Table of Contents

• Mechanism and Background of RTK Surveying

• The Emergence of Smartphone RTK Surveying and the Technical Features of LRTK

• The Basis for Achieving Centimeter-Level Accuracy with Smartphone RTK

• Applications to Point Cloud Measurement, As-Built Management, CAD Integration, AR Guidance, and Cloud Sharing

• On-Site Implementation Cases and Their Effects (Labor Reduction, Time Savings, Improved Safety)

• Barriers to Adoption and Practical Tips for On-Site Use

• Introduction to Simple Surveying Using LRTK

• FAQ

Mechanism and Background of RTK Surveying

An indispensable technique for conducting precise surveying is RTK surveying. RTK stands for *Real Time Kinematic（リアルタイムキネマティック）* and refers to a real-time high-precision positioning method based on satellite positioning (GNSS). Positioning using ordinary GPS or GNSS typically results in errors of several meters on common smartphones. This is because various factors such as atmospheric effects, satellite orbital errors, and receiver clock errors cause discrepancies in positioning. However, in RTK surveying, the differences between GNSS signals received by the reference station (a receiver placed at a known accurate position) and the rover (the device whose position is to be measured) are used in real time to correct these error factors. As a result, even in the field, you can determine your position with centimeter-level accuracy (cm level accuracy, half-inch accuracy).

In Japan, the Geospatial Information Authority of Japan has deployed a nationwide Continuously Operating Reference Station network (GEONET) and a network RTK is in place that can receive correction information (differential data) over the Internet via a mechanism called Ntrip. If the positioning terminal is an RTK-capable receiver and you connect to a correction information service with a smartphone or similar device, you can obtain a high-precision solution called a "fixed solution" in about 1 minute, after which location information continues to be updated with an astonishing accuracy of approximately within 1–2 cm (0.4–0.8 in).

Furthermore, in recent years a centimeter-level augmentation service (CLAS) using the Quasi-Zenith Satellite System (QZSS, nickname: Michibiki), operated by the Cabinet Office, has been introduced; with a compatible receiver, correction information can be obtained directly from the satellites even in mountainous areas where mobile signals do not reach, allowing high-precision positioning to be maintained. Even at disaster sites or in regions with unstable communications infrastructure, as long as devices are CLAS-compatible, stable centimeter-level positioning (half-inch accuracy) becomes possible, and the usefulness of RTK technology is increasing.

Thus, thanks to the mechanism of RTK surveying, even positioning using GNSS satellites can have errors thoroughly eliminated, making it possible to determine absolute position to centimeter-level accuracy. Traditionally, achieving this level of accuracy required optical surveying instruments such as total stations or an expensive GNSS receiver set, and operation by a skilled surveyor was assumed. However, recent technological developments have produced new solutions that make this easier to realize. A representative example is RTK surveying using smartphones, and among these, LRTK has attracted attention as a groundbreaking product.

Emergence of Smartphone RTK Surveying and the Technical Characteristics of LRTK

Smartphone RTK surveying, as the name implies, is a method of performing RTK positioning using a smartphone. When you hear "high-precision positioning" many imagine specialized equipment, but in recent years it has become possible to achieve centimeter-level positioning simply by attaching an external device to a smartphone. A pioneer in this area is Lefixea (Refixia), a startup originating from Tokyo Institute of Technology, which developed the LRTK Phone. This is an ultra-compact RTK-GNSS receiver that fits in a pocket and is designed to attach to an iPhone or iPad in one touch via a dedicated case. Weighing approximately 125 g and with a thickness of approximately 13 mm (0.51 in), it is slim and lightweight yet houses a high-performance antenna and battery, so it does not add bulk when attached to the back of a smartphone. It communicates with the smartphone via Bluetooth connection or Lightning cable, and supports both the aforementioned network RTK (Ntrip method) and reception of CLAS corrections from the Quasi-Zenith Satellite System, allowing high-precision real-time positioning anywhere in Japan. The built-in battery runs continuously for approximately 6 hours, and operation can be extended via USB power supply. It also has dustproof and waterproof performance, so it can be used with confidence even in harsh outdoor field environments.

LRTK provides more than just hardware receivers. LRTK system consists of three main components and offers comprehensive support for field surveying. The first is the dedicated GNSS terminal mentioned earlier, the "LRTK Phone." The second is the LRTK app for iPhone/iPad. Using this dedicated app, all necessary tasks—from initiating positioning to data recording, AR display, and navigation—can be completed on a single smartphone. For example, you can switch with a single button between single point measurements (single point measurement) and continuous positioning of up to 10 points per second, and because it includes a multi-point averaging function, it also contributes to improved accuracy. Acquired points are plotted on the map in real time, and conversions to the Japan Plane Rectangular Coordinate System and automatic calculation of geoid height are performed in the background. Furthermore, photos taken with the smartphone camera are automatically tagged with high-precision location information and shooting direction, and each photo can be uploaded to the cloud with a single tap. It also features the ability to set recorded points as target locations and navigate to them on the map or in AR. By using AR to display virtual stakes and markings, it supports guidance for stake-driving and batter-board positions, enables non-contact positioning of distant targets, and makes on-site tasks such as measuring, indicating, and recording possible with a single smartphone.

The third component is the web-based LRTK Cloud. Survey data collected on site (survey point coordinates, point clouds, photos, etc.) can be synced to the cloud with a single button press and are shared and stored instantly without returning to the office. As long as there is an internet connection, there is no need to install dedicated software, and data can be viewed on browser-based maps and a 3D viewer. If you send a sharing link to stakeholders, external partner companies or clients who do not hold licenses can view the data without a high-spec PC or a specialized point-cloud viewer. It is also easy to track chronologically organized survey points and photos, or to download CSV coordinate data and point-cloud data for use in your company's CAD software.

As such, LRTK is offered as a comprehensive platform—dedicated device＋app＋cloud—and supports the entire workflow, not only measuring high-precision coordinates with GNSS but also the subsequent data processing, utilization, and sharing. The approach of turning a handheld smartphone into a surveying instrument makes it possible to achieve accuracy and functionality comparable to conventional expensive surveying instruments more easily, which can be described as revolutionary.

The Basis for Achieving Centimeter-Level Accuracy with Smartphone RTK

When people hear "smartphone RTK surveying," some may be skeptical and wonder, "Can a smartphone really achieve that level of accuracy?" However, the basis for that centimeter-level accuracy (cm level accuracy (half-inch accuracy)) lies in the use of the RTK technology described above. The GPS chips built into typical smartphones are not designed for high-precision positioning and therefore suffer errors of several meters (several ft), but if you connect an RTK-capable dedicated receiver such as an LRTK, you can use a professional-grade GNSS antenna and positioning engine, which quickly improves positioning accuracy. Furthermore, by receiving correction information from the Geospatial Information Authority of Japan's network of continuously operating reference stations via Ntrip, or by directly receiving and augmenting with CLAS satellite signals, the majority of the error is removed. As a result, even when using a smartphone, positioning in principle can be achieved at levels equivalent to conventional surveying GNSS units.

In fact, LRTK's positioning accuracy has been confirmed to be comparable to that of traditional fixed receivers. For example, when a fixed LRTK unit performed stationary positioning and coordinates were calculated using an averaging function, data show that the horizontal standard deviation, which was approximately 12 mm (0.47 in) for single positioning, was reduced to about 8 mm (0.31 in) by averaging 60 observations. This means the position could be determined with an accuracy below 10 mm (0.39 in), a result that rivals professional surveying instruments. Of course, accuracy depends on the positioning environment (such as sky visibility and surrounding obstructions), but generally outdoors in fair weather one can expect horizontal accuracy of about 1-2 cm (0.4-0.8 in) and vertical accuracy of about 3-5 cm (1.2-2.0 in). That such accuracy can be obtained with a smartphone is a technological leap that would have been unthinkable not long ago.

Various measures implemented in the LRTK app also help support accuracy. For example, multiple sampling of positioning points with automatic averaging, and tilt correction using the smartphone’s attitude sensors (applying vertical correction even when mounted on a pole at an angle) address common field situations and prevent accuracy degradation. Furthermore, as mentioned above, CLAS support allows positioning to continue even outside cellular coverage, so centimeter-level positions can be maintained regardless of distance from the reference point, even in signal-deprived locations such as forested areas or under overpasses. These elements combine to create a system that ensures centimeter-level reliability at all times even with smartphone RTK.

Applications of Point Cloud Measurement to As-built Management, CAD Integration, AR Guidance, and Cloud Sharing

Smartphone RTK surveying enabled by LRTK is not limited to simply measuring point coordinates; it also holds potential for a wide range of on-site operational applications. Here we introduce the representative five application areas.

• Point cloud measurement: You can acquire surrounding 3D point cloud data using the LiDAR scanner and cameras of iPhone and iPad. By using LRTK together, you can assign high-precision geographic (global) coordinates to all scanned points, eliminating the distortions and scale instabilities that are common with scans made by smartphones alone. For example, if you walk around a development site scanning the terrain and structures, you can obtain an accurate 3D point cloud model without positional shifts in a short time. The acquisition distance covers the range reachable by smartphone LiDAR (about 5 m (16.4 ft)) and, by combining photogrammetry techniques, can cover targets up to 50-60 m (164.0-196.9 ft) away. This allows anyone to easily create point cloud data with absolute coordinates over a wide area, which is useful for large-scale earthwork volume calculations and terrain analysis.

• As-built management: Smartphone RTK also proves powerful for as-built management, which checks the post-construction shape in civil and building works. Traditionally, after construction surveying teams measured many point elevations and then returned to the office to compare them with design drawings and reference elevations to assess excesses or deficiencies in fills and excavations. Using LRTK, this process can be performed on-site in real time. For example, immediately after grading a development site, you can scan the ground surface point cloud on the spot with the smartphone's LiDAR while simultaneously assigning absolute coordinates to each point with RTK. The acquired current point cloud can then be overlaid on the completed design model (3D design data) or the specified design elevations on the spot, allowing intuitive identification by color coding of areas with excessive fill and areas with insufficient excavation. A cloud-based point-cloud viewer lets you check the elevation and cross-sections at any point, and it also has automatic calculation functions for fill and cut volumes, drastically reducing the time required for as-built inspections. Being able to perform quality control immediately helps prevent rework and enables safe and efficient construction management.

• CAD integration: Point cloud data and survey point coordinates acquired with LRTK can be used seamlessly for subsequent CAD drawing creation and BIM model construction. From the LRTK cloud you can export them as CSV-format coordinate data or point cloud files in LAS format, etc., and import them into common civil engineering design CAD or 3D software. Conversely, you can upload 3D models and drawing data from the design phase to the LRTK cloud and overlay them with point clouds acquired on site. This makes it possible to detect discrepancies between the design model and actual conditions in advance and to use them as material when considering design changes. Data flows between the field and the design office, and by digitally linking the surveying→design→construction cycle, tasks that were previously separate processes become integrated and speed up.

• AR guidance: AR (augmented reality) technology that overlays information onto the real world through a smartphone screen also becomes a more practical tool thanks to LRTK. For example, virtual stakes or markings can be displayed in AR at pre-set coordinate positions to support position setting for stake driving and batter board layout (installation of reference lines). Tasks that used to require determining positions on site with tape measures or surveying instruments based on dimensions on drawings will be replaced by simply walking following on-screen arrows with a smartphone in hand to be guided to the exact point. Even in locations where driving stakes is physically difficult, such as on slopes or bedrock, AR markers can indicate positions, reducing the workload. Also, if the positions of underground pipes and cables are pre-scanned and recorded as point clouds, that data can be projected via AR during the next excavation to visualize areas not to be dug. AR guidance can be expected to prevent on-site mistakes and improve efficiency.

• Cloud sharing: Any data acquired using LRTK (measured points, point clouds, photographs, notes, etc.) can be shared instantly via the cloud. Because information measured on site can be shared on the spot with internal stakeholders and clients, real-time information sharing is achieved. For example, if a site representative uploads points or photos measured with LRTK to the cloud, designers or supervisors in remote locations can immediately check the content and issue instructions as needed. This greatly shortens the traditional time lag of "measure on site, take data back, report/consult, and then make decisions." Also, because data are centrally managed in the cloud, it is easy to review daily survey results and construction progress on a timeline. There is no longer a need to exchange data via paper field books or USB memory sticks, and the speed and accuracy of information sharing improve dramatically.

Case Studies of On-Site Implementation and Their Effects (Labor Savings, Time Reduction, and Improved Safety)

From actual field implementations of smartphone RTK, effects such as labor savings, time savings, and improved safety have been reported compared with conventional methods. Let's look at some specific cases.

Case 1: Stake-setting work at a road construction site – At a certain paving project site, positioning had previously taken a team of two more than half a day to lay string lines from reference points. After introducing LRTK, one worker could hold a smartphone and use the AR guidance feature to identify stake positions and simply mark them on the spot. The same task was completed in a matter of tens of minutes, and with the workforce halved this led to a significant efficiency improvement. "Because one person can do it, we can reassign other personnel to different tasks and productivity has increased," the site supervisor said, surprised.

Case 2: As-built inspection in site formation work – For verifying the as-built condition of earthworks, such as embankments and cuttings, the conventional approach was to measure hundreds of survey points and perform volume calculations back at the office. On one site, LRTK was used to acquire the as-built point cloud immediately after construction, and the volume differences against the design model were calculated instantly in the cloud. As a result, the as-built verification work that used to take a full day was completed in a few hours, and identification of corrective areas and instructions for additional fill were issued on the same day. This not only prevented situations of "discovering a mistake later and having to redo work," but also sped up reporting to the client because progress could be checked numerically on a daily basis.

Case 3: Safe surveying at disaster sites – In one municipality, LRTK was used for the restoration design of a slope (embankment) that collapsed during heavy rain. Because surveying a landslide site carries the risk of secondary disasters, it had traditionally been difficult to obtain detailed terrain information until repair work began. Therefore, immediately after the disaster the person in charge used a smartphone equipped with LRTK to perform a point-cloud scan of the collapsed area from a safe distance, and instantly calculated an estimate of the collapsed soil volume and the slope angle of the cliff face. One person completed the work in a matter of tens of minutes without entering dangerous locations, greatly contributing to speeding up the initial response and ensuring staff safety. The municipality plans to continue using smartphone surveying as a disaster-response tool, saying that “surveying is completed simply by carrying it.”

As described above, the on-site introduction of smartphone RTK has demonstrated effects such as alleviating labor shortages, shortening work time, and ensuring worker safety. In terms of equipment costs, solutions like LRTK are more affordable than conventional surveying instruments, so if they become widespread at many sites, efficient operations with one device per person will become possible.

Implementation Hurdles and Practical Tips for On-site Use

Smartphone RTK is innovative, but there are some hurdles to consider when introducing it to the field. First, among field staff there may be anxiety and resistance along the lines of "Is it really okay to use a smartphone?" Surveyors and technicians who have become accustomed to conventional equipment through many years of experience can be particularly skeptical of new technologies. This concern can only be dispelled by actually using the technology and experiencing its effectiveness, so it's best to start by gradually using it for supplementary purposes. For example, while carrying out important control point surveys with a total station as before, you could try smartphone RTK for other simple surveys and progress checks — the key is to build trust gradually.

Next, cost considerations. Devices and services for smartphone RTK require initial costs, but they are significantly cheaper compared with expensive total stations or 3D laser scanners. For organizations already using iPhone or iPad on site, in many cases you can start simply by purchasing an additional receiver. Also, because they may be eligible for national or municipal subsidies promoting ICT in construction, the financial barriers have been decreasing year by year. From a return on investment perspective, compare the labor and time savings with the equipment costs, and use that comparison to persuade management.

As a technical hurdle, attention must also be paid to the constraints specific to GNSS positioning. Even though it offers high accuracy, because it relies on satellites, positioning can become unstable or impossible in locations where the sky is not open (urban canyons between high-rise buildings, wooded areas with dense trees, inside tunnels, etc.). In such environments, do not force it; respond by combining conventional methods or by using measures such as the remote positioning function from an open area later (a function to measure distant targets with a camera). Also, for workers who are inexperienced with smartphone operation, it is important to provide sufficient initial operation training and to prepare manuals. Fortunately, the LRTK app is designed with an intuitive UI, and basic usage can be learned in a short training session, but practicing beforehand is reassuring so that staff are not confused on site.

A practical tip for on-site use is to appoint an internal champion and share know-how. Staff who are knowledgeable about digital surveying should take the lead in mastering the tools, and by sharing and rolling out the collected data within the company, other members will more easily understand, thinking, "Is it really this easy to get results?" It is also effective to start with small-scale sites or pilot deployments and accumulate successful experiences. Doing so will help the benefits of using smartphone RTK permeate the entire site and lead to smooth adoption.

Introduction to Simple Surveying Using LRTK

As we have seen, the combination of smartphones and RTK technology is set to bring a major transformation to the world of surveying. By leveraging solutions like LRTK, tasks that used to be entrusted to specialized survey teams are being transformed into simple surveys that anyone can easily perform. An era has arrived in which on-site personnel themselves can acquire the necessary data on the spot and immediately put it to use, without being burdened by complex equipment operation or cumbersome procedures.

The advantage of being able to perform RTK surveying with a smartphone is not just that it improves work efficiency; it also has the potential to change the way work is done on site. For example, processes that used to have to wait for a surveyor to arrive can now be advanced quickly at the discretion of the on-site technician. In addition, real-time digital sharing of survey results makes collaborative work—where multiple people participate remotely and provide immediate feedback—easy. Indeed, under the slogan "One surveying device per person", smartphone RTK is poised to become the on-site standard.

It's natural to feel uneasy about adopting new technologies, but with LRTK one of its appeals is the ease of getting started—you only need a smartphone. If you're interested, why not try this simple smartphone-based surveying on a small site or in a trial operation? Once you can freely handle position information with centimeter-level accuracy (cm level accuracy, half-inch accuracy), on-site productivity and creativity should improve dramatically. As a step toward pioneering the surveying style of the future, please consider using LRTK.

FAQ

Q: Can smartphone RTK really achieve centimeter-level accuracy? A: Yes, it is possible with appropriate equipment and conditions. By using RTK-capable receivers such as LRTK and correction information (Ntrip services or CLAS satellites), high-precision positioning that keeps horizontal errors within about 1-2 cm (0.4-0.8 in) can be achieved. Of course, there are some conditions, such as maintaining clear sky visibility, but it has been confirmed that in clear outdoor conditions accuracy comparable to conventional high-precision GNSS receivers can be obtained.

Q: What equipment and preparations are needed to get started with smartphone RTK? A: Basically, you can start simply by attaching a dedicated RTK receiver to your smartphone (currently iPhone and iPad are supported) and installing a compatible app. For LRTK, the dedicated receiver "LRTK Phone" and a smartphone case are sold as a set and can be attached with one touch. After that, you need an environment to receive correction information via the internet (using the smartphone's 4G/5G connection on-site), but even in locations without mobile coverage, if CLAS is supported you can receive corrections directly from satellites. There is no need to prepare a dedicated base station yourself, so it can be implemented with relatively simple preparations.

Q: Can it be used in mountainous areas or on sites outside mobile coverage? A: Yes. The LRTK receiver supports three-frequency GNSS and can be augmented by the QZSS Michibiki CLAS signal. Therefore, even in remote mountain interiors or isolated islands where mobile signals do not reach, as long as the sky is open, centimeter-level positioning (half-inch level) can be maintained. There are actual examples of its use in forest surveying and disaster sites, and it is useful as a surveying tool that does not rely on communications infrastructure. However, it is difficult to use in environments where satellites cannot be acquired, such as inside tunnels or in the shadow of buildings; in such cases, combining it with conventional methods or applying other measures is necessary.

Q: Is operating it difficult? Can people who aren't good with machines use it? A: If you're used to operating smartphone apps, it's not difficult. The LRTK app has an intuitive user interface and is designed so you can perform positioning and recording simply by looking at the map and tapping buttons. For example, "record a survey point" is a single tap, and "take a photo and save it" is also a single tap, so no complicated setup work is required. Even people who aren't comfortable with machines can start surveying right away after receiving basic instruction. Also, the app includes in-app help and a support system, so you can feel reassured if you run into any problems.

Q: Can smartphone RTK replace traditional surveying equipment (total stations and GNSS receivers)? A: It depends on the application. For many situations—general topographic surveying, as-built measurement, staking out positions for pile driving, and so on—smartphone RTK can be used as a substitute. Especially for wide-area surveys, sequential point recording, and point-cloud measurements, smartphone RTK is advantageous because a single person can work nimbly. On the other hand, for precision displacement measurements that require millimeter-level accuracy, or for cases that require positioning indoors or underground, traditional total stations or specialized instruments are still necessary. Therefore smartphone RTK should be regarded as one tool among others, and it is desirable to use it alongside conventional equipment according to site needs. At present, it is more commonly used as a new option that complements and improves the efficiency of traditional equipment, rather than as a single device that replaces everything.

Q: Can it be used on Android smartphones? A: As of 2024, LRTK is provided for iOS (iPhone/iPad). Because Apple devices offer stable access to high-precision GNSS and stable sensor performance, the product was first released for iOS. However, demand for an Android version is high, so development may be underway (please check official information). At present, Android users would use LRTK by pairing an iOS device with an LRTK receiver. It can also be used on tablets, and there are cases where an iPad has been introduced and used on-site.