNSS receiver aboard the vessel are linked with depth data from an acoustic echo sounder. The following are technical points and benefits in detail.

• Real-time tidal correction: Using the height information obtained from RTK, tidal correction for depth data can be performed in real time. If the ellipsoidal height of the rover GNSS antenna and the offset from the antenna to the echo sounder are known, and if the height difference between the survey’s reference water level (for example, Tokyo Bay Datum = T.P. or mean sea level) and the ellipsoid is precomputed (geoid separation / difference between ellipsoidal height and water-level datum), then tidal variations do not need to be observed continuously during sounding, and depth can be corrected to the chosen reference surface. In other words, “tide-free surveying using RTK” becomes possible. This eliminates the need to install tide gauges and assign personnel for tide observation, enabling immediate normalized depth values and greatly improving operational efficiency and immediacy.

• High-precision vessel positioning and labor savings: RTK-GNSS continuously traces the vessel’s position at centimeter accuracy. This removes the need for onshore visual guidance or setting out survey points, allowing a single operator on board to perform line-following surveys. Especially on large lakes, dam reservoirs, or complex bays, the vessel’s real-time position displayed on a map enables efficient running of survey lines. As a result, tasks that previously required multiple people can often be completed by a small crew (in some cases 1–2 people).

• Integrated installation of GPS antenna and echo sounder: A key technical point is the spatial arrangement of the GNSS antenna and the acoustic transducer. Ideally, they are placed on the same vertical line (directly above one another) so that the height difference from the reference surface is known. For example, on a small boat install the echo sounder transducer on the hull bottom near the center and mount the GNSS antenna directly above it to minimize displacement between the sounding point and positioning point due to roll and pitch. In such an integrated system, the RTK-derived height and acoustic depth data are linked in real time, drastically reducing post-processing. Within the vessel’s system, depth and position are integrated, enabling efficiencies like near-real-time charting.

• Improved survey accuracy and reliability: By using RTK height information, short-term vessel vertical motion due to waves is reflected in the GNSS solution and, in theory, wave effects can be corrected in real time. For example, if the vessel moves up and down with waves, the instantaneous antenna height captures that motion, stabilizing the resulting depth values relative to the still-water surface (note the caveat about severe waves described below). Consequently, scatter in single-pass depth measurements is reduced, enabling the creation of more precise cross-sections and seabed maps. Data with linked vessel position and depth are highly reproducible and make it clear which point corresponds to which depth during later analysis. This enhances field reliability and increases stakeholders’ confidence in survey results.

• Reduced work time and cost: Because RTK enables labor savings and immediacy as described, total work time is shortened. Eliminating waits for tides and multiple averaging runs means surveys can be completed within favorable weather windows. Reduced personnel and time directly translate into cost savings. Although initial investment in RTK-capable equipment and software is required, frequent hydrographic survey operations can expect good cost-effectiveness over the medium to long term.

The above summarizes the technical overview and benefits of RTK-based hydrographic surveying. Next, we will explain in detail the challenges and points to note when applying this approach in the field.

Points to Note for RTK Hydrographic Surveying (Challenges and Countermeasures)

RTK hydrographic surveying is highly useful, but there are several issues to be aware of when applying it in the field. To maintain high accuracy and carry out safe, efficient surveys, pay attention to the following points.

• Satellite signal reception environment: Although line-of-sight is generally good on water, the antenna can be affected by wind-induced sway and motion. Fix the GNSS antenna in as stable a location as possible and use a high-performance antenna with strong multipath rejection. In areas with tall structures or bridges, multipath (signal reflection) is likely and can degrade positioning accuracy or cause phase breaks. In such areas, measures include fitting a ground plane under the antenna, selecting times with less interference, or, if possible, planning survey lines to avoid problematic spots.

• Distance from the RTK base station: RTK positioning accuracy depends on distance from the base station. Generally, high accuracy is maintained within a few km, but beyond 10 km accuracy gradually degrades due to ionospheric and tropospheric errors. For extensive lake surveys or coastal areas far from a base station, consider using network RTK (VRS information calculated from nationwide electronic reference stations) or deploying your own mobile base stations along the shoreline to follow the survey area. In inland narrow valleys or forested dam lakes where direct radio from the base may be obstructed, consider installing relay stations or using pocket Wi‑Fi or satellite communications to receive Ntrip correction information.

• GNSS and echo sounder offset settings: The height and position relationship (offset) between the GNSS antenna and the echo sounder transducer must be measured precisely beforehand and configured in the system. Entering incorrect horizontal or vertical offsets from the antenna reference point to the transducer will directly introduce systematic errors into the bathymetric data. Vertical offset in particular is critical for successful tidal correction. Before surveying, measure the distance between antenna and transducer accurately on land and register it correctly in the sounding software. If possible, perform a validation measurement at a shallow spot with known depth (for example, a pier edge measured by tape) and compare the RTK sounding system’s indicated depth to verify accuracy.

• Vessel motion and attitude correction: Small boats experience motion from waves and engine vibration. As noted, aligning the antenna and transducer vertically reduces errors from roll and pitch but does not eliminate them entirely. For multibeam echosounders measuring a wide swath at once, it is necessary to use attitude sensors (IMU) or gyros to measure roll, pitch, and heading and correct beam incidence angles. Even with single-beam sounding, heave sensors are advisable in rough seas to correct vertical motion. While RTK height information compensates for some wave effects, GNSS update rates (typically 1 Hz–5 Hz) cannot fully track high-frequency motion, so combine dedicated motion sensors as needed to stabilize data.

• Communication outages and backups: RTK positioning relies on correction data. If communication with the base station is lost or the Ntrip cellular signal goes out, the solution can degrade to a float solution or positioning may stop entirely. In large offshore or mountainous lake areas with unstable connectivity, consider the risk of positioning interruptions during critical operations. As countermeasures, use a receiver capable of switching to PPP (precise point positioning) mode when RTK is lost (positioning continues though accuracy degrades), or record all observation data for later PPK (post-processing kinematic) correction. Also maintain a backup plan such as concurrently recording tide data with a conventional tide gauge in case RTK becomes unreliable.

• Waterproofing and power management of equipment: For work on water, equipment waterproofing and reliable power supply are important. Protect GNSS receivers, communications devices, and laptops with waterproof cases or select waterproof-rated equipment to prevent failure from spray or rain. Since exposure to saltwater is common, perform post-use cleaning and anti-corrosion measures. Prepare batteries with capacity sufficient for mission duration to avoid power loss during long surveys. If deploying your own RTK base station, confirm power supply arrangements (large-capacity batteries or a generator) before heading to the site.

With careful attention to these points, RTK-equipped hydrographic surveys can be conducted safely and effectively. Next we look at practical use cases and fields where RTK hydrographic surveying has been applied.

Use Cases of RTK Hydrographic Surveying

RTK hydrographic surveying is already being utilized in various fields. Below are major use cases and application areas leveraging its high precision and immediacy.

• River cross-section surveys and channel inspections: River improvement or dredging planning requires detailed understanding of riverbed cross-sections. Using a boat equipped with RTK-GNSS and an echo sounder to traverse river cross-sections enables rapid, high-precision creation of riverbed cross-section charts. Previously, personnel had to be stationed on both banks to measure positions with tapes or total stations, but RTK allows fewer people to measure efficiently and safely. It is particularly effective for timely surveys after flood events when changes to the riverbed must be assessed.

• Sediment surveys in dam reservoirs and regulatory ponds: Reservoirs and ponds require periodic surveys of sediment accumulation. Wide-surface surveys are streamlined by RTK-based autopilot navigation and positioning. Small boats or unmanned surface vessels (radio-controlled boats) equipped with RTK receivers and fishfinders have been used to collect data in shallow or hazardous areas where people cannot safely enter. Unmanned survey boats using VRS network RTK offer compact, one-person-operable systems. This enables accurate sediment quantification across water bodies from small agricultural ponds to large dam reservoirs.

• As-built management for harbor works and dredging: For harbor channel dredging and quay construction, as-built surveys verify that excavation meets design depths. Systems where working vessels are equipped with RTK-GPS to monitor position and dredge depth in real time are already in practical use. This allows operators to monitor vessel position and excavation depth and accurately reach target depths. Post-construction inspection surveys can also be instantly charted and shared, enabling rapid acceptance processes and transitions to subsequent work stages.

• Emergency waterway surveys after disasters: RTK hydrographic surveying is useful for rapid response surveys after floods or seismic liquefaction that change water depths. For instance, small teams with small boats and RTK equipment have quickly collected bathymetric data to determine changes in riverbeds or areas near breached levees. Real-time corrected depth data enable on-the-spot map generation, supporting prompt decision-making in disaster response.

• Fisheries and environmental surveys: High-precision bathymetric maps are required in fisheries and lake environmental monitoring. RTK simplifies the previously laborious survey processes in these fields. Examples include fisheries cooperatives using small boats with survey equipment to map seabed features in bays, or researchers using RTK-equipped kayaks to study depth distributions in marshes. There are also reports of low-cost seafloor mapping combining consumer fishfinders with RTK-GNSS, offering a low-cost method to obtain environmental data while maintaining acceptable accuracy.

Thus, RTK hydrographic surveying is being adopted across civil engineering, disaster management, and environmental fields, demonstrating its usefulness. The next section examines the accuracy of positioning and depth measurements with concrete numbers and validation results.

Accuracy and Validation of RTK Hydrographic Surveying

Although RTK boasts high accuracy, a common question is, “How accurate is it in practice?” Accuracy should be considered separately for horizontal positioning and vertical (depth) measurements. We discuss theoretical values and field-measured values.

● Horizontal positioning accuracy: GNSS-RTK theoretically offers horizontal accuracy on the order of ±1–2 cm. Field validations have shown that, at close distances to the base station, errors can remain under a few centimeters. Horizontal accuracy matters in hydrographic surveys where you need to accurately capture local depressions or deposits in narrow channels, or ensure high reproducibility between survey lines measured multiple times. RTK coordinates are obtained in a global geodetic frame, so data from different days can be compared on the same basis. In one validation, repeatedly passing a known reference point (e.g., a pier edge) with an RTK-equipped boat produced positional scatter within a radius of a few centimeters. This is a dramatic improvement compared to conventional DGPS, which often had errors on the order of meters. However, as distance from the base station increases, horizontal accuracy can worsen to ± several tens of centimeters, so for tasks requiring high horizontal accuracy it is preferable to work as close as possible to the reference station.

● Vertical (depth) accuracy: Vertical accuracy tends to be slightly worse than horizontal but is still theoretically on the order of ±3–5 cm (±1.2–2.0 in). When using RTK height for tidal correction, remaining errors stem mainly from GNSS height random errors, incorrect antenna–transducer offset settings, and vessel motion compensation errors. In one verification, a boat floated in a tank with known still-water depth and measurements from an RTK hydrographic system showed mean errors within a few centimeters. In real sea conditions on calm days, repeated measurements at the same point produced depth values with a standard deviation of about 5 cm (2.0 in). This is clearly more precise than residual errors typical of conventional tide-corrected methods (which can be on the order of 10–20 cm in some cases). However, during high waves or strong winds the GNSS antenna can sway and momentarily produce unstable solutions, so such data should be excluded or smoothed in post-processing as needed.

● Combined system accuracy evaluation: The overall accuracy of a bathymetric system combining RTK-GNSS and an acoustic echosounder is the composite of the above factors. One domestic surveying company reported for its RTK bathymetry system "depth accuracy: several centimeters to over ten centimeters; horizontal positioning accuracy: several centimeters to over ten centimeters." The range reflects variability in sea state and survey conditions, but under normal conditions one can generally expect accuracy better than 10 cm (3.9 in). The Geospatial Information Authority’s operational guidelines also position RTK-based marine surveys as achieving accuracies comparable to or better than tide-gauged methods. To confirm data quality in the field, it is recommended to perform independent checks at key points, such as comparing measured values to known depth points within the survey area or comparing against measurements taken on different days by alternative methods (e.g., long staff and level from shore). Repeated validation builds confidence that RTK hydrographic survey results are reliable and provides material for explaining results to clients and stakeholders.

Future Prospects of RTK Hydrographic Surveying

RTK-based hydrographic surveying is expected to become more widespread and sophisticated. Below are technological trends and outlooks.

On the technical side, improvements in GNSS performance are favorable. Using Japan’s quasi-zenith satellite “Michibiki,” centimeter-class augmentation services (CLAS) enable high-precision positioning without installing a dedicated base station, and the development of network RTK and PPP-RTK could make centimeter-level positioning stable even offshore or in remote areas. Fusion of satellite positioning with other sensors (IMU, acoustic gyro) is advancing, improving inertial compensation technologies that can maintain high accuracy for short periods when GNSS is interrupted. Future systems are expected to be more compact and robust while maintaining precision.

On the operational side, automation and unmanned systems are trending. Small unmanned survey vessels (USVs) equipped with RTK and autopilots for autonomous route-following are already practical. In the future, integrated solutions combining aerial surveys from drones (UAVs) and USV-based hydrographic surveys could provide comprehensive terrain measurements from both air and water for rivers and coastal zones. RTK’s high-precision positioning will remain a core technology in these solutions.

Cost barriers are also lowering. RTK-GNSS systems once cost several million yen, but recently low-cost high-precision GNSS modules and receivers have appeared. RTK devices that work with smartphones are becoming available. As these lower-cost devices proliferate, small survey firms, local governments, and university labs will find it easier to try RTK hydrographic surveys.

Overall, RTK hydrographic surveying is shifting from “specialized surveys requiring high precision” toward a "general-purpose technology accessible to many." As technicians’ skills improve and know-how is shared, higher-quality bathymetric information will become more common, contributing to disaster risk reduction, infrastructure management, and environmental conservation.

Possibility of Simple Surveys with LRTK

Recently introduced LRTK (low-cost RTK) systems are attracting attention as an approach that makes RTK positioning more accessible. For example, LRTK Phone is a smartphone-integrated high-precision GNSS receiver that enables centimeter-level positioning simply by attaching it to a phone. Compared to traditional professional equipment it is compact, lightweight, and cost-effective, and is easy to use even by non-specialists.

With systems like LRTK, surveying tasks that previously required professional contractors may be performed by a single person in a simplified manner. For instance, to measure the depth of a small pond, combining an LRTK device with a fishfinder on a simple boat may enable individuals or researchers to obtain high-quality data. There are user reports of mounting an LRTK terminal on a monopod to measure point elevations solo, or combining it with a drone or smartphone camera for 3D surveying.

From a field perspective, LRTK functions as a "portable surveying instrument" and is promising for rapid on-site inspections during disasters or routine infrastructure checks. Its portability and responsiveness make it suitable for simple bathymetric measurements from small boats or rafts, and such use may become more common.

Importantly, these simplified systems are beginning to produce results comparable to traditional high-end survey equipment. In experiments, LRTK Phone reportedly achieved near-professional accuracy (horizontal ±1–2 cm (±0.4–0.8 in), vertical ±3 cm (±1.2 in)). Achieving such accuracy from pocket-sized devices is remarkable and signals a democratization of surveying.

Of course, full-scale hydrographic surveys still require high-performance equipment and experienced technicians, but the spread of LRTK promises an environment where you can survey when needed. In the future it may become commonplace for field personnel to take quick measurements with a smartphone and share data immediately.

FAQ

Q: What is the difference between RTK and GPS? A: GPS refers broadly to satellite positioning systems; standalone positioning typically has errors of several meters. RTK incorporates relative observations with a base station to correct those errors and reduce them to the centimeter level. In short, RTK is a "high-precision GPS method," and its main difference from ordinary GPS is that it provides high-precision coordinates in real time.

Q: Why is RTK advantageous for hydrographic surveying? A: The greatest advantages are real-time tidal correction and precise vessel positioning. With RTK you can correct depth data to the reference surface simultaneously with sounding, eliminating the need for separate tide observations. The markedly improved vessel positioning also allows surveys to be conducted with fewer personnel. As a result, high-quality data are obtained immediately and total work time is reduced.

Q: What equipment is needed for RTK hydrographic surveying? A: Basic components include RTK-capable GNSS receivers (a base and a rover), an acoustic echosounder (single-beam or multibeam), a survey computer (for data recording and processing), communications equipment (radio or mobile router), and a survey vessel. For small surveys, consumer fishfinders can sometimes be used. Recently, methods that obtain correction data over the network without placing a base station have become available. The most important points are to securely mount the antenna and echo sounder and to measure offsets correctly.

Q: Does RTK really make tide gauges unnecessary? A: Properly configured, tide gauges are in principle unnecessary. RTK-derived height data can be used to correct depths on-site. However, tide gauges and leveling measurements still help when preparing the initial reference surface and as a backup in case RTK is unavailable. During system introduction, it is advisable to run tests combining traditional tide measurements to compare results and ensure confidence before fully switching to a no-tide approach.

Q: Can RTK hydrographic surveying be done in rain or high waves? A: Light rain is acceptable with waterproofing measures, but heavy rain or high waves should be avoided because they reduce survey accuracy and pose safety risks. Heavy rain can degrade radio reception and increase equipment failure risk. High waves cause large vessel motions and unstable GNSS solutions. If operations are necessary under such conditions, use attitude sensors to correct motions and apply post-processing filters to data, but ideally perform surveys in calm weather.

Q: What is LRTK? A: LRTK refers to low-cost RTK systems. A notable example is the “LRTK Phone,” a high-precision GNSS receiver that attaches to a smartphone. Also called low-cost RTK, it offers centimeter-class positioning at lower cost and in a compact form, expanding RTK use beyond professional contexts. In hydrographic surveying, LRTK enables individuals to attempt small-scale depth measurements.

Q: How can data obtained with RTK surveys be used? A: RTK hydrographic data feed directly into digital terrain models and bathymetric charts. They are used to draw longitudinal and cross-sectional river profiles for design comparison, plan dredging works, update nautical charts, and more. Increasingly, data are handled as 3D point clouds and integrated with terrestrial LiDAR for visualization. High-precision, georeferenced bathymetric data also enhance the accuracy of simulations and modeling.

Q: I am worried about introducing RTK hydrographic surveying for the first time. How can I learn it? A: The quickest route is to attend training and technical seminars offered by equipment manufacturers and survey vendors. You can learn RTK setup, sounding software operation, and water-specific precautions in a structured way. Start by practicing RTK positioning on land, then move to calm water for test surveys. With the emergence of LRTK and other simple equipment, you can also experiment on a small pond. Although it may seem daunting at first, following procedures carefully and verifying results step by step will build competence. Gaining field experience accumulates know-how and confidence for reliable operations.