RTK positioning made accessible! The secret behind achieving centimeter-level accuracy with compact surveying instruments

In recent years, the advent of compact surveying devices has made centimeter-class high-precision positioning (cm-level accuracy, half-inch accuracy) increasingly accessible. RTK positioning, which previously required specialized surveying equipment and expert technicians, can now be easily achieved with a palm-sized device and a smartphone. On construction sites and in surveying operations, the spread of such compact surveying devices is bringing significant changes to workflows and role allocation.

In this article, we will explain in simple terms the transformation in surveying work brought about by compact surveying devices, the mechanism by which RTK positioning achieves centimeter-level accuracy (half-inch accuracy), and the technical background. We will also introduce the specifications and usage of the compact surveying device LRTK, and, while touching on the benefits as an easy surveying tool anyone can use and concrete on-site use cases, propose a future in which RTK positioning becomes an everyday tool.

Background of the Proliferation of Compact Surveying Instruments and Changes in Surveying Operations

In the past, surveying typically meant that expensive, heavy equipment—such as total stations mounted on tripods and large GNSS receivers—was operated by specialized surveyors. Obtaining survey results required both manpower and time, and field personnel were not in an environment where they could easily carry out surveying work. However, in recent years, driven by technological innovation and the wave of digitization, compact, easy-to-use surveying instruments have begun to appear.

Especially the spread of small surveying instruments that work with smartphones is having a major impact on on-site operations. Smartphones are devices many people use daily and are equipped with high-performance cameras, sensors, and communication functions. The introduction of small surveying instruments that can be used in combination with these smartphones has enabled site managers and workers themselves to quickly carry out surveying and position checks when needed. Even in the construction industry's DX (digital transformation), such as i-Construction promoted by the Ministry of Land, Infrastructure, Transport and Tourism, expectations for these easy-to-use positioning technologies are rising. The era in which each person carries a small surveying instrument in their pocket and can use it immediately when needed is becoming a reality.

Changes in on-site operations are also noticeable. For example, tasks such as verification of as-built conditions and setting-out (stake-driving) work, which had previously been requested from surveying teams, are increasingly being handled by on-site personnel themselves using compact surveying instruments. Interruptions to construction due to waiting for surveying and communication losses have decreased, improving work efficiency. Also, thanks to devices that even less-experienced technicians can operate intuitively, they are contributing to alleviating labor shortages and the transfer of skills. The spread of compact surveying instruments is transforming surveying work from something reserved for specialists into a familiar task for everyone working on site.

Basics of RTK Positioning and the Principles Behind Achieving cm level Accuracy (half-inch accuracy)

An indispensable technology when discussing the advances of compact surveying instruments is RTK positioning (Real Time Kinematic positioning). RTK positioning is a method that obtains highly accurate positions by comparing in real time the GNSS satellite signals received at both a mobile unit (rover) and a base station installed at a known point, and correcting the errors. Typically, common GNSS positioning such as smartphone-built-in GPS has errors on the order of several meters. These errors arise from various factors such as signal delay errors caused by the atmosphere (ionosphere and troposphere) and satellite clock offsets, but RTK cancels these in real time using correction information from the base station.

To explain the principle simply, a base station is a receiver whose precise position is known. By comparing the GNSS signal information received by that base station with the information received by the rover (the receiver at the point to be measured), the deviation of the satellite signals is calculated. The amount of that deviation (correction data) is then transmitted to the rover, which applies the correction to its own positioning results. Because errors are canceled out through this relative comparison between the two, the positioning accuracy is dramatically improved. Using RTK positioning, horizontal positions can achieve approximately ±2–3 cm (±0.8–1.2 in), and vertical positions are also on the order of ±a few cm (±a few in), yielding centimeter-level accuracy (half-inch accuracy). Since conventional GPS positioning used to have errors on the order of several meters (several ft), RTK reduces them to within a few centimeters (within a few in), so it is widely used in fields that require precise positioning such as surveying, civil engineering and construction, agriculture, and autonomous driving.

Because RTK positioning basically requires real-time transmission of correction information, the following methods were used in the past. One was the local reference station method, in which the user sets a control point with known coordinates on site and installs a reference-station receiver, communicating wirelessly with the rover to send correction data. In this case, installation and management of the base station equipment was labor-intensive, and there was the constraint that accuracy degraded as one moved away from the base station. The other was the network RTK method, which obtains correction information from a network of reference stations on the Internet (for example, the Geospatial Information Authority of Japan’s network of continuously operating reference stations) via the mobile communications network. This method has the advantage that users do not need to provide their own base station, and because virtual reference points are set near the user and correction information is distributed, stable accuracy can be obtained anywhere. However, there were also issues such as the need to subscribe to correction information services and pay monthly fees, and that it cannot be used outside of communication coverage.

Thus, while RTK positioning is highly accurate, it has required either setting up a base station or obtaining corrections via communications infrastructure. However, advances in technology have produced new approaches that greatly relax these requirements. The key to this is the Centimeter-Level Positioning Augmentation Service (CLAS) provided by Japan's Quasi-Zenith Satellite System "Michibiki".

Technical advances that have enabled RTK to be used on compact surveying instruments

The reason small surveying instruments have made RTK positioning possible lies in technological advances in both hardware and infrastructure. On the hardware side, the miniaturization and performance improvements of GNSS receivers and antennas are notable. A decade ago, centimeter-class GNSS receivers were housed in large cases that included batteries, but recently high-sensitivity GNSS modules have come to fit on boards a few centimeters (a few in) in size. Moreover, advances in multi-GNSS and multi-frequency chip technology have led to practical small receivers that can simultaneously receive multiple frequencies—not only L1 but also bands such as L5—and multiple satellite constellations beyond GPS, such as GLONASS, Galileo, BeiDou, and QZSS (Michibiki). This increases the number of satellites that can be tracked even in urban canyons and mountainous areas, enabling more stable positioning than before.

At the same time, improvements in the positioning performance of smartphones themselves should not be overlooked. On Android smartphones, models supporting dual-frequency GNSS (supporting the L1 and L5 bands) began appearing around 2018, and other smartphones such as the iPhone have also increasingly added devices that support high-precision positioning. In addition, Android OS has published APIs that allow access to raw GNSS data (carrier phase and pseudorange, etc.), and there are attempts to run advanced positioning algorithms on smartphones. As a result, standalone smartphones are becoming capable of higher-precision positioning than before (sub-meter-level (sub-1 m (sub-3.3 ft)), and in some cases decimeter-level (0.1 m (3.9 in))). However, because standalone smartphones have issues such as the sensitivity and noise of their built-in antennas, there remain challenges to stably achieving centimeter-level accuracy (half-inch accuracy) in practical use.

This led to the approach of combining a smartphone with an external high-performance GNSS receiver. The compact surveying device LRTK was developed exactly with this concept. By combining the smartphone's usability and communication functions with the compact surveying device's high-sensitivity GNSS, it easily achieves centimeter-level positioning that was difficult to accomplish with either alone.

There has also been major progress on the infrastructure side. That is the practical implementation of the aforementioned CLAS. CLAS (Centimeter Level Augmentation Service) is a satellite-communication-type RTK correction distribution service that covers all of Japan, and as long as you have a compatible receiver, you can achieve centimeter-level positioning without internet communication. Because correction information can be applied simply by receiving L6-band signals transmitted from Michibiki, high-precision positioning can be maintained in real time even in remote mountain areas or sites outside of communications coverage.

For example, during the 2023 Noto Peninsula earthquake, there were reports that compact CLAS-compatible RTK receivers were active and demonstrated their effectiveness in field surveys even as mobile networks in the affected areas were down. This is the benefit of a technology that can obtain correction information directly from satellites without relying on network-based RTK. Thanks to technical advances in both hardware and software, RTK positioning has entered an era in which it can be used without special large-scale equipment.

Compact Surveying Instrument LRTK: Specifications, Size, Centimeter-level Accuracy, Smartphone Connectivity, and CLAS Support

A representative example of the compact surveying instruments born from the aforementioned technological advances is LRTK. LRTK is a series of high-precision GNSS receivers developed by a startup originating from the Tokyo Institute of Technology, and among them the smartphone-integrated "LRTK Phone" is a revolutionary device small enough to fit in a pocket. It weighs approximately 165 g and is only about 13 mm (0.51 in) thick, with a built-in battery and antenna, and can be attached to a smartphone (mainly iPhone) with one touch via a dedicated phone case. No cable connection is required; once attached it links with the phone via Bluetooth, and you operate the positioning app on the phone's screen. Simply attaching this single unit turns your everyday smartphone into a centimeter-level accuracy (half-inch accuracy) surveying instrument.

LRTK is equipped with a high-performance GNSS module and, as noted above, is a receiver that supports multi-GNSS such as GPS, GLONASS, Galileo, and Michibiki. Because it can simultaneously receive satellite signals on multiple frequency bands such as L1, L2, and L5, it has the advantage of being able to obtain stable positioning solutions even in challenging environments. In practice, positioning using LRTK can stably obtain an RTK fixed solution outdoors (errors within a few centimeters (a few inches)), and in some cases accuracy can improve to the millimeter order (mm (0.04 in)). Another major feature is CLAS support. Because LRTK can receive Michibiki’s L6-band signal, in covered areas it can perform standalone RTK positioning without connecting to the Internet. Of course, when a communication environment is available, it can also connect to Ntrip-based network RTK correction services via a smartphone connection. In other words, “where there is communication, via the network; where there is no communication, via satellites”, allowing flexible acquisition of correction information according to the situation.

Ruggedness is also important for equipment used in the field. LRTK series devices are designed with environmental robustness such as dust and water resistance in mind, and operate reliably even under the harsh conditions of construction sites. Although they are compact surveying instruments, they pack professional-grade specifications, and some models include a tilt compensation function that can calculate the precise coordinates of the tip even when mounted on a slightly tilted pole. This allows measurement of points even in situations where the pole cannot be held vertically because of trees or obstacles, making them effective for surveying locations that were previously unmeasurable.

To summarize, LRTK is a compact, all-in-one surveying instrument designed for easy use in conjunction with a smartphone. It is pocket-sized and easy to carry, so you can take it out on site when needed and immediately begin centimeter-level positioning (cm level accuracy (half-inch accuracy)). With the dedicated app, no complicated setup is required—you can start positioning with a single button, and the coordinates of measured points can be plotted on a map on the spot or saved to the cloud and shared with the office. Compared with expensive conventional surveying equipment, the initial cost is also lower, and by using the free CLAS, maintenance costs are kept to a minimum. Combining this level of specification and ease of use, the LRTK is poised to revolutionize surveying workflows in the field.

On-site positioning workflow using LRTK

Now let's look at the workflow to see which tasks and to what extent they can be simplified when using the compact LRTK surveying device in the field. With just the LRTK and a smartphone, you can consistently perform tasks that previously required separate devices and procedures. The main features and operational flow are as follows.

• Single-point RTK positioning (point surveying): Set the LRTK at the point you want to measure, and just press a button on the smartphone app to record that point’s high-precision latitude, longitude, and height. For example, attach the supplied monopod or pole to the LRTK, touch the tip to the ground point, and tap the positioning button to instantly obtain centimeter-level (half-inch accuracy) coordinates. The recorded coordinate data for the point automatically includes the time and the positioning method (whether an RTK fixed solution was achieved), and point names are auto-numbered. There is no need for cumbersome coordinate calculations or handwritten notes: the app automatically performs conversions to the plane rectangular coordinate system and computes the geoid height on site, so anyone can obtain accurate survey coordinates.

• AR staking and layout marking (survey guidance using AR): Staking and layout marking work based on design drawings and existing coordinate data can also be made more efficient with LRTK. If you preload the design reference points and positions of structures into the LRTK app, the smartphone will navigate you to those positions on site. By following the AR arrows and guides displayed on the smartphone screen, you will be guided to the target position with centimeter-level accuracy (cm level accuracy, half-inch accuracy). When you reach the designated point, you can also place a virtual AR stake on the screen. For example, even in places where you normally cannot physically enter to set stakes, such as mid-slope on an embankment, you can place a stake in AR from a distance to record the location. Even if you cannot physically drive a stake into a hard concrete surface, you can mark it virtually with an AR stake, enabling safe and reliable surveying work. AR technology allows anyone to lay out stake positions and perform layout marking without getting lost, and it also supports solo staking operations.

• 3D point cloud acquisition (smartphone LiDAR measurement and absolute coordinate assignment): By combining the LiDAR scanner built into the latest iPhone and iPad with LRTK, you can easily perform three-dimensional measurements of surrounding terrain and structures. Conventional standalone smartphone LiDAR scans had the problem that positional errors caused distortions in the scanned geometry. However, if you perform a walking scan while continuously correcting the self-position to cm level accuracy (half-inch accuracy) with LRTK, you can assign accurate absolute coordinates to the point cloud data. For example, simply walking and scanning across a large development site can produce a high-density terrain point cloud model comprising tens of millions of points. Because that point cloud aligns with a known coordinate system, it can be used immediately after acquisition for comparison with design data or for volume calculations. A major advantage is that even in narrow spaces inaccessible to heavy machinery or in urban areas where drones cannot be flown, an operator can perform detailed 3D surveying simply by walking. The acquired point cloud can be uploaded to the cloud and shared immediately, or, if necessary, cross-sectional drawings can be generated and exported.

• Photo records and location information tags: LRTK is also effective for recording site conditions and maintenance management tasks. When you take a photo with a smartphone camera, the high-precision coordinates of the shooting location and the camera's orientation (azimuth) can be automatically recorded at the same time. This makes it easy to check the shooting location for each photo on a map in the office afterward, or to re-shoot in the same place and from the same angle as a previous photo. The LRTK app can even use AR guide displays to help you hold the smartphone in the "same position and angle as last time," making it convenient for recording long-term changes in fixed-point observations. Because cracks and equipment deterioration can be photographed and compared every time using exactly the same framing without any shift, the traditional hassle of comparing photos pasted in paper forms is eliminated. Photo records with coordinates will also be of great help for as-built inspection reports and disaster survey records.

• Report Output & Cloud Sharing: All data acquired with LRTK is stored in digital form, so subsequent document creation is just one click. Lists of survey points, coordinate values, point cloud data, and photos with location information are automatically plotted on a map when uploaded to the LRTK cloud. Office staff can simply access the cloud via a browser to immediately share and review the latest survey results obtained on site. For example, because calculations such as distances between survey points and area and volume can be performed on the cloud, you can smoothly proceed from measured data on site directly to calculation of as-built quantities and report creation. This eliminates the need to copy data into paper field notebooks or manually enter it into surveying software, enabling seamlessly link the process from surveying to reporting.

As described above, using LRTK allows the entire series of positioning and surveying tasks to be completed within a single system. From point observation and comparison with design values to 3D scanning, management of record photos, and report generation, it can handle every surveying need that arises on site with a single device, and can be called an all-in-one surveying tool.

User-friendly operation for surveying beginners and on-site supervisors

Even high-performance surveying equipment is meaningless if it can't be used effectively in the field. In that respect, the LRTK is thoroughly designed with a focus on ease of use. One advantage of linking with a smartphone is that positioning tasks can be carried out through an interface everyone is already familiar with. The dedicated app's screen is intuitive and easy to understand, allowing surveying to begin without having to think about difficult technical terms. For example, when you press the "Start Positioning" button, it automatically handles everything from connecting to correction data to calculating coordinates, completing positioning in just a few seconds. Whether an RTK fixed solution has been obtained is also visible at a glance on the screen, so even beginners can use it with confidence.

Because you can check maps and AR displays on a smartphone’s large screen, it is easier to grasp the situation visually than by reading the small screens and arrays of numbers on conventional devices. The AR navigation function requires no instruction manual—just follow the indicated direction—so even those not specialized in surveying, such as site supervisors, can use it without hesitation. The device itself is lightweight, compact, and cable-free, so carrying and setting it up does not take time. The convenience of taking it out of your pocket and attaching it to your smartphone to start measuring immediately is a feature that allows stress-free use even during breaks in field work.

Furthermore, managing positioning data is simple. Because it integrates with a cloud service, even people unfamiliar with operating the device need not worry about losing or mixing up data. Measured data is automatically synced to the cloud, preventing mistakes such as "accidentally losing a note" or "forgetting to transfer data via USB." By enabling anyone to respond immediately to sudden surveying needs on site, this reduces waiting times and human errors, contributing to improved productivity and safety.

Thus, LRTK combines the ease of use that allows even people without surveying expertise to operate it and the reliability to withstand on-site practical work. If positioning tasks that were previously entrusted to surveying companies or qualified personnel can be handled by your own field staff, you can expect significant cost reductions and faster operations.

Practical use cases (as-built, stakeout, boundary confirmation, disaster surveying, etc.)

Small surveying instruments like the LRTK are used in a variety of on-site scenarios. Here we present some representative use cases.

• As-built surveying and as-built management: In surveys to confirm the as-built condition (the shape after construction) after project completion, traditionally many points were measured with a total station, and in some cases cross-sections were measured manually. Using LRTK, the finished terrain and structures can be captured in a short time as 3D point clouds and overlaid on the design model to check the as-built condition. For example, calculating the volumes of embankments and excavations from point cloud data on site and comparing them with the design quantities is easy. Because the acquired as-built data can be shared instantly via the cloud, inspections with the client proceed smoothly. As-built surveying with small surveying instruments greatly reduces the effort required to prepare for inspections and contributes to shorter construction schedules.

• Pile driving and layout/positioning tasks: LRTK proves powerful even for indispensable pile driving and layout (positioning) tasks in building and civil engineering works. With LRTK’s AR guidance you can pinpoint positions to preset design coordinates, eliminating the need for survey staff and construction staff to separately confirm pile positions. A single person can pinpoint and mark pile positions, so work can proceed efficiently even on sites with limited personnel or during nighttime operations. In addition, the AR virtual piling feature enables the positioning of hazardous or hard-to-access areas to be carried out remotely and safely. Even the traditionally time-consuming installation of batter boards can be completed quickly and accurately with LRTK, allowing construction to start sooner.

• Boundary verification and land surveying: Confirming boundary stakes and site boundaries is a delicate survey task where mistakes are not acceptable. LRTK can register known boundary point coordinate data (for example, public coordinate values) to the cloud in advance and be used to guide you to those points in the field. For example, if you upload a CSV or geojson file of boundary coordinate lists to the LRTK cloud, you can navigate on-site to each boundary point synced to the app. Even when searching for a buried, unseen boundary stake, following the arrow on the smartphone screen will allow you to locate the target within an error range of a few centimeters (a few in). At boundary inspection meetings, being able to provide digital corroboration alongside visual confirmation increases reliability. Because accurate boundary verification can be performed without surveying expertise, it contributes to simplifying land surveying and preventing boundary disputes.

• Surveying and inspection at disaster sites: At sites where disasters such as earthquakes and landslides occur, rapid situation assessment and recovery planning are required. Because LRTK operates by receiving corrections from satellites even in environments where mobile communications are down, it proves useful in isolated areas immediately after a disaster. It is used for tasks such as walking through collapsed terrain in affected areas to capture point clouds, calculating the volume of collapsed soil on the spot to help arrange heavy machinery, and conducting fixed-point monitoring of the progression of cracks and tilting. When drones cannot be flown due to bad weather or radio conditions are unstable, LRTK— which can be used by personnel walking the site—becomes a valuable surveying method. In actual disaster response, there have been cases where site data obtained with LRTK was shared via the cloud, enabling municipalities and support teams to exchange information in real time and thus support rapid decision-making. On disaster surveying sites, small surveying instruments can be considered nimble, reliable partners.

As described above, simplified surveying using LRTK is being put into practical use across a wide range of scenarios, from construction management to maintenance and emergency response. Surveying work that previously required significant time and cost has been dramatically streamlined, accelerating on-site DX.

How simplified surveying with LRTK paves the way for a future in which RTK becomes an everyday tool

The compact surveying device LRTK has made easy-to-use, high-precision positioning a reality, transforming RTK positioning into an everyday tool. Centimeter-level positioning, which used to be advanced technology reserved for specialists, has now become an accessible instrument that anyone on site can use. This is not merely a convenience; it has the power to transform the very nature of surveying. When high-precision position information is available in real time at any time, construction site workflows and management methods evolve significantly. For example, performing fine adjustments and as-built checks on the spot during construction and providing immediate feedback can minimize rework and errors. Sharing real-time on-site information via the cloud also gives rise to new forms of collaboration, such as experts being able to provide advice remotely.

LRTK combines high performance with a low barrier to adoption, and it has the potential to become the new norm in the construction and surveying industries going forward. Its price is markedly more reasonable than traditional surveying equipment, and maintenance costs are low, making it a realistic option for many sites. Individuals carrying their own personal surveying devices and taking measurements whenever needed——that future is just around the corner. If RTK positioning becomes an everyday tool, on-site productivity and safety will be dramatically improved, and it will, in turn, contribute to reforming work practices across the industry.

Simple surveying using LRTK, which brings centimeter-level positioning closer to users, is precisely that first step. It is a technology we hope even those who have until now regarded surveying as outside their expertise will pick up and experience for themselves. By introducing the compact surveying device LRTK to the field, surveying tasks that used to be cumbersome become remarkably simple, and the value of the data obtained increases. Now that RTK positioning is becoming more commonplace, why not leverage LRTK to take your site to the next stage?