Is Hydrography Possible with RTK? Summary of Points to Note

Table of Contents

• Introduction

• What is RTK: Basics of Real Time Kinematic Positioning

• Conventional Methods and Challenges of Hydrographic Surveying (Bathymetry)

• Techniques and Benefits of Hydrographic Surveying Using RTK

• Points to Note for Marine RTK Surveying (Issues and Countermeasures)

• Use Cases of Marine RTK Surveying

• Accuracy and Verification of Marine RTK Surveying

• Future Prospects of Marine RTK Surveying

• Possibility of Simple Surveying Using LRTK

• FAQ

Introduction

Surveying on water (hydrography, bathymetry) 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 boats 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 for applying it to marine surveying, foreseeable issues and countermeasures, concrete use cases, accuracy verification results, and future prospects, this article provides a comprehensive explanation. It will be useful not only for engineers interested in GNSS surveying on water but also for those considering implementation.

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 perform real-time centimeter-level high-precision positioning. Ordinary standalone GPS positioning typically has errors of several meters, but RTK exchanges observational data between a base station (reference) and a rover (mobile unit) via radio or communication networks to correct satellite signal error factors in real time. As a result, it can determine current position with horizontal accuracy of several centimeters and vertical accuracy of several centimeters.

RTK positioning systems generally consist of the following components:

• Base station: A GNSS receiver installed at a known accurate coordinate position. It generates differential correction information based on phase data received from satellites. While it used to be common to set up a personal base station, today many systems utilize permanently installed Continuously Operating Reference Stations (CORS) or services that provide correction data over the Internet (network RTK: Ntrip, etc.).

• Rover: The GNSS receiver carried to the survey point. It receives correction information from the base station and applies it to its own observations to compute high-precision coordinates. Rovers include survey antennas and receivers, plus radios or communication modems (when using mobile networks) to receive corrections.

• Data communication: The means of connecting base and rover. Historically, specific low-power radios or UHF radios covering several kilometers to a dozen kilometers were common, but recently receiving correction data distributed over the Internet via mobile networks (VRS or mountpoint methods) has become more prevalent.

The strength of RTK is that it is both real-time and highly accurate. Surveyors can obtain high-precision coordinates on site immediately, reducing post-processing workload and improving work efficiency while maintaining accuracy. However, RTK has limitations: accuracy degrades when the distance from the base station becomes too large, and if radio signals are interrupted the fixed solution must be reacquired. These and other detailed concerns are discussed later, but first let’s look at conventional hydrographic surveying methods and how RTK changes them.

Conventional Methods and Challenges of Hydrographic Surveying (Bathymetry)

Hydrographic surveying refers to measuring underwater terrain and depth in lakes, rivers, and sea areas. In Japan it is also called bathymetry and is indispensable for securing navigation channels, port construction, dam and river maintenance, and verification of dredging works. Conventional marine surveying had to address several challenges different from land surveying.

• Vessel positioning and guidance: In the past, to accurately determine a survey vessel’s position, a total station installed on land would track a prism, or in river cross-section surveys a wire rope with distance markings would be used to guide the boat. After the spread of DGPS (Differential GPS), satellite positioning for vessel position became more common, but planar position accuracy on the order of several meters can be insufficient for narrow waterways or small-scale projects.

• Depth measurement: Water depth is measured with echo sounders (single-beam echo sounders or multibeam echo sounders). Since these devices alone provide only relative distance to the seabed, tide corrections are required afterward. Traditionally, tide gauges were installed near the survey area to record water level changes during surveying, and depth data were corrected for tidal variation. Such surveys requiring tide observation necessitated additional personnel and equipment, increasing cost and labor.

• Efficiency and manpower: Conventional methods required many steps—setting up control points, observing tides, and guiding the vessel—so survey teams tended to be large. For example, personnel were needed to operate total stations on land, read tide gauges, and handle echo sounders and navigation on the boat. Preparing and coordinating these tasks took time and hindered efficiency improvements.

• Data accuracy and reliability: Even minor errors in vessel guidance could be hard to detect later in depth data. If positional records are offset on maps, visual inspection often cannot reveal it, so the outcome depended heavily on operator skill. Additionally, wave-induced heave and rolling cause variation in single depth measurements. Traditionally, when waves were present, the same line was measured multiple times and averaged, but real-time detailed error assessment was difficult and ensuring accuracy was challenging.

As shown above, conventional hydrographic surveying involved significant effort and complex corrections. Next, we will examine in detail how adopting RTK can resolve these challenges.

Techniques and Benefits of Hydrographic Surveying Using RTK

Applying RTK’s high-precision positioning to hydrographic surveying has made great strides in solving issues of conventional methods. In RTK-based hydrographic surveying, the real-time high-precision position (X, Y, Z) obtained from an RTK-GNSS receiver mounted on the vessel is linked with depth data from an echo sounder. The following describes technical points and benefits.

• Real-time tide correction: By using altitude (height) information obtained from RTK, tide correction of depth data can be performed in real time. If the ellipsoidal height of the rover GNSS antenna and the offset to the echo sounder are known, and the height difference between the ellipsoid and the survey datum (geoid separation or difference between ellipsoidal height and water level datum, such as Tokyo Peil (T.P.) or mean sea level) is known in advance, depth can be corrected to the reference surface without continuously observing tide during sounding. In other words, RTK enables "sounding without tide-gauge observation." This eliminates the need to install tide gauges or assign personnel for tide measurement, providing normalized depth values immediately and dramatically improving operational efficiency and timeliness.

• High-precision vessel positioning and labor reduction: RTK-GNSS continuously traces the vessel position at centimeter-level accuracy. This removes the need for land-based visual guidance or placing control points, enabling a single vessel operator to conduct surveys along planned lines. Especially on large lakes, reservoirs, or complex bays, real-time display of vessel position on a map allows efficient line-running. Consequently, tasks that once required multiple personnel can often be completed by a small team on the boat (sometimes just 1–2 people).

• Integrated installation of GNSS antenna and echo sounder: A key technical factor is the spatial arrangement of the GNSS antenna and the echo sounder. Ideally, they should be aligned vertically (one above the other) with a known height difference from the reference surface. For example, on a small boat you can mount the transducer on the hull bottom near the center and the GNSS antenna directly above it on the cabin or deck, minimizing displacement between sounding and positioning points caused by rolling or pitching. In such an integrated system, RTK-derived heights and echo sounder data link in real time, significantly reducing post-processing. Essentially, depth and position are integrated within the onboard system, enabling near real-time bathymetric mapping.

• Improved survey accuracy and reliability: Using RTK height information, temporary vessel vertical motion due to waves will be reflected in the GNSS solution, so in theory wave effects can be corrected in real time. For example, if the boat moves vertically due to waves, the antenna height captures that motion, stabilizing depth values relative to the still-water surface (note the caution regarding large waves and swell discussed later). As a result, variation in depth values from a single pass is reduced, enabling creation of more precise cross-sections and seabed maps. Linked position and depth data produce highly reproducible datasets, making it clear at which location each depth was taken during later analysis. These aspects increase on-site reliability and stakeholder confidence in survey results.

• Reduced time and cost: Because RTK enables labor and time savings and immediate corrections, overall survey time is shortened. Eliminating tide-waiting and repeated averaging means surveys can be completed within favorable weather windows. Manpower and time reductions translate directly into cost savings. Although initial investments in RTK-capable equipment and software are necessary, frequent marine surveying projects can expect good long-term cost-effectiveness.

The above summarizes the technical overview and benefits of RTK-based hydrographic surveying. Next, we describe in detail the issues and precautions to consider when applying this method in the field.

Points to Note for Marine RTK Surveying (Issues and Countermeasures)

RTK-based marine surveying is highly useful but there are several issues to be aware of when applying it in the field. To maintain high accuracy and conduct safe, efficient surveying, consider the following points.

• Satellite signal reception environment: While visibility is often good on water, antennas may be subject to wind-induced motion. Fix the GNSS antenna in the most stable location possible and use a high-performance antenna with strong multipath rejection. In areas with tall structures or bridges, satellite signal multipath (reflections) can easily occur, reducing positioning accuracy or interrupting phase tracking. In such locations, use a ground plane on the antenna, choose times with less interference, or if possible route the survey path to avoid problematic spots.

• Distance from RTK base station: RTK accuracy depends on distance from the base station. Generally, high accuracy is maintained within a few kilometers, but accuracy gradually degrades beyond about 10 km due to ionospheric and tropospheric errors. For large lakes or coastal surveys far from the base station, consider using network RTK (VRS computed from nationwide CORS networks) or deploying your own mobile base stations along the shoreline to follow the survey area. In narrow inland valleys or forested reservoir areas where direct radio from the base is difficult, set up relay stations or use pocket Wi‑Fi or satellite communication to receive Ntrip corrections.

• GNSS and echo sounder offset settings: The vertical and horizontal offset between the GNSS antenna and the echo sounder transducer must be measured accurately and entered into the system before surveying. Incorrect input of the offset from the antenna reference point to the transducer will introduce systematic errors in bathymetric data. Vertical offset in particular is crucial for tide correction. Before starting, precisely measure the distance between antenna and transducer on land and register it accurately in the sonar software. If possible, perform verification measurements at a known shallow point (for example, a pier depth measured with a tape) to compare the RTK bathymetric system’s readings with known values.

• Vessel motion and attitude correction: Small boats experience motion from waves and engine vibration. As noted, aligning antenna and transducer on one vertical axis reduces roll and pitch errors, but does not eliminate them. For multibeam echosounders that measure wide swaths, use attitude sensors (IMU) or gyros to measure roll, pitch, and heading and correct each beam’s angle. Even for single-beam sounding, in substantial swell it is desirable to use a heave sensor to correct vertical motion. RTK height data compensates for some wave effects, but GNSS update rates (typically 1–5 Hz) cannot fully follow high-frequency motion; combine dedicated motion sensors as needed to stabilize data.

• Communication loss and backups: RTK depends on correction data. If communications with the base station are lost or mobile Ntrip connections drop out, the solution may degrade to a float solution or positioning may be impossible. In open sea or mountain reservoirs with unstable radio conditions, plan for the risk of positioning interruption at critical moments. Countermeasures include using receivers capable of switching to PPP (precise point positioning) mode—allowing continued positioning with reduced accuracy—or logging all observation data for later PPK (post-processed kinematic) correction. Also, maintain a backup plan such as running traditional tide-gauge measurements in parallel to mitigate risk if RTK becomes unstable.

• Waterproofing and power management of equipment: On-water operations require attention to equipment waterproofing and stable power supply. Put GNSS receivers, communication devices, and laptops in waterproof cases or choose waterproof-rated equipment to protect against spray and rain. Because saltwater exposure is common, perform post-use cleaning and anti-corrosion measures. Prepare batteries with capacity adequate for operation time to avoid power loss during long surveys. If deploying your own RTK base station, confirm power arrangements (large-capacity batteries or generators) before going to the field.

By preparing with the above points in mind, you can conduct RTK-based marine surveys safely and effectively. Next, let’s look at real-world use cases where RTK has been applied.

Use Cases of Marine RTK Surveying

RTK hydrographic surveying is already being utilized in various field applications. Below are major use cases and sectors taking advantage of its high precision and immediacy.

• River cross-section surveys and channel investigations: For river improvement and dredging planning, detailed knowledge of riverbed cross-section is necessary. Using a boat equipped with RTK-GNSS and an echo sounder to traverse a river enables rapid, high-precision cross-sectional mapping. Where previously surveyors were stationed on both banks with tapes or total stations, RTK allows fewer personnel to survey more safely and efficiently. It is especially effective for post-flood bed-change surveys that require timely measurement.

• Sedimentation surveys in dam reservoirs and retention ponds: Periodic surveys of sediment accumulation in reservoirs and ponds are necessary. Wide-area lake surveying is made more efficient by RTK-enabled autopilot and positioning. Small boats or unmanned boats (radio-controlled) equipped with RTK receivers and fish-finders can collect data in shallow or hazardous areas. Unmanned survey boats using VRS network RTK and compact platforms have been developed, enabling one-person operation. This allows accurate sediment volume estimation across scales from small agricultural ponds to large dam reservoirs.

• As-built control for harbor construction and dredging: For managing dredging and quay construction, as-built surveys confirm whether excavation meets design. Systems with RTK-GPS on work vessels enable real-time monitoring of vessel position and dredge depth. Operators can monitor position and depth to accurately excavate to the target depth. Post-construction inspection surveys can immediately map results with RTK data for rapid acceptance and transition to next stages.

• Emergency waterway surveys after disasters: Following floods or earthquakes that alter channel morphology, RTK hydrographic surveys provide fast reconnaissance. For example, teams using small boats and RTK gear quickly collected bathymetric data after floods to assess bed changes or near-breach terrain. Because depth data are corrected to reference heights in real time, approximate maps can be produced on site and used for rapid disaster response decision-making.

• Fisheries and environmental monitoring: High-precision depth maps are required in fisheries and lake environment monitoring. RTK simplifies labor-intensive surveying tasks here as well. Examples include fishing cooperatives using small boats with surveying gear to map bay seabeds, or researchers using RTK-equipped kayaks to map marsh depth distributions. Low-cost approaches combining consumer fish-finders with RTK-GNSS for simple seabed mapping have been reported, offering low-cost, accurate environmental data collection.

Thus, RTK marine surveying is being applied across civil engineering, disaster response, and environmental fields, demonstrating its usefulness. The next section examines achievable positioning and depth accuracy with concrete numbers and verification results.

Accuracy and Verification of Marine RTK Surveying

Although RTK-based marine surveying claims high precision, a key question is “How accurate is it in practice?” Accuracy should be considered separately for horizontal position and vertical (depth) direction, and we compare theoretical and measured field values.

● Horizontal position accuracy: GNSS-RTK theoretically offers ±1–2 cm (±0.4–0.8 in) horizontal accuracy. Field verifications report that when close to a base station, errors stabilize within a few centimeters or less. Horizontal accuracy matters when you need to precisely detect local depressions or depositional areas in narrow channels, or when you require high reproducibility among multiple survey lines. RTK coordinates are obtained in a global geodetic frame, so data from different days can be compared with a common reference. In one verification, repeated passes over a known point (such as a pier end) with an RTK-equipped boat produced plot scatter within a radius of a few centimeters (a few inches). This is a dramatic improvement over conventional DGPS, which had errors on the order of meters. However, accuracy can worsen to several tenths of a meter if the base station distance increases, so when high horizontal precision is required, work as close to the reference as possible.

● Vertical (depth) accuracy: Vertical accuracy tends to be somewhat lower than horizontal but is still theoretically around ±3–5 cm (±1.2–2.0 in). When using RTK height data for tide correction, remaining error sources are GNSS height random error, antenna–transducer offset input error, and vessel motion correction error. In one test, a boat floated in a tank with known still-water depth and the RTK bathymetric system measured depths with mean errors within a few centimeters. In sea trials on calm days, repeated measurements at the same point showed a standard deviation of about 5 cm (2.0 in). This is notably more accurate than traditional tide-based corrections, which can exhibit residual errors of 10–20 cm in some cases. Nevertheless, in high-wave or strong-wind conditions the GNSS antenna may be shaken and momentarily destabilize the solution; such data may need to be excluded or post-processed with smoothing.

● Combined system accuracy: The overall accuracy of a bathymetric system combining RTK-GNSS and echo sounder depends on the combined factors above. One survey company in Japan reported for its proprietary RTK bathymetry system "depth accuracy: several cm to a dozen-plus cm, horizontal position accuracy: several cm to a dozen-plus cm." The range reflects variation due to sea conditions and survey circumstances, but under typical conditions sub-10 cm accuracy is reasonable to expect. The Geospatial Information Authority of Japan also notes in its procedural guidelines that accuracy obtained by RTK-based marine surveying is at least equivalent to tide-gauge-based methods. To confirm data quality in the field, it is recommended to perform independent checks at key points—for example, compare measured depths at known points within the survey area or measure shallow areas from land with a leveling rod and compare results. Repeated verification builds confidence in RTK hydrographic results and provides evidence for stakeholders.

Future Prospects of Marine RTK Surveying

RTK-based hydrographic surveying is expected to become more widespread and evolve further. Here are prospects regarding technology trends and adoption.

On the technical side, improvements in GNSS performance support this trend. Using Japan’s quasi-zenith satellite Michibiki and its centimeter-level augmentation service (CLAS), high-precision positioning can be achieved without dedicated local base stations. As network RTK and PPP-RTK technologies evolve, centimeter-level positioning over offshore and remote areas will become more stable. Sensor fusion of satellite positioning with IMUs and acoustic gyros is progressing, and inertial compensation technology will likely improve so that short GNSS outages can be bridged without losing high accuracy. In the future, more compact and robust marine surveying systems that maintain accuracy are expected.

Operationally, automation and unmanned systems are trending. Small unmanned surface vehicles (USVs) equipped with RTK and autopilots are already commercially used to follow pre-set routes for autonomous sounding. Future systems could integrate aerial drone (UAV) surveys with USV water surveys to create holistic mappings of river and coastal topography from air and water. RTK’s high-precision positioning will be central to these solutions.

Cost barriers are also lowering. RTK-GNSS receiver systems once cost many hundreds of thousands of dollars, but affordable high-precision GNSS modules and low-cost receivers are emerging. RTK devices that interface with smartphones are appearing. As more cost-effective gear becomes available, small survey firms, local governments, and university labs will be more likely to try marine RTK surveying.

Overall, RTK marine surveying is transforming from a technology for specialized high-precision surveys to a widely usable general technique. As practitioner skills and knowledge sharing grow, higher-quality bathymetric information will be delivered to society, aiding disaster prevention, infrastructure management, and environmental conservation.

Possibility of Simple Surveying Using LRTK

Emerging LRTK (low-cost RTK) systems are attracting attention as an approach that makes RTK 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 professional equipment, LRTK devices are smaller, lighter, and less expensive, and are easier for non-specialists to use.

Using systems like LRTK, survey tasks that formerly required contractors can be performed simply by a single person. For example, to measure the depth of a small pond, one could combine an LRTK device with a fish-finder on a small boat to obtain relatively high-precision data for personal or research purposes. User cases already include mounting an LRTK terminal on a monopod to perform single-point elevation surveying alone, or combining LRTK with a drone or smartphone camera for 3D surveying.

From a field perspective, LRTK serves as "a survey instrument anyone can carry," and is promising for quick reconnaissance during disasters and routine infrastructure inspections. Its portability and responsiveness make it suitable for small boats or rafts to perform simple sounding. As LRTK becomes widespread, people may routinely take quick measurements at waterside sites with a smartphone and share the data.

Importantly, these simple systems are approaching the performance of traditional high-end instruments. Experiments report that LRTK Phone achieved comparable accuracy to professional survey instruments (horizontal ±1–2 cm (±0.4–0.8 in), vertical ±3 cm (±1.2 in) range). Achieving such accuracy from a pocket-sized device is remarkable and represents democratization of surveying.

Of course, professional marine surveys still require high-performance equipment and experienced personnel, but wider adoption of LRTK will enable “measurements when needed,” making aquatic data collection more commonplace. In the future, field staff may routinely take quick waterside measurements with a phone in hand and share results.

FAQ

Q: What is the difference between RTK and GPS? A: GPS refers to satellite positioning systems in general; ordinary standalone positioning typically has errors of several meters. RTK adds relative observations to a base station and corrects for those errors, reducing them to a few centimeters. In short, RTK is a high-precision GPS technique and differs from ordinary GPS by providing high-precision coordinates in real time.

Q: Why is RTK advantageous for hydrographic surveying? A: The main advantages are real-time tide correction and precise vessel positioning. Using RTK, depth data can be corrected to a reference surface during surveying, eliminating separate tide observations. Vessel positional accuracy improves markedly, enabling fewer personnel to guide survey boats efficiently. Consequently, high-quality data are obtained immediately and survey time is reduced.

Q: What equipment is needed for marine RTK surveying? A: Basic components are an RTK-capable GNSS receiver (base and rover set), an echo sounder (single-beam or multibeam), a survey computer (for recording and processing), communication devices (radio or mobile router), and a survey boat. For small-scale surveys, a fish-finder can sometimes be repurposed. Nowadays, correction data can also be obtained over networks without placing a local base station. The important points are securely mounting the antenna and echo sounder and correctly measuring offsets.

Q: If RTK is used, are tide gauges really unnecessary? A: Properly configured, tide gauges are generally unnecessary. Using RTK-derived heights, you can perform on-site tide correction. However, to prepare for the rare case that RTK is unavailable or to initially determine the offset to the reference surface, traditional leveling and tide-gauge data remain useful. During system introduction, it is advisable to run trial surveys that compare RTK and tide-gauge results before fully switching to a tide-free method.

Q: Can RTK marine surveys be performed in rain or rough seas? A: Light rain is acceptable with waterproofing measures, but heavy rain or high waves should be avoided due to degraded accuracy and safety risks. Heavy rain can degrade radio reception and increase equipment failure risk. High waves cause strong vessel motion and GNSS solutions can become unstable. If operation is unavoidable, use attitude sensors for correction and apply post-processing filters, but it is preferable to operate in calm conditions.

Q: What is LRTK? A: LRTK refers to low-cost RTK systems; recently, products like LRTK Phone—high-precision GNSS receivers attachable to smartphones—have become known. Also called Low-cost RTK, these systems are cheaper, smaller, and easier to use than professional GNSS gear, and they enable centimeter-level positioning for non-specialist users. LRTK has the potential to bring RTK to a broad user base; in marine surveying, it makes small-scale depth measurements feasible for individuals.

Q: How can RTK-surveyed data be used? A: RTK hydrographic data feed directly into digital terrain models and bathymetric charts. Use cases include producing longitudinal and cross-sectional river profiles for planning, designing dredging operations, updating nautical charts, and more. These high-precision underwater terrain datasets are also used as 3D point clouds integrated with terrestrial lidar data for visualization. Accurately georeferenced seabed data improve the precision of simulations and models.

Q: I’m anxious about introducing RTK marine surveying for the first time. How can I learn it? A: Attending workshops and technical seminars offered by equipment manufacturers and dealers is the fastest route. You can systematically learn RTK setup, sonar software operation, and marine-specific precautions. Start by practicing RTK positioning on land, then perform test surveys on calm water. LRTK-like simple gear also makes experimentation on small ponds feasible. Begin cautiously, verify procedures, and you will gain proficiency. Accumulating field experience builds know-how and confidence in operations.